From b6468bff55e94784b0332e77fdb06ffd69a236c9 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 5 Nov 2025 14:13:04 +0100
Subject: [PATCH 01/67] Rename test class mill.api.Opt to TestOpt

---
 .../test/src/mill/api/ApplicativeTests.scala  | 114 +++++++++---------
 core/api/test/src/mill/api/Opt.scala          |  30 -----
 core/api/test/src/mill/api/TestOpt.scala      |  30 +++++
 3 files changed, 87 insertions(+), 87 deletions(-)
 delete mode 100644 core/api/test/src/mill/api/Opt.scala
 create mode 100644 core/api/test/src/mill/api/TestOpt.scala

diff --git a/core/api/test/src/mill/api/ApplicativeTests.scala b/core/api/test/src/mill/api/ApplicativeTests.scala
index ea0da51eea14..f85db1e5e378 100644
--- a/core/api/test/src/mill/api/ApplicativeTests.scala
+++ b/core/api/test/src/mill/api/ApplicativeTests.scala
@@ -23,86 +23,86 @@ object ApplicativeTests extends TestSuite {
 
     test("selfContained") {
 
-      test("simple") - assert(Opt("lol " + 1) == Opt.some("lol 1"))
-      test("singleSome") - assert(Opt("lol " + Opt.some("hello")()) == Opt.some("lol hello"))
+      test("simple") - assert(TestOpt("lol " + 1) == TestOpt.some("lol 1"))
+      test("singleSome") - assert(TestOpt("lol " + TestOpt.some("hello")()) == TestOpt.some("lol hello"))
       test("twoSomes") - assert(
-        Opt(Opt.some("lol ")() + Opt.some("hello")()) == Opt.some("lol hello")
+        TestOpt(TestOpt.some("lol ")() + TestOpt.some("hello")()) == TestOpt.some("lol hello")
       )
-      test("singleNone") - assert(Opt("lol " + Opt.none()) == Opt.none)
-      test("twoNones") - assert(Opt("lol " + Opt.none() + Opt.none()) == Opt.none)
+      test("singleNone") - assert(TestOpt("lol " + TestOpt.none()) == TestOpt.none)
+      test("twoNones") - assert(TestOpt("lol " + TestOpt.none() + TestOpt.none()) == TestOpt.none)
       test("moreThan22") {
         assert(
-          Opt(
+          TestOpt(
             "lol " +
-              Opt.none() + Opt.none() + Opt.none() + Opt.none() + Opt.none() +
-              Opt.none() + Opt.none() + Opt.none() + Opt.none() + Opt.some(" world")() +
-              Opt.none() + Opt.none() + Opt.none() + Opt.none() + Opt.none() +
-              Opt.none() + Opt.none() + Opt.none() + Opt.none() + Opt.none() +
-              Opt.none() + Opt.none() + Opt.none() + Opt.none() + Opt.some(" moo")()
-          ) == Opt.none
+              TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() +
+              TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.some(" world")() +
+              TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() +
+              TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() +
+              TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.some(" moo")()
+          ) == TestOpt.none
         )
         assert(
-          Opt(
+          TestOpt(
             "lol " +
-              Opt.some("a")() + Opt.some("b")() + Opt.some("c")() + Opt.some("d")() +
-              Opt.some("e")() + Opt.some("a")() + Opt.some("b")() + Opt.some("c")() +
-              Opt.some("d")() + Opt.some("e")() + Opt.some("a")() + Opt.some("b")() +
-              Opt.some("c")() + Opt.some("d")() + Opt.some("e")() + Opt.some("a")() +
-              Opt.some("b")() + Opt.some("c")() + Opt.some("d")() + Opt.some("e")() +
-              Opt.some("a")() + Opt.some("b")() + Opt.some("c")() + Opt.some("d")() +
-              Opt.some("e")() + Opt.some("a")() + Opt.some("b")() + Opt.some("c")() +
-              Opt.some("d")() + Opt.some("e")() + Opt.some("a")() + Opt.some("b")() +
-              Opt.some("c")() + Opt.some("d")() + Opt.some("e")() + Opt.some("a")() +
-              Opt.some("b")() + Opt.some("c")() + Opt.some("d")() + Opt.some("e")() +
-              Opt.some("a")() + Opt.some("b")() + Opt.some("c")() + Opt.some("d")() +
-              Opt.some("e")() + Opt.some("a")() + Opt.some("b")() + Opt.some("c")() +
-              Opt.some("d")() + Opt.some("e")()
-          ) == Opt.some("lol abcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcde")
+              TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() +
+              TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() +
+              TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() +
+              TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some("a")() +
+              TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some("e")() +
+              TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() +
+              TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() +
+              TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() +
+              TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some("a")() +
+              TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some("e")() +
+              TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() +
+              TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() +
+              TestOpt.some("d")() + TestOpt.some("e")()
+          ) == TestOpt.some("lol abcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcde")
         )
       }
     }
     test("context") {
-      assert(Opt(Opt.ctx() + Opt.some("World")()) == Opt.some("hellooooWorld"))
+      assert(TestOpt(TestOpt.ctx() + TestOpt.some("World")()) == TestOpt.some("hellooooWorld"))
     }
     test("capturing") {
       val lol = "lol "
       def hell(o: String) = "hell" + o
-      test("simple") - assert(Opt(lol + 1) == Opt.some("lol 1"))
-      test("singleSome") - assert(Opt(lol + Opt.some(hell("o"))()) == Opt.some("lol hello"))
+      test("simple") - assert(TestOpt(lol + 1) == TestOpt.some("lol 1"))
+      test("singleSome") - assert(TestOpt(lol + TestOpt.some(hell("o"))()) == TestOpt.some("lol hello"))
       test("twoSomes") - assert(
-        Opt(Opt.some(lol)() + Opt.some(hell("o"))()) == Opt.some("lol hello")
+        TestOpt(TestOpt.some(lol)() + TestOpt.some(hell("o"))()) == TestOpt.some("lol hello")
       )
-      test("singleNone") - assert(Opt(lol + Opt.none()) == Opt.none)
-      test("twoNones") - assert(Opt(lol + Opt.none() + Opt.none()) == Opt.none)
+      test("singleNone") - assert(TestOpt(lol + TestOpt.none()) == TestOpt.none)
+      test("twoNones") - assert(TestOpt(lol + TestOpt.none() + TestOpt.none()) == TestOpt.none)
     }
     test("allowedLocalDef") {
-      // Although x is defined inside the Opt{...} block, it is also defined
+      // Although x is defined inside the TestOpt{...} block, it is also defined
       // within the LHS of the Applyable#apply call, so it is safe to life it
       // out into the `zipMap` arguments list.
-      val res = Opt { "lol " + new Opt(Some("hello").flatMap(x => Some(x))).apply() }
-      assert(res == Opt.some("lol hello"))
+      val res = TestOpt { "lol " + new TestOpt(Some("hello").flatMap(x => Some(x))).apply() }
+      assert(res == TestOpt.some("lol hello"))
     }
     test("upstreamAlwaysEvaluated") {
       // Whether or not control-flow reaches the Applyable#apply call inside an
-      // Opt{...} block, we always evaluate the LHS of the Applyable#apply
+      // TestOpt{...} block, we always evaluate the LHS of the Applyable#apply
       // because it gets lifted out of any control flow statements
       val counter = new Counter()
-      def up = Opt { "lol " + counter() }
-      val down = Opt { if ("lol".length > 10) up() else "fail" }
+      def up = TestOpt { "lol " + counter() }
+      val down = TestOpt { if ("lol".length > 10) up() else "fail" }
       assert(
-        down == Opt.some("fail"),
+        down == TestOpt.some("fail"),
         counter.value == 1
       )
     }
     test("upstreamEvaluatedOnlyOnce") {
       // Even if control-flow reaches the Applyable#apply call more than once,
-      // it only gets evaluated once due to its lifting out of the Opt{...} block
+      // it only gets evaluated once due to its lifting out of the TestOpt{...} block
       val counter = new Counter()
-      def up = Opt { "lol " + counter() }
+      def up = TestOpt { "lol " + counter() }
       def runTwice[T](t: => T) = (t, t)
-      val down = Opt { runTwice(up()) }
+      val down = TestOpt { runTwice(up()) }
       assert(
-        down == Opt.some(("lol 1", "lol 1")),
+        down == TestOpt.some(("lol 1", "lol 1")),
         counter.value == 1
       )
     }
@@ -110,38 +110,38 @@ object ApplicativeTests extends TestSuite {
       // This required some fiddling with owner chains inside the macro to get
       // working, so ensure it doesn't regress
       val counter = new Counter()
-      def up = Opt { "hello" + counter() }
-      val down1 = Opt { (() => up())() }
-      val down2 = Opt { Seq(1, 2, 3).map(n => up() * n) }
+      def up = TestOpt { "hello" + counter() }
+      val down1 = TestOpt { (() => up())() }
+      val down2 = TestOpt { Seq(1, 2, 3).map(n => up() * n) }
       assert(
-        down1 == Opt.some("hello1"),
-        down2 == Opt.some(Seq("hello2", "hello2hello2", "hello2hello2hello2"))
+        down1 == TestOpt.some("hello1"),
+        down2 == TestOpt.some(Seq("hello2", "hello2hello2", "hello2hello2hello2"))
       )
     }
     test("appliesEvaluatedOncePerLexicalCallsite") {
       // If you have multiple Applyable#apply() lexically in the source code of
-      // your Opt{...} call, each one gets evaluated once, even if the LHS of each
+      // your TestOpt{...} call, each one gets evaluated once, even if the LHS of each
       // apply() call is identical. It's up to the downstream zipMap()
       // implementation to decide if it wants to dedup them or do other things.
       val counter = new Counter()
-      def up = Opt { s"hello${counter()}" }
-      val down = Opt { Seq(1, 2, 3).map(n => n + up() + up()) }
-      assert(down == Opt.some(Seq("1hello1hello2", "2hello1hello2", "3hello1hello2")))
+      def up = TestOpt { s"hello${counter()}" }
+      val down = TestOpt { Seq(1, 2, 3).map(n => n + up() + up()) }
+      assert(down == TestOpt.some(Seq("1hello1hello2", "2hello1hello2", "3hello1hello2")))
     }
     test("appliesEvaluateBeforehand") {
-      // Every Applyable#apply() within a Opt{...} block evaluates before any
+      // Every Applyable#apply() within a TestOpt{...} block evaluates before any
       // other logic within that block, even if they would happen first in the
       // normal Scala evaluation order
       val counter = new Counter()
-      def up = Opt { counter() }
-      val down = Opt {
+      def up = TestOpt { counter() }
+      val down = TestOpt {
         val res = counter()
         val one = up()
         val two = up()
         val three = up()
         (res, one, two, three)
       }
-      assert(down == Opt.some((4, 1, 2, 3)))
+      assert(down == TestOpt.some((4, 1, 2, 3)))
     }
   }
 }
diff --git a/core/api/test/src/mill/api/Opt.scala b/core/api/test/src/mill/api/Opt.scala
deleted file mode 100644
index 2d1dc97100a3..000000000000
--- a/core/api/test/src/mill/api/Opt.scala
+++ /dev/null
@@ -1,30 +0,0 @@
-package mill.api
-
-import mill.api.internal.Applicative
-
-import scala.quoted.*
-
-case class Opt[+T](self: Option[T]) extends Applicative.Applyable[Opt, T]
-object Opt {
-  def none: Opt[Nothing] = new Opt(None)
-  def some[T](t: T): Opt[T] = new Opt(Some(t))
-  val injectedCtx = "helloooo"
-
-  def ctx()(using c: String): String = c
-  inline def apply[T](inline t: T): Opt[T] = ${ applyImpl[T]('t) }
-
-  def traverseCtx[I, R](xs: Seq[Opt[I]])(f: (Seq[I], String) => Applicative.Id[R])
-      : Opt[R] = {
-    new Opt(
-      if (xs.exists(_.self.isEmpty)) None
-      else Some(f(xs.map(_.self.get).toVector, Opt.injectedCtx))
-    )
-  }
-  def applyImpl[T: Type](t: Expr[T])(using
-      Quotes
-  ): Expr[Opt[T]] =
-    Applicative.impl[Opt, Opt, Applicative.Id, T, String](
-      (args, fn) => '{ traverseCtx($args)($fn) },
-      t
-    )
-}
diff --git a/core/api/test/src/mill/api/TestOpt.scala b/core/api/test/src/mill/api/TestOpt.scala
new file mode 100644
index 000000000000..c9ac7d35029a
--- /dev/null
+++ b/core/api/test/src/mill/api/TestOpt.scala
@@ -0,0 +1,30 @@
+package mill.api
+
+import mill.api.internal.Applicative
+
+import scala.quoted.*
+
+case class TestOpt[+T](self: Option[T]) extends Applicative.Applyable[TestOpt, T]
+object TestOpt {
+  def none: TestOpt[Nothing] = new TestOpt(None)
+  def some[T](t: T): TestOpt[T] = new TestOpt(Some(t))
+  val injectedCtx = "helloooo"
+
+  def ctx()(using c: String): String = c
+  inline def apply[T](inline t: T): TestOpt[T] = ${ applyImpl[T]('t) }
+
+  def traverseCtx[I, R](xs: Seq[TestOpt[I]])(f: (Seq[I], String) => Applicative.Id[R])
+      : TestOpt[R] = {
+    new TestOpt(
+      if (xs.exists(_.self.isEmpty)) None
+      else Some(f(xs.map(_.self.get).toVector, TestOpt.injectedCtx))
+    )
+  }
+  def applyImpl[T: Type](t: Expr[T])(using
+      Quotes
+  ): Expr[TestOpt[T]] =
+    Applicative.impl[TestOpt, TestOpt, Applicative.Id, T, String](
+      (args, fn) => '{ traverseCtx($args)($fn) },
+      t
+    )
+}

From 6d5c78114f3d2abf78256b904f36584e67f9e6b3 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 5 Nov 2025 14:14:02 +0100
Subject: [PATCH 02/67] Introduce new mill.api.opt with Opt, OptGroup, Opts and
 OptSyntax

---
 .../mill/api/daemon/internal/OptsApi.scala    |  13 ++
 core/api/src/mill/api/opt/Opt.scala           |  93 +++++++++++++++
 core/api/src/mill/api/opt/OptGroup.scala      |  54 +++++++++
 core/api/src/mill/api/opt/OptSyntax.scala     |  16 +++
 core/api/src/mill/api/opt/Opts.scala          |  32 +++++
 .../api/test/src/mill/api/opt/OptsTests.scala | 112 ++++++++++++++++++
 6 files changed, 320 insertions(+)
 create mode 100644 core/api/daemon/src/mill/api/daemon/internal/OptsApi.scala
 create mode 100644 core/api/src/mill/api/opt/Opt.scala
 create mode 100644 core/api/src/mill/api/opt/OptGroup.scala
 create mode 100644 core/api/src/mill/api/opt/OptSyntax.scala
 create mode 100644 core/api/src/mill/api/opt/Opts.scala
 create mode 100644 core/api/test/src/mill/api/opt/OptsTests.scala

diff --git a/core/api/daemon/src/mill/api/daemon/internal/OptsApi.scala b/core/api/daemon/src/mill/api/daemon/internal/OptsApi.scala
new file mode 100644
index 000000000000..37700ac9450e
--- /dev/null
+++ b/core/api/daemon/src/mill/api/daemon/internal/OptsApi.scala
@@ -0,0 +1,13 @@
+package mill.api.daemon.internal
+
+trait OptsApi {
+  def toStringSeq: Seq[String]
+  def value: Seq[OptGroupApi]
+}
+
+trait OptGroupApi {
+}
+
+trait OptApi {
+  def toString(): String
+}
diff --git a/core/api/src/mill/api/opt/Opt.scala b/core/api/src/mill/api/opt/Opt.scala
new file mode 100644
index 000000000000..e3f9d7305a63
--- /dev/null
+++ b/core/api/src/mill/api/opt/Opt.scala
@@ -0,0 +1,93 @@
+package mill.api.opt
+
+import mill.api.daemon.internal.OptApi
+import mill.api.JsonFormatters.given
+
+import scala.annotation.targetName
+import scala.language.implicitConversions
+
+case class Opt private (value: Seq[Opt.OptTypes]) extends OptApi {
+  override def toString(): String = value.mkString("")
+
+  def map(conv: Opt.OptTypes => Opt.OptTypes): Opt = Opt(value.map(conv)*)
+
+  private def startStrings: Seq[String] =
+    value.takeWhile(_.isInstanceOf[String]).collect { case s: String => s }
+
+  def startsWith(prefix: String): Boolean = startStrings.mkString("").startsWith(prefix)
+
+  def mapStartString(rep: String => String): Opt = {
+    val rest = value.dropWhile(_.isInstanceOf[String])
+    Opt((rep(startStrings.mkString("")) +: rest)*)
+  }
+
+  def containsPaths: Boolean = value.exists {
+    case _: os.Path => true
+    case _ => false
+  }
+}
+
+object Opt {
+
+  type OptTypes = (String | os.Path)
+
+  @targetName("applyVarArg")
+  def apply(value: OptTypes*): Opt = {
+    // TODO: merge sequential strings
+    new Opt(value.filter {
+      case s: String if s.isEmpty => false
+      case _ => true
+    })
+  }
+
+  /**
+   * Constructs a path from multiple path elements and a separator string.
+   * Can be used to render classpaths.
+   * Each path component will still be handled properly, e.g. mapped according to the current [[MappedPaths]] mapping.
+   */
+  def mkPath(paths: Seq[os.Path], prefix: String = "", sep: String, suffix: String = ""): Opt = {
+    var needSep = false
+    Opt(
+      (
+        Seq(prefix) ++
+          paths.flatMap { path =>
+            if (needSep)
+              Seq(sep, path)
+            else {
+              needSep = true
+              Seq(path)
+            }
+          } ++ Seq(suffix)
+      )*
+    )
+  }
+
+  def mkPlatformPath(paths: Seq[os.Path]): Opt = mkPath(paths, sep = java.io.File.pathSeparator)
+
+  given jsonReadWriter: upickle.ReadWriter[Opt] =
+    upickle.readwriter[Seq[(Option[String], Option[os.Path])]].bimap(
+      _.value.map {
+        case path: os.Path => (None, Some(path))
+        case str: String => (Some(str), None)
+      },
+      seq =>
+        Opt(seq.map {
+          case (Some(str), _) => str
+          case (_, Some(path)) => path
+        }*)
+    )
+
+//  given stringToOpt: Conversion[String, Opt] = (value: String) => Opt(value)
+//  given osPathToOpt: Conversion[os.Path, Opt] = (value: os.Path) => Opt(value)
+
+//  implicit def IterableToOpt[T](s: Iterable[T])(using f: T => Opt): Opt =
+//    Opt(s.toSeq.flatMap(f(_).value))
+
+
+  implicit def StringToOpt(s: String): Opt = Opt(s)
+
+  implicit def OsPathToOpt(p: os.Path): Opt = Opt(p)
+
+  implicit def OptToOpt(o: Opt): Opt = o
+
+}
diff --git a/core/api/src/mill/api/opt/OptGroup.scala b/core/api/src/mill/api/opt/OptGroup.scala
new file mode 100644
index 000000000000..83fefd77b59f
--- /dev/null
+++ b/core/api/src/mill/api/opt/OptGroup.scala
@@ -0,0 +1,54 @@
+package mill.api.opt
+
+import mill.api.daemon.internal.OptGroupApi
+
+import scala.annotation.targetName
+import scala.language.implicitConversions
+
+/**
+ * A set of options, which are used together
+ */
+case class OptGroup private (value: Seq[Opt]) extends OptGroupApi
+    derives upickle.ReadWriter {
+  override def toString(): String = value.mkString("(", ", ", ")")
+
+  def isEmpty: Boolean = value.isEmpty
+
+  def containsPaths: Boolean = value.exists(_.containsPaths)
+
+  def head: Opt = value.head
+
+  def toStringSeq: Seq[String] = value.map(_.toString())
+
+  def concat(suffix: OptGroup): OptGroup = new OptGroup(value ++ suffix.value)
+
+  @`inline` final def ++(suffix: OptGroup): OptGroup = concat(suffix)
+
+}
+
+object OptGroup {
+  @targetName("applyVarAar")
+  def apply(opts: Opt*): OptGroup = new OptGroup(opts)
+  @targetName("applyIterable")
+  def apply[T](opts: T*)(using f: T => Opt): OptGroup = new OptGroup(opts.map(f(_)))
+
+  def when(cond: Boolean)(value: Opt*): OptGroup = if (cond) OptGroup(value*) else OptGroup()
+
+//  given optsToOptGroup: Conversion[(OptTypes, OptTypes), OptGroup] =
+//    (tuple: (OptTypes, OptTypes)) =>
+//      OptGroup(Opt(tuple._1), Opt(tuple._2))
+
+  implicit def StringToOptGroup(s: String): OptGroup = OptGroup(Seq(Opt(s)))
+
+  implicit def OsPathToOptGroup(p: os.Path): OptGroup = OptGroup(Seq(Opt(p)))
+
+  implicit def OptToOptGroup(o: Opt): OptGroup = OptGroup(Seq(o))
+
+  implicit def IterableToOptGroup[T](s: Iterable[T])(using f: T => OptGroup): OptGroup =
+    OptGroup(s.toSeq.flatMap(f(_).value))
+
+//  implicit def ArrayToOptGroup[T](s: Array[T])(using f: T => OptGroup): OptGroup =
+//    OptGroup(s.flatMap(f(_).value))
+
+
+}
diff --git a/core/api/src/mill/api/opt/OptSyntax.scala b/core/api/src/mill/api/opt/OptSyntax.scala
new file mode 100644
index 000000000000..e0816fcaca07
--- /dev/null
+++ b/core/api/src/mill/api/opt/OptSyntax.scala
@@ -0,0 +1,16 @@
+package mill.api.opt
+
+import scala.language.implicitConversions
+
+implicit class OptSyntax(ctx: StringContext) extends AnyVal {
+  def opt(opts: Any*): Opt = {
+    val vals = ctx.parts.take(opts.length).zip(opts).flatMap { case (p, a) => Seq(p, a) } ++
+      ctx.parts.drop(opts.length)
+
+    val elems: Seq[(String | os.Path)] = vals.flatMap {
+      case path: os.Path => Seq(path)
+      case s => Seq(s.toString).filter(_.nonEmpty)
+    }
+    Opt(elems*)
+  }
+}
diff --git a/core/api/src/mill/api/opt/Opts.scala b/core/api/src/mill/api/opt/Opts.scala
new file mode 100644
index 000000000000..6d5958c09821
--- /dev/null
+++ b/core/api/src/mill/api/opt/Opts.scala
@@ -0,0 +1,32 @@
+package mill.api.opt
+
+import mill.api.daemon.internal.OptsApi
+
+import scala.language.implicitConversions
+
+case class Opts private (override val value: OptGroup*) extends OptsApi
+    derives upickle.ReadWriter {
+
+  def toStringSeq: Seq[String] = value.flatMap(_.toStringSeq)
+  override def toString(): String = value.mkString("Opts(", ", ", ")")
+
+  def concat(suffix: Opts): Opts = Opts(value ++ suffix.value)
+  @`inline` final def ++(suffix: Opts): Opts = concat(suffix)
+
+  def containsPaths: Boolean = value.exists(_.containsPaths)
+
+  def isEmpty: Boolean = value.isEmpty
+  def filterGroup(pred: OptGroup => Boolean): Opts = Opts(value.filter(pred)*)
+  def mapGroup(f: OptGroup => OptGroup): Opts = Opts(value.map(f)*)
+  def flatMap(f: OptGroup => Seq[OptGroup]): Opts = Opts(value.flatMap(f)*)
+}
+
+object Opts {
+  def apply(value: OptGroup*): Opts = new Opts(value.filter(!_.isEmpty))
+//  @targetName("applyUnion")
+//  def apply(value: (Opt | OptGroup | Seq[Opt])*): Opts = Opts(value.flatMap {
+//    case a: Opt => Seq(OptGroup(a))
+//    case a: OptGroup => Seq(a)
+//    case s: Iterable[Opt] => s.map(OptGroup(_))
+//  })
+}
diff --git a/core/api/test/src/mill/api/opt/OptsTests.scala b/core/api/test/src/mill/api/opt/OptsTests.scala
new file mode 100644
index 000000000000..b015bd867f40
--- /dev/null
+++ b/core/api/test/src/mill/api/opt/OptsTests.scala
@@ -0,0 +1,112 @@
+package mill.api.opt
+
+import utest.*
+import mill.api.opt.*
+
+class OptsTests extends TestSuite {
+
+  val homeDir = os.home
+  val workDir = homeDir / "work"
+  val outDir = workDir / "out"
+
+  val srcDir1 = workDir / "src1"
+  val srcDir2 = workDir / "src2"
+  val sources1 = Seq(srcDir1, srcDir2)
+
+  val srcDir3 = workDir / "src3"
+  val srcDir4 = workDir / "src4"
+  val sources2 = Seq(srcDir3, srcDir4)
+
+  val plugin1 = homeDir / ".cache" / "plugin1"
+
+  val opts1 = Opts(
+    // single arg
+    Opt("-deprecation"),
+    // implicit single args
+    "-verbose",
+    // two args as group
+    OptGroup("--release", "17"),
+    // an option including a file via ArgParts
+    Opt("-Xplugin=", plugin1),
+    // an option including a file via arg string interpolator
+    opt"-Xplugin:${plugin1}",
+    // some files
+    sources1,
+    // some files as ArgGroup
+    OptGroup(sources2*),
+    // Mixed ArgGroup
+    OptGroup(opt"--extra", opt"-Xplugin=${plugin1}") ++ OptGroup(sources1*)
+  )
+
+  val expectedOpts1 = Opts(
+    Opt("-deprecation"),
+    Opt("-verbose"),
+    OptGroup(
+      Opt("--release"),
+      Opt("17")
+    ),
+    Opt("-Xplugin=", plugin1),
+    Opt("-Xplugin:", plugin1),
+    Opt(srcDir1),
+    Opt(srcDir2),
+    Opt(srcDir3),
+    Opt(srcDir4),
+    OptGroup(
+      Opt("--extra"),
+      Opt("-Xplugin=", plugin1),
+      Opt(srcDir1),
+      Opt(srcDir2)
+    )
+  )
+
+  val expectedSeq1 = Seq(
+    "-deprecation",
+    "-verbose",
+    "--release",
+    "17",
+    s"-Xplugin=${plugin1.toString()}",
+    // an option including a file via arg string interpolator
+    s"-Xplugin:${plugin1.toString()}"
+  ) ++
+    sources1.map(_.toString())
+    ++
+    sources2.map(_.toString())
+    ++
+    Seq(
+      "--extra",
+      s"-Xplugin=${plugin1.toString()}"
+    ) ++
+    sources1.map(_.toString())
+
+  override def tests: Tests = Tests {
+    test("structure") {
+      assert(opts1 == expectedOpts1)
+    }
+    test("toStringSeq") {
+      val str = opts1.toStringSeq
+      assert(str == expectedSeq1)
+    }
+    test("jsonify") {
+      test("without-mapping") {
+        val json = upickle.write(opts1)
+        assertGoldenLiteral(
+          json,
+          "{\"value\":[{\"value\":[[[\"-deprecation\",null]],[[\"-verbose\",null]],[[\"--release\",null]],[[\"17\",null]],[[\"-Xplugin=\",null],[null,\"/home/lefou/.cache/plugin1\"]],[[\"-Xplugin:\",null],[null,\"/home/lefou/.cache/plugin1\"]],[[null,\"/home/lefou/work/src1\"]],[[null,\"/home/lefou/work/src2\"]],[[null,\"/home/lefou/work/src3\"]],[[null,\"/home/lefou/work/src4\"]],[[\"--extra\",null]],[[\"-Xplugin=\",null],[null,\"/home/lefou/.cache/plugin1\"]],[[null,\"/home/lefou/work/src1\"]],[[null,\"/home/lefou/work/src2\"]]]}]}"
+        )
+        assert(json.split("\\Q$HOME\\E").size == 1)
+        val back = upickle.read[Opts](json)
+        assert(opts1 == back)
+      }
+//      test("with-mapping-home") {
+//        val json = upickle.write(opts1)
+//        assertGoldenLiteral(
+//          json,
+//          "{\"value\":[{\"value\":[[[\"-deprecation\",null]],[[\"-verbose\",null]],[[\"--release\",null]],[[\"17\",null]],[[\"-Xplugin=\",null],[null,\"/home/lefou/.cache/plugin1\"]],[[\"-Xplugin:\",null],[null,\"/home/lefou/.cache/plugin1\"]],[[null,\"/home/lefou/work/src1\"]],[[null,\"/home/lefou/work/src2\"]],[[null,\"/home/lefou/work/src3\"]],[[null,\"/home/lefou/work/src4\"]],[[\"--extra\",null]],[[\"-Xplugin=\",null],[null,\"/home/lefou/.cache/plugin1\"]],[[null,\"/home/lefou/work/src1\"]],[[null,\"/home/lefou/work/src2\"]]]}]}"
+//        )
+//        assert(json.split("\\Q$HOME\\E").size == 10)
+//        val back = upickle.read[Opts](json)
+//        assert(opts1 == back)
+//      }
+    }
+  }
+}

From 7fbd722fb07b44dcaef4b5d00ceae9454fa75996 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 5 Nov 2025 14:16:09 +0100
Subject: [PATCH 03/67] WIP Replace `Seq[String]` used for options by
 `mill.api.opt.Opts`

---
 .../jmh/src/mill/contrib/jmh/JmhModule.scala  |  2 +-
 .../contrib/scoverage/ScoverageModule.scala   | 21 ++++---
 .../api/daemon/internal/JavaModuleApi.scala   |  4 +-
 .../internal/bsp/BspJavaModuleApi.scala       | 14 +++--
 .../daemon/internal/bsp/BspRunModuleApi.scala | 10 ++--
 .../androidlib/AndroidAppKotlinModule.scala   | 19 ++++---
 .../mill/androidlib/AndroidKotlinModule.scala | 29 +++++-----
 .../src/mill/androidlib/AndroidModule.scala   |  4 +-
 .../androidlib/hilt/AndroidHiltSupport.scala  | 11 ++--
 .../src/mill/javalib/AssemblyModule.scala     | 13 +++--
 .../javalib/src/mill/javalib/JavaModule.scala | 24 +++-----
 .../src/mill/javalib/JavacOptions.scala       |  9 +++
 libs/javalib/src/mill/javalib/RunModule.scala | 19 +++----
 .../mill/javalib/SemanticDbJavaModule.scala   | 29 ++++++----
 .../javalib/src/mill/javalib/TestModule.scala | 19 +++----
 .../src/mill/javalib/bsp/BspJavaModule.scala  | 18 +++---
 .../src/mill/javalib/bsp/BspRunModule.scala   | 10 ++--
 .../javalib/errorprone/ErrorProneModule.scala | 19 ++++---
 .../src/mill/javalib/idea/GenIdeaModule.scala | 10 ++--
 .../javalib/repackage/RepackageModule.scala   |  6 +-
 .../test/src/mill/javalib/LauncherTests.scala |  3 +-
 .../javalib/errorprone/ErrorProneTests.scala  | 10 ++--
 .../src/mill/kotlinlib/KotlinModule.scala     | 46 ++++++++--------
 .../src/mill/kotlinlib/KotlincOptions.scala   |  7 +++
 .../mill/kotlinlib/js/KotlinJsModule.scala    | 27 ++++-----
 .../mill/kotlinlib/kover/KoverModule.scala    | 13 ++---
 .../src/mill/kotlinlib/ksp/KspModule.scala    | 26 ++++-----
 .../contrib/kover/KoverModuleTests.scala      |  5 +-
 .../src/mill/scalajslib/ScalaJSModule.scala   |  9 +--
 .../mill/scalajslib/CompileLinkTests.scala    | 10 ++--
 .../src/mill/scalalib/ScalaModule.scala       | 51 +++++++++--------
 .../src/mill/scalalib/ScalacOptions.scala     |  9 +++
 .../mill/scalalib/idea/GenIdeaModule.scala    |  1 +
 .../src/mill/scalalib/CrossVersionTests.scala |  3 +-
 .../src/mill/scalalib/HelloWorldTests.scala   | 21 +++----
 .../mill/scalalib/ScalaColorOutputTests.scala |  3 +-
 .../src/mill/scalalib/ScalaFlagsTests.scala   |  5 +-
 .../src/mill/scalalib/ScalaMacrosTests.scala  |  3 +-
 .../mill/scalalib/ScalaScaladocTests.scala    | 15 ++---
 .../scalanativelib/ScalaNativeModule.scala    | 10 ++--
 .../mill/scalanativelib/CompileRunTests.scala |  3 +-
 mill-itest-plugin.adoc                        | 55 +++++++++++++++++++
 .../mill/bsp/worker/MillJavaBuildServer.scala |  2 +-
 .../mill/bsp/worker/MillJvmBuildServer.scala  | 30 +++++-----
 .../bsp/worker/MillScalaBuildServer.scala     | 10 ++--
 .../src/mill/meta/MillBuildRootModule.scala   | 13 ++---
 46 files changed, 393 insertions(+), 287 deletions(-)
 create mode 100644 libs/javalib/src/mill/javalib/JavacOptions.scala
 create mode 100644 libs/kotlinlib/src/mill/kotlinlib/KotlincOptions.scala
 create mode 100644 libs/scalalib/src/mill/scalalib/ScalacOptions.scala
 create mode 100644 mill-itest-plugin.adoc

diff --git a/contrib/jmh/src/mill/contrib/jmh/JmhModule.scala b/contrib/jmh/src/mill/contrib/jmh/JmhModule.scala
index 6cb760182bfd..44fc0a52bb8d 100644
--- a/contrib/jmh/src/mill/contrib/jmh/JmhModule.scala
+++ b/contrib/jmh/src/mill/contrib/jmh/JmhModule.scala
@@ -77,7 +77,7 @@ trait JmhModule extends JavaModule {
   def generateBenchmarkSources =
     Task {
       val dest = Task.ctx().dest
-      val forkedArgs = forkArgs().toSeq
+      val forkedArgs = forkArgs().toStringSeq
       val sourcesDir = dest / "jmh_sources"
       val resourcesDir = dest / "jmh_resources"
 
diff --git a/contrib/scoverage/src/mill/contrib/scoverage/ScoverageModule.scala b/contrib/scoverage/src/mill/contrib/scoverage/ScoverageModule.scala
index 522db75abc4f..a624711b14fa 100644
--- a/contrib/scoverage/src/mill/contrib/scoverage/ScoverageModule.scala
+++ b/contrib/scoverage/src/mill/contrib/scoverage/ScoverageModule.scala
@@ -1,10 +1,9 @@
 package mill.contrib.scoverage
 
 import coursier.Repository
-import mill._
-import mill.api.{PathRef}
-import mill.api.BuildCtx
-import mill.api.{Result}
+import mill.*
+import mill.api.{BuildCtx, PathRef, Result}
+import mill.api.opt.*
 import mill.contrib.scoverage.api.ScoverageReportWorkerApi2.ReportType
 import mill.util.BuildInfo
 import mill.javalib.api.JvmWorkerUtil
@@ -168,18 +167,18 @@ trait ScoverageModule extends ScalaModule { outer: ScalaModule =>
       Task { outer.scalacPluginMvnDeps() ++ outer.scoveragePluginDeps() }
 
     /** Add the scoverage specific plugin settings (`dataDir`). */
-    override def scalacOptions: T[Seq[String]] =
+    override def scalacOptions: T[Opts] =
       Task {
         val extras =
           if (isScala3()) {
-            Seq(
-              s"-coverage-out:${data().path.toIO.getPath()}",
-              s"-sourceroot:${BuildCtx.workspaceRoot}"
+            Opts(
+              opt"-coverage-out:${data().path}",
+              opt"-sourceroot:${BuildCtx.workspaceRoot}"
             )
           } else {
-            Seq(
-              s"-P:scoverage:dataDir:${data().path.toIO.getPath()}",
-              s"-P:scoverage:sourceRoot:${BuildCtx.workspaceRoot}"
+            Opts(
+              opt"-P:scoverage:dataDir:${data().path}",
+              opt"-P:scoverage:sourceRoot:${BuildCtx.workspaceRoot}"
             )
           }
 
diff --git a/core/api/daemon/src/mill/api/daemon/internal/JavaModuleApi.scala b/core/api/daemon/src/mill/api/daemon/internal/JavaModuleApi.scala
index 2c94a5baebb9..73f49fdc6b18 100644
--- a/core/api/daemon/src/mill/api/daemon/internal/JavaModuleApi.scala
+++ b/core/api/daemon/src/mill/api/daemon/internal/JavaModuleApi.scala
@@ -13,8 +13,8 @@ trait JavaModuleApi extends ModuleApi with GenIdeaModuleApi {
 
   def transitiveModuleCompileModuleDeps: Seq[JavaModuleApi]
 
-  def javacOptions: TaskApi[Seq[String]]
-  def mandatoryJavacOptions: TaskApi[Seq[String]]
+  def javacOptions: TaskApi[OptsApi]
+  def mandatoryJavacOptions: TaskApi[OptsApi]
 
   // BSP Tasks that sometimes need to be customized
 
diff --git a/core/api/daemon/src/mill/api/daemon/internal/bsp/BspJavaModuleApi.scala b/core/api/daemon/src/mill/api/daemon/internal/bsp/BspJavaModuleApi.scala
index a83955d8ae50..27971f63c998 100644
--- a/core/api/daemon/src/mill/api/daemon/internal/bsp/BspJavaModuleApi.scala
+++ b/core/api/daemon/src/mill/api/daemon/internal/bsp/BspJavaModuleApi.scala
@@ -1,6 +1,6 @@
 package mill.api.daemon.internal.bsp
 
-import mill.api.daemon.internal.{EvaluatorApi, ModuleApi, TaskApi}
+import mill.api.daemon.internal.{EvaluatorApi, ModuleApi, OptApi, OptsApi, TaskApi}
 
 import java.nio.file.Path
 
@@ -34,7 +34,7 @@ trait BspJavaModuleApi extends ModuleApi {
   )
       : TaskApi[EvaluatorApi => (
           classesPath: Path,
-          javacOptions: Seq[String],
+          javacOptions: OptsApi,
           classpath: Seq[String]
       )]
 
@@ -42,13 +42,17 @@ trait BspJavaModuleApi extends ModuleApi {
       needsToMergeResourcesIntoCompileDest: Boolean,
       enableJvmCompileClasspathProvider: Boolean,
       clientWantsSemanticDb: Boolean
-  ): TaskApi[(Seq[String], EvaluatorApi => Seq[String], EvaluatorApi => java.nio.file.Path)]
+  ): TaskApi[(
+      scalacOptionsTask: OptsApi,
+      compileClasspathTask: EvaluatorApi => Seq[String],
+      classPathTask: EvaluatorApi => java.nio.file.Path
+  )]
 
   private[mill] def bspBuildTargetScalaMainClasses
       : TaskApi[(
           classes: Seq[String],
-          forkArgs: Seq[String],
-          forkEnv: Map[String, String]
+          forkArgs: OptsApi,
+          forkEnv: Map[String, OptApi]
       )]
 
   private[mill] def bspLoggingTest: TaskApi[Unit]
diff --git a/core/api/daemon/src/mill/api/daemon/internal/bsp/BspRunModuleApi.scala b/core/api/daemon/src/mill/api/daemon/internal/bsp/BspRunModuleApi.scala
index 62d1a7d2a128..86f62b63f7f0 100644
--- a/core/api/daemon/src/mill/api/daemon/internal/bsp/BspRunModuleApi.scala
+++ b/core/api/daemon/src/mill/api/daemon/internal/bsp/BspRunModuleApi.scala
@@ -1,6 +1,6 @@
 package mill.api.daemon.internal.bsp
 
-import mill.api.daemon.internal.{ModuleApi, TaskApi}
+import mill.api.daemon.internal.{ModuleApi, OptApi, OptsApi, TaskApi}
 
 import java.nio.file.Path
 
@@ -10,18 +10,18 @@ trait BspRunModuleApi extends ModuleApi {
 
   private[mill] def bspJvmRunEnvironment: TaskApi[(
       runClasspath: Seq[Path],
-      forkArgs: Seq[String],
+      forkArgs: OptsApi,
       forkWorkingDir: Path,
-      forkEnv: Map[String, String],
+      forkEnv: Map[String, OptApi],
       mainClass: Option[String],
       localMainClasses: Seq[String]
   )]
 
   private[mill] def bspJvmTestEnvironment: TaskApi[(
       runClasspath: Seq[Path],
-      forkArgs: Seq[String],
+      forkArgs: OptsApi,
       forkWorkingDir: Path,
-      forkEnv: Map[String, String],
+      forkEnv: Map[String, OptApi],
       mainClass: Option[String],
       testEnvVars: Option[(
           mainClass: String,
diff --git a/libs/androidlib/src/mill/androidlib/AndroidAppKotlinModule.scala b/libs/androidlib/src/mill/androidlib/AndroidAppKotlinModule.scala
index b94d379263fd..5e976318d626 100644
--- a/libs/androidlib/src/mill/androidlib/AndroidAppKotlinModule.scala
+++ b/libs/androidlib/src/mill/androidlib/AndroidAppKotlinModule.scala
@@ -2,6 +2,7 @@ package mill.androidlib
 
 import coursier.params.ResolutionParams
 import mill.api.{ModuleRef, PathRef, Task}
+import mill.api.opt.*
 import mill.kotlinlib.{Dep, DepSyntax}
 import mill.javalib.TestModule.Junit5
 import mill.javalib.{JavaModule, TestModule}
@@ -293,7 +294,7 @@ trait AndroidAppKotlinModule extends AndroidKotlinModule, AndroidAppModule { out
       PathRef(diffImageDir)
     }
 
-    override def forkArgs: T[Seq[String]] = super.forkArgs() ++ testJvmArgs()
+    override def forkArgs: T[Opts] = super.forkArgs() ++ testJvmArgs()
     override def runClasspath: T[Seq[PathRef]] =
       super.runClasspath() ++ androidPreviewScreenshotTestEngineClasspath() ++ compileClasspath()
 
@@ -315,18 +316,18 @@ trait AndroidAppKotlinModule extends AndroidKotlinModule, AndroidAppModule { out
     As defined in
     [[https://android.googlesource.com/platform/tools/base/+/61923408e5f7dc20f0840844597f9dde17453a0f/preview/screenshot/screenshot-test-gradle-plugin/src/main/java/com/android/compose/screenshot/tasks/PreviewScreenshotValidationTask.kt?#84]]
      */
-    private def testJvmArgs: T[Seq[String]] = Task {
+    private def testJvmArgs: T[Opts] = Task {
       val params = Map(
-        "previews-discovered" -> androidDiscoveredPreviews().previewsDiscoveredJsonFile.path.toString(),
-        "referenceImageDirPath" -> screenshotResults().path.toString(),
-        "diffImageDirPath" -> diffImageDirPath().path.toString,
-        "renderResultsFilePath" -> androidScreenshotGeneratedResults().path.toString,
-        "renderTaskOutputDir" -> screenshotResults().path.toString(),
-        "resultsDirPath" -> androidScreenshotTestResultDir().path.toString(),
+        "previews-discovered" -> androidDiscoveredPreviews().previewsDiscoveredJsonFile.path,
+        "referenceImageDirPath" -> screenshotResults().path,
+        "diffImageDirPath" -> diffImageDirPath().path,
+        "renderResultsFilePath" -> androidScreenshotGeneratedResults().path,
+        "renderTaskOutputDir" -> screenshotResults().path,
+        "resultsDirPath" -> androidScreenshotTestResultDir().path,
         "threshold" -> androidScreenshotTestDiffThreshold.toString
       )
 
-      params.map { case (k, v) => s"$JvmTestArg$k=$v" }.toSeq
+      Opts(params.map { case (k, v) => opt"$JvmTestArg$k=$v" })
     }
 
     private val JvmTestArg = "-Dcom.android.tools.preview.screenshot.junit.engine."
diff --git a/libs/androidlib/src/mill/androidlib/AndroidKotlinModule.scala b/libs/androidlib/src/mill/androidlib/AndroidKotlinModule.scala
index c71449a631c6..0c23379dbe77 100644
--- a/libs/androidlib/src/mill/androidlib/AndroidKotlinModule.scala
+++ b/libs/androidlib/src/mill/androidlib/AndroidKotlinModule.scala
@@ -2,15 +2,11 @@ package mill.androidlib
 
 import mill.*
 import mill.api.{ModuleRef, PathRef, Result}
+import mill.api.opt.*
 import mill.javalib.{CoursierModule, Dep}
-import mill.kotlinlib.{Dep, DepSyntax, KotlinModule}
+import mill.kotlinlib.{Dep, DepSyntax, KotlinModule, KotlincOptions}
 import mill.{T, Task}
-import mill.androidlib.databinding.{
-  AndroidDataBindingWorker,
-  GenerateBindingSourcesArgs,
-  ProcessResourcesArgs,
-  AndroidDataBindingWorkerModule
-}
+import mill.androidlib.databinding.{AndroidDataBindingWorker, AndroidDataBindingWorkerModule, GenerateBindingSourcesArgs, ProcessResourcesArgs}
 import mill.util.Jvm
 
 // TODO expose Compose configuration options
@@ -185,21 +181,26 @@ trait AndroidKotlinModule extends KotlinModule with AndroidModule { outer =>
    * If this module has any module dependencies, we need to tell the kotlin compiler to
    * handle the compiled output as a friend path so top level declarations are visible.
    */
-  def kotlincFriendPaths: T[Option[String]] = Task {
+  def kotlincFriendPaths: T[Opts] = Task {
     val compiledCodePaths = Task.traverse(transitiveModuleCompileModuleDeps)(m =>
       Task.Anon {
         Seq(m.compile().classes.path)
       }
     )().flatten
 
-    val friendlyPathFlag: Option[String] =
-      compiledCodePaths.headOption.map(_ => s"-Xfriend-paths=${compiledCodePaths.mkString(",")}")
-
-    friendlyPathFlag
+    Opts(
+      OptGroup.when(compiledCodePaths.nonEmpty)(
+        Opt.mkPath(
+          compiledCodePaths,
+          prefix = KotlincOptions.`-Xfriend-paths`,
+          sep = KotlincOptions.friendPathSeparator
+        )
+      )
+    )
   }
 
-  override def kotlincOptions: T[Seq[String]] = Task {
-    super.kotlincOptions() ++ kotlincFriendPaths().toSeq
+  override def kotlincOptions: T[Opts] = Task {
+    super.kotlincOptions() ++ kotlincFriendPaths()
   }
 
   def kspDependencyResolver: Task[CoursierModule.Resolver] = Task.Anon {
diff --git a/libs/androidlib/src/mill/androidlib/AndroidModule.scala b/libs/androidlib/src/mill/androidlib/AndroidModule.scala
index c754872fa69e..0e93003dd768 100644
--- a/libs/androidlib/src/mill/androidlib/AndroidModule.scala
+++ b/libs/androidlib/src/mill/androidlib/AndroidModule.scala
@@ -516,7 +516,7 @@ trait AndroidModule extends JavaModule { outer =>
    * The Java compiled classes of [[androidResources]]
    */
   def androidCompiledRClasses: T[CompilationResult] = Task(persistent = true) {
-    val jOpts = JavaCompilerOptions.split(javacOptions() ++ mandatoryJavacOptions())
+    val jOpts = JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
     val worker = jvmWorker().internalWorker()
     worker.apply(
       ZincCompileJava(
@@ -728,7 +728,7 @@ trait AndroidModule extends JavaModule { outer =>
 
     val rJar = Task.dest / "R.jar"
 
-    val jOpts = JavaCompilerOptions.split(javacOptions() ++ mandatoryJavacOptions())
+    val jOpts = JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
     val worker = jvmWorker().internalWorker()
     val classesDest = worker
       .apply(
diff --git a/libs/androidlib/src/mill/androidlib/hilt/AndroidHiltSupport.scala b/libs/androidlib/src/mill/androidlib/hilt/AndroidHiltSupport.scala
index 506dee24e9d7..7186bf86c154 100644
--- a/libs/androidlib/src/mill/androidlib/hilt/AndroidHiltSupport.scala
+++ b/libs/androidlib/src/mill/androidlib/hilt/AndroidHiltSupport.scala
@@ -1,6 +1,7 @@
 package mill.androidlib.hilt
 
 import mill.androidlib.AndroidKotlinModule
+import mill.api.opt.*
 import mill.api.{ModuleRef, PathRef}
 import mill.kotlinlib.ksp.KspModule
 import mill.javalib.Dep
@@ -24,12 +25,12 @@ import mill.{T, Task}
 @mill.api.experimental
 trait AndroidHiltSupport extends KspModule, AndroidKotlinModule {
 
-  override def kspProcessorOptions: T[Map[String, String]] = Task {
+  override def kspProcessorOptions: T[Map[String, Opt]] = Task {
     super.kspProcessorOptions() ++ Map(
-      "dagger.fastInit" -> "enabled",
-      "dagger.hilt.android.internal.disableAndroidSuperclassValidation" -> "true",
-      "dagger.hilt.android.internal.projectType" -> "APP",
-      "dagger.hilt.internal.useAggregatingRootProcessor" -> "true"
+      "dagger.fastInit" -> opt"enabled",
+      "dagger.hilt.android.internal.disableAndroidSuperclassValidation" -> opt"true",
+      "dagger.hilt.android.internal.projectType" -> opt"APP",
+      "dagger.hilt.internal.useAggregatingRootProcessor" -> opt"true"
     )
   }
 
diff --git a/libs/javalib/src/mill/javalib/AssemblyModule.scala b/libs/javalib/src/mill/javalib/AssemblyModule.scala
index a3ef4a185f4d..8e04af1cadad 100644
--- a/libs/javalib/src/mill/javalib/AssemblyModule.scala
+++ b/libs/javalib/src/mill/javalib/AssemblyModule.scala
@@ -4,6 +4,7 @@ import mill.api.PathRef
 import mill.api.Result
 import mill.util.JarManifest
 import mill.api.*
+import mill.api.opt.*
 import mill.api.Task.Simple as T
 import mill.javalib.Assembly.UnopenedInputStream
 import mill.util.Jvm
@@ -16,17 +17,17 @@ trait AssemblyModule extends mill.api.Module {
 
   def finalMainClassOpt: T[Either[String, String]]
 
-  def forkArgs: T[Seq[String]]
+  def forkArgs: T[Opts]
 
   /**
    * Similar to `forkArgs` but only applies to the `sh` launcher script
    */
-  def forkShellArgs: T[Seq[String]] = Task { Seq.empty[String] }
+  def forkShellArgs: T[Opts] = Task { Opts() }
 
   /**
    * Similar to `forkArgs` but only applies to the `bat` launcher script
    */
-  def forkCmdArgs: T[Seq[String]] = Task { Seq.empty[String] }
+  def forkCmdArgs: T[Opts] = Task { Opts() }
 
   /**
    * Creates a manifest representation which can be modified or replaced
@@ -54,10 +55,10 @@ trait AssemblyModule extends mill.api.Module {
           mainClass = cls,
           shellClassPath = Seq("$0"),
           cmdClassPath = Seq("%~dpnx0"),
-          jvmArgs = forkArgs(),
+          jvmArgs = forkArgs().toStringSeq,
           shebang = false,
-          shellJvmArgs = forkShellArgs(),
-          cmdJvmArgs = forkCmdArgs()
+          shellJvmArgs = forkShellArgs().toStringSeq,
+          cmdJvmArgs = forkCmdArgs().toStringSeq
         )
     }
   }
diff --git a/libs/javalib/src/mill/javalib/JavaModule.scala b/libs/javalib/src/mill/javalib/JavaModule.scala
index aaab34c76938..bde994b4975a 100644
--- a/libs/javalib/src/mill/javalib/JavaModule.scala
+++ b/libs/javalib/src/mill/javalib/JavaModule.scala
@@ -7,19 +7,13 @@ import coursier.params.ResolutionParams
 import coursier.parse.{JavaOrScalaModule, ModuleParser}
 import coursier.util.{EitherT, ModuleMatcher, Monad}
 import mainargs.Flag
-import mill.api.{MillException, Result}
+import mill.api.{DefaultTaskModule, MillException, ModuleRef, PathRef, Result, Segment, Task, TaskCtx}
+import mill.api.opt.*
 import mill.api.daemon.internal.{EvaluatorApi, JavaModuleApi, internal}
-import mill.api.daemon.internal.bsp.{
-  BspBuildTarget,
-  BspJavaModuleApi,
-  BspModuleApi,
-  BspUri,
-  JvmBuildTarget
-}
+import mill.api.daemon.internal.bsp.{BspBuildTarget, BspJavaModuleApi, BspModuleApi, BspUri, JvmBuildTarget}
 import mill.api.daemon.internal.eclipse.GenEclipseInternalApi
 import mill.javalib.*
 import mill.api.daemon.internal.idea.GenIdeaInternalApi
-import mill.api.{DefaultTaskModule, ModuleRef, PathRef, Segment, Task, TaskCtx}
 import mill.javalib.api.CompilationResult
 import mill.javalib.api.internal.{JavaCompilerOptions, ZincCompileJava}
 import mill.javalib.bsp.{BspJavaModule, BspModule}
@@ -263,12 +257,12 @@ trait JavaModule
   /**
    * Options to pass to the java compiler
    */
-  override def javacOptions: T[Seq[String]] = Task { Seq.empty[String] }
+  override def javacOptions: T[Opts] = Task { Opts() }
 
   /**
    * Additional options for the java compiler derived from other module settings.
    */
-  override def mandatoryJavacOptions: T[Seq[String]] = Task { Seq.empty[String] }
+  override def mandatoryJavacOptions: T[Opts] = Task { Opts() }
 
   /**
    *  The direct dependencies of this module.
@@ -863,7 +857,7 @@ trait JavaModule
     val jOpts = JavaCompilerOptions.split(Seq(
       "-s",
       compileGenSources.toString
-    ) ++ javacOptions() ++ mandatoryJavacOptions())
+    ) ++ javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
 
     val worker = jvmWorker().internalWorker()
 
@@ -1105,7 +1099,7 @@ trait JavaModule
    * You should not set the `-d` setting for specifying the target directory,
    * as that is done in the [[docJar]] task.
    */
-  def javadocOptions: T[Seq[String]] = Task { Seq[String]() }
+  def javadocOptions: T[Opts] = Task { Opts() }
 
   /**
    * Directories to be processed by the API documentation tool.
@@ -1150,7 +1144,7 @@ trait JavaModule
           classPath.mkString(java.io.File.pathSeparator)
         )
 
-      val options = javadocOptions() ++
+      val options = javadocOptions().toStringSeq ++
         Seq("-d", javadocDir.toString) ++
         cpOptions ++
         files.map(_.toString)
@@ -1228,7 +1222,7 @@ trait JavaModule
       val cmd = Seq(Jvm.jdkTool("jshell", javaHome().map(_.path))) ++ jshellArgs
       os.call(
         cmd = cmd,
-        env = allForkEnv(),
+        env = allForkEnv().view.mapValues(_.toString).toMap,
         cwd = forkWorkingDir(),
         stdin = os.Inherit,
         stdout = os.Inherit
diff --git a/libs/javalib/src/mill/javalib/JavacOptions.scala b/libs/javalib/src/mill/javalib/JavacOptions.scala
new file mode 100644
index 000000000000..89d5d13aef40
--- /dev/null
+++ b/libs/javalib/src/mill/javalib/JavacOptions.scala
@@ -0,0 +1,9 @@
+package mill.javalib
+
+object JavacOptions {
+
+  val `-target` = "-target"
+  val `-source` = "-source"
+  val `-deprecation` = "-deprecation"
+  
+}
diff --git a/libs/javalib/src/mill/javalib/RunModule.scala b/libs/javalib/src/mill/javalib/RunModule.scala
index cec40c553c6d..5f3680a8e600 100644
--- a/libs/javalib/src/mill/javalib/RunModule.scala
+++ b/libs/javalib/src/mill/javalib/RunModule.scala
@@ -2,14 +2,13 @@ package mill.javalib
 
 import java.lang.reflect.Modifier
 import scala.util.control.NonFatal
-import mill.api.BuildCtx
+import mill.api.{BuildCtx, ModuleCtx, ModuleRef, PathRef, Result, Task, TaskCtx}
+import mill.api.opt.*
 import mainargs.arg
-import mill.api.Result
 import mill.api.daemon.internal.RunModuleApi
 import mill.api.daemon.internal.bsp.BspRunModuleApi
 import mill.constants.DaemonFiles
 import mill.api.JsonFormatters.pathReadWrite
-import mill.api.{ModuleCtx, ModuleRef, PathRef, Task, TaskCtx}
 import mill.javalib.bsp.BspRunModule
 import mill.javalib.classgraph.ClassgraphWorkerModule
 import mill.util.Jvm
@@ -36,21 +35,21 @@ trait RunModule extends WithJvmWorkerModule with RunModuleApi {
   /**
    * Any command-line parameters you want to pass to the forked JVM.
    */
-  def forkArgs: T[Seq[String]] = Task { Seq.empty[String] }
+  def forkArgs: T[Opts] = Task { Opts() }
 
   /**
    * Any environment variables you want to pass to the forked JVM.
    */
-  def forkEnv: T[Map[String, String]] = Task { Map.empty[String, String] }
+  def forkEnv: T[Map[String, Opt]] = Task { Map.empty[String, Opt] }
 
   /**
    * Environment variables to pass to the forked JVM.
    *
    * Includes [[forkEnv]] and the variables defined by Mill itself.
    */
-  def allForkEnv: T[Map[String, String]] = Task {
+  def allForkEnv: T[Map[String, Opt]] = Task {
     forkEnv() ++ Map(
-      EnvVars.MILL_WORKSPACE_ROOT -> BuildCtx.workspaceRoot.toString
+      EnvVars.MILL_WORKSPACE_ROOT -> Opt(BuildCtx.workspaceRoot)
     )
   }
 
@@ -171,8 +170,8 @@ trait RunModule extends WithJvmWorkerModule with RunModuleApi {
     new RunModule.RunnerImpl(
       finalMainClassOpt(),
       runClasspath().map(_.path),
-      forkArgs(),
-      allForkEnv(),
+      forkArgs().toStringSeq,
+      allForkEnv().view.mapValues(_.toString()).toMap,
       runUseArgsFile(),
       javaHome().map(_.path),
       propagateEnv()
@@ -247,7 +246,7 @@ trait RunModule extends WithJvmWorkerModule with RunModuleApi {
         Seq(classpathJar)
       }
 
-    Jvm.createLauncher(finalMainClass(), launchClasspath, forkArgs(), Task.dest)
+    Jvm.createLauncher(finalMainClass(), launchClasspath, forkArgs().toStringSeq, Task.dest)
   }
 
   /**
diff --git a/libs/javalib/src/mill/javalib/SemanticDbJavaModule.scala b/libs/javalib/src/mill/javalib/SemanticDbJavaModule.scala
index e7c39b07906f..1b593a3d7575 100644
--- a/libs/javalib/src/mill/javalib/SemanticDbJavaModule.scala
+++ b/libs/javalib/src/mill/javalib/SemanticDbJavaModule.scala
@@ -1,7 +1,16 @@
 package mill.javalib
 
-import mill.api.{BuildCtx, Discover, ExternalModule, ModuleRef, PathRef, Result, experimental}
+import mill.api.{
+  BuildCtx,
+  Discover,
+  ExternalModule,
+  ModuleRef,
+  PathRef,
+  Result,
+  experimental
+}
 import mill.api.daemon.internal.SemanticDbJavaModuleApi
+import mill.api.opt.*
 import mill.constants.CodeGenConstants
 import mill.util.BuildInfo
 import mill.javalib.api.{CompilationResult, JvmWorkerUtil}
@@ -34,8 +43,8 @@ trait SemanticDbJavaModule extends CoursierModule with SemanticDbJavaModuleApi
     }
 
   private[mill] def bspBuildTarget: BspBuildTarget
-  def javacOptions: T[Seq[String]]
-  def mandatoryJavacOptions: T[Seq[String]]
+  def javacOptions: T[Opts]
+  def mandatoryJavacOptions: T[Opts]
   private[mill] def compileClasspathTask(compileFor: CompileFor): Task[Seq[PathRef]]
   def moduleDeps: Seq[JavaModule]
 
@@ -99,11 +108,11 @@ trait SemanticDbJavaModule extends CoursierModule with SemanticDbJavaModuleApi
   /**
    * Scalac options to activate the compiler plugins.
    */
-  protected def semanticDbEnablePluginScalacOptions: T[Seq[String]] = Task {
+  protected def semanticDbEnablePluginScalacOptions: T[Opts] = Task {
     val resolvedJars = defaultResolver().classpath(
       semanticDbPluginMvnDeps().map(_.exclude("*" -> "*"))
     )
-    resolvedJars.iterator.map(jar => s"-Xplugin:${jar.path}").toSeq
+    Opts(resolvedJars.map(jar => opt"-Xplugin:${jar.path}"))
   }
 
   protected def semanticDbPluginClasspath: T[Seq[PathRef]] = Task {
@@ -118,7 +127,7 @@ trait SemanticDbJavaModule extends CoursierModule with SemanticDbJavaModuleApi
     val javacOpts = SemanticDbJavaModule.javacOptionsTask(
       javacOptions() ++ mandatoryJavacOptions(),
       semanticDbJavaVersion()
-    )
+    ).toStringSeq
 
     Task.log.debug(s"effective javac options: ${javacOpts}")
 
@@ -225,15 +234,15 @@ object SemanticDbJavaModule extends ExternalModule with CoursierModule {
     ))
   }
 
-  def javacOptionsTask(javacOptions: Seq[String], semanticDbJavaVersion: String)(using
+  def javacOptionsTask(javacOptions: Opts, semanticDbJavaVersion: String)(using
       ctx: mill.api.TaskCtx
-  ): Seq[String] = {
+  ): Opts = {
     val isNewEnough =
       Version.isAtLeast(semanticDbJavaVersion, "0.8.10")(using Version.IgnoreQualifierOrdering)
     val buildTool = s" -build-tool:${if (isNewEnough) "mill" else "sbt"}"
     val verbose = if (ctx.log.debugEnabled) " -verbose" else ""
-    javacOptions ++ Seq(
-      s"-Xplugin:semanticdb -sourceroot:${ctx.workspace} -targetroot:${ctx.dest / "classes"}${buildTool}${verbose}"
+    javacOptions ++ Opts(
+      opt"-Xplugin:semanticdb -sourceroot:${BuildCtx.workspaceRoot} -targetroot:${ctx.dest / "classes"}${buildTool}${verbose}"
     )
   }
 
diff --git a/libs/javalib/src/mill/javalib/TestModule.scala b/libs/javalib/src/mill/javalib/TestModule.scala
index ce2559fb03db..eee0f367a0ab 100644
--- a/libs/javalib/src/mill/javalib/TestModule.scala
+++ b/libs/javalib/src/mill/javalib/TestModule.scala
@@ -1,14 +1,11 @@
 package mill.javalib
 
 import mill.T
-import mill.api.Result
+import mill.api.{DefaultTaskModule, PathRef, Result, Task, TaskCtx}
+import mill.api.opt.*
 import mill.api.daemon.internal.TestModuleApi
 import mill.api.daemon.internal.TestReporter
 import mill.api.daemon.internal.bsp.{BspBuildTarget, BspModuleApi}
-import mill.api.PathRef
-import mill.api.Task
-import mill.api.TaskCtx
-import mill.api.DefaultTaskModule
 import mill.javalib.bsp.BspModule
 import mill.api.JsonFormatters.given
 import mill.constants.EnvVars
@@ -216,9 +213,9 @@ trait TestModule
    */
   def testSandboxWorkingDir: T[Boolean] = true
 
-  override def allForkEnv: T[Map[String, String]] = Task {
+  override def allForkEnv: T[Map[String, Opt]] = Task {
     super.allForkEnv() ++ Map(
-      EnvVars.MILL_TEST_RESOURCE_DIR -> resources().iterator.map(_.path).mkString(";")
+      EnvVars.MILL_TEST_RESOURCE_DIR -> Opt.mkPath(resources().map(_.path), sep = ";")
     )
   }
 
@@ -232,7 +229,7 @@ trait TestModule
     Task.Anon {
       val testModuleUtil = new TestModuleUtil(
         testUseArgsFile(),
-        forkArgs(),
+        forkArgs().toStringSeq,
         globSelectors(),
         jvmWorker().scalalibClasspath(),
         resources(),
@@ -242,7 +239,7 @@ trait TestModule
         args(),
         testForkGrouping(),
         jvmWorker().testrunnerEntrypointClasspath(),
-        allForkEnv(),
+        allForkEnv().view.mapValues(_.toString()).toMap,
         testSandboxWorkingDir(),
         forkWorkingDir(),
         testReportXml(),
@@ -453,7 +450,7 @@ object TestModule {
     def specs2Version: T[String] = Task { "" }
     override def testFramework: T[String] = "org.specs2.runner.Specs2Framework"
     override def scalacOptions = Task {
-      super.scalacOptions() ++ Seq("-Yrangepos")
+      super.scalacOptions() ++ Opts("-Yrangepos")
     }
     override def mandatoryMvnDeps: T[Seq[Dep]] = Task {
       super.mandatoryMvnDeps() ++
@@ -577,7 +574,7 @@ object TestModule {
   }
 
   trait ScalaModuleBase extends mill.Module {
-    def scalacOptions: T[Seq[String]] = Seq()
+    def scalacOptions: T[Opts] = Opts()
   }
 
 }
diff --git a/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala b/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala
index e112c81b6fa4..ed96f47f3477 100644
--- a/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala
+++ b/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala
@@ -1,11 +1,11 @@
 package mill.javalib.bsp
 
 import java.nio.file.Path
-
 import mill.api.daemon.internal.bsp.BspJavaModuleApi
 import mill.Task
-import mill.api.daemon.internal.{EvaluatorApi, TaskApi, internal}
+import mill.api.daemon.internal.{EvaluatorApi, OptsApi, TaskApi, internal}
 import mill.api.ModuleCtx
+import mill.api.opt.*
 import mill.javalib.{JavaModule, SemanticDbJavaModule}
 import mill.api.JsonFormatters.given
 
@@ -30,7 +30,7 @@ trait BspJavaModule extends mill.api.Module with BspJavaModuleApi {
       clientWantsSemanticDb: Boolean
   ): Task[EvaluatorApi => (
       classesPath: Path,
-      javacOptions: Seq[String],
+      javacOptions: OptsApi,
       classpath: Seq[String]
   )] = {
     val classesPathTask = jm match {
@@ -105,13 +105,17 @@ trait BspJavaModule extends mill.api.Module with BspJavaModuleApi {
     jm.resources().map(_.path.toNIO)
   }
 
-  def scalacOptionsTask = Task.Anon(Seq.empty[String])
+  def scalacOptionsTask: Task[Opts] = Task.Anon(Opts())
 
   override private[mill] def bspBuildTargetScalacOptions(
       needsToMergeResourcesIntoCompileDest: Boolean,
       enableJvmCompileClasspathProvider: Boolean,
       clientWantsSemanticDb: Boolean
-  ): Task[(Seq[String], EvaluatorApi => Seq[String], EvaluatorApi => java.nio.file.Path)] = {
+  ): Task[(
+      scalacOptionsTask: Opts,
+      compileClasspathTask: EvaluatorApi => Seq[String],
+      classPathTask: EvaluatorApi => java.nio.file.Path
+  )] = {
 
     val compileClasspathTask: Task[EvaluatorApi => Seq[String]] =
       if (enableJvmCompileClasspathProvider) {
@@ -144,8 +148,8 @@ trait BspJavaModule extends mill.api.Module with BspJavaModuleApi {
   override private[mill] def bspBuildTargetScalaMainClasses
       : Task.Simple[(
           classes: Seq[String],
-          forkArgs: Seq[String],
-          forkEnv: Map[String, String]
+          forkArgs: Opts,
+          forkEnv: Map[String, Opt]
       )] =
     Task {
       (jm.allLocalMainClasses(), jm.forkArgs(), jm.allForkEnv())
diff --git a/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala b/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala
index f18a191229ba..bc486e4bddfb 100644
--- a/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala
+++ b/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala
@@ -1,11 +1,11 @@
 package mill.javalib.bsp
 
 import java.nio.file.Path
-
 import mill.api.daemon.internal.bsp.BspRunModuleApi
 import mill.api.daemon.internal.internal
 import mill.api.{Discover, ExternalModule, ModuleCtx}
 import mill.api.JsonFormatters.given
+import mill.api.opt.*
 import mill.javalib.{JavaModule, RunModule, TestModule}
 import mill.{Args, Task}
 
@@ -33,9 +33,9 @@ private[mill] object BspRunModule extends ExternalModule {
 
       override private[mill] def bspJvmRunEnvironment: Task.Simple[(
           runClasspath: Seq[Path],
-          forkArgs: Seq[String],
+          forkArgs: Opts,
           forkWorkingDir: Path,
-          forkEnv: Map[String, String],
+          forkEnv: Map[String, Opt],
           mainClass: Option[String],
           localMainClasses: Seq[String]
       )] =
@@ -52,9 +52,9 @@ private[mill] object BspRunModule extends ExternalModule {
 
       override private[mill] def bspJvmTestEnvironment: Task.Simple[(
           runClasspath: Seq[Path],
-          forkArgs: Seq[String],
+          forkArgs: Opts,
           forkWorkingDir: Path,
-          forkEnv: Map[String, String],
+          forkEnv: Map[String, Opt],
           mainClass: Option[String],
           testEnvVars: Option[(
               mainClass: String,
diff --git a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
index fab493c3e65a..6053471d1adf 100644
--- a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
+++ b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
@@ -1,6 +1,7 @@
 package mill.javalib.errorprone
 
 import mill.api.PathRef
+import mill.api.opt.*
 import mill.javalib.{Dep, DepSyntax, JavaModule}
 import mill.{T, Task}
 
@@ -37,15 +38,16 @@ trait ErrorProneModule extends JavaModule {
   /**
    * Options used to enable and configure the `error-prone` plugin in the Java compiler.
    */
-  def errorProneJavacEnableOptions: T[Seq[String]] = Task {
-    val processorPath = errorProneClasspath().map(_.path).mkString(File.pathSeparator)
-    val enableOpts = Seq(
+  def errorProneJavacEnableOptions: T[Opts] = Task {
+    val processorPath: Seq[os.Path] = errorProneClasspath().map(_.path)
+
+    val enableOpts: Opts = Opts(
       "-XDcompilePolicy=simple",
       "-processorpath",
       processorPath,
-      (Seq("-Xplugin:ErrorProne") ++ errorProneOptions()).mkString(" ")
+      (Seq("-Xplugin:ErrorProne") ++ errorProneOptions().toStringSeq).mkString(" ")
     )
-    val java17Options = Option.when(scala.util.Properties.isJavaAtLeast(16))(Seq(
+    val java17Options: Seq[String] = Option.when(scala.util.Properties.isJavaAtLeast(16))(Seq(
       "--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED",
       "--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED",
       "--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED",
@@ -57,7 +59,8 @@ trait ErrorProneModule extends JavaModule {
       "--add-opens=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED",
       "--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED"
     ).map(o => s"-J${o}")).toSeq.flatten
-    java17Options ++ enableOpts
+
+    Opts(java17Options) ++ enableOpts
   }
 
   /**
@@ -65,12 +68,12 @@ trait ErrorProneModule extends JavaModule {
    *
    * Those are documented as "flags" at https://errorprone.info/docs/flags
    */
-  def errorProneOptions: T[Seq[String]] = Task { Seq.empty[String] }
+  def errorProneOptions: T[Opts] = Task { Opts() }
 
   /**
    * Appends the [[errorProneJavacEnableOptions]] to the Java compiler options.
    */
-  override def mandatoryJavacOptions: T[Seq[String]] = Task {
+  override def mandatoryJavacOptions: T[Opts] = Task {
     super.mandatoryJavacOptions() ++ errorProneJavacEnableOptions()
   }
 }
diff --git a/libs/javalib/src/mill/javalib/idea/GenIdeaModule.scala b/libs/javalib/src/mill/javalib/idea/GenIdeaModule.scala
index 7c6d8fdc31c6..3a1d380df87e 100644
--- a/libs/javalib/src/mill/javalib/idea/GenIdeaModule.scala
+++ b/libs/javalib/src/mill/javalib/idea/GenIdeaModule.scala
@@ -1,14 +1,14 @@
 package mill.javalib.idea
 
 import java.nio.file.Path
-
 import mill.Task
-import mill.api.Segments
+import mill.api.{ModuleCtx, PathRef, Segments}
+import mill.api.opt.*
 import mill.api.daemon.internal.idea.{GenIdeaInternalApi, ResolvedModule}
 import mill.api.daemon.internal.internal
-import mill.api.{ModuleCtx, PathRef}
 import mill.javalib.{BoundDep, Dep, JavaModule}
 import mill.api.JsonFormatters.given
+
 trait GenIdeaModule extends mill.api.Module with GenIdeaInternalApi {
   private[mill] val jarCollector: PartialFunction[PathRef, Path] = {
     case p if p.path.ext == "jar" => p.path.toNIO
@@ -48,7 +48,7 @@ trait GenIdeaModule extends mill.api.Module with GenIdeaInternalApi {
 
   private[mill] def scalacPluginsMvnDeps = Task.Anon(Seq.empty[Dep])
 
-  private[mill] def allScalacOptions = Task.Anon(Seq.empty[String])
+  private[mill] def allScalacOptions: Task[Opts] = Task.Anon(Opts())
 
   private[mill] def scalaVersion = Task.Anon(Option.empty[String])
 
@@ -73,7 +73,7 @@ trait GenIdeaModule extends mill.api.Module with GenIdeaInternalApi {
 
     val scopedCpEntries = scopedClasspathEntries()
     val pluginClasspath = scalacPluginDependencies().collect(jarCollector)
-    val scalacOpts = allScalacOptions()
+    val scalacOpts = allScalacOptions().toStringSeq
     val resolvedCompilerCp = scalaCompilerClasspath().map(_.path.toNIO)
     val resolvedLibraryCp = externalLibraryDependencies().map(_.path.toNIO)
     val facets = javaModule.ideaJavaModuleFacets(ideaConfigVersion)()
diff --git a/libs/javalib/src/mill/javalib/repackage/RepackageModule.scala b/libs/javalib/src/mill/javalib/repackage/RepackageModule.scala
index dd8b775091be..b291d7cd4830 100644
--- a/libs/javalib/src/mill/javalib/repackage/RepackageModule.scala
+++ b/libs/javalib/src/mill/javalib/repackage/RepackageModule.scala
@@ -52,9 +52,9 @@ trait RepackageModule extends mill.api.Module {
             shellClassPath = Seq("$0"),
             cmdClassPath = Seq("%~dpnx0"),
             shebang = false,
-            jvmArgs = m.forkArgs(),
-            shellJvmArgs = m.forkShellArgs(),
-            cmdJvmArgs = m.forkCmdArgs()
+            jvmArgs = m.forkArgs().toStringSeq,
+            shellJvmArgs = m.forkShellArgs().toStringSeq,
+            cmdJvmArgs = m.forkCmdArgs().toStringSeq
           )
         ).filter(_.nonEmpty)
       }
diff --git a/libs/javalib/test/src/mill/javalib/LauncherTests.scala b/libs/javalib/test/src/mill/javalib/LauncherTests.scala
index b8e7eab672ed..21d44db5f69d 100644
--- a/libs/javalib/test/src/mill/javalib/LauncherTests.scala
+++ b/libs/javalib/test/src/mill/javalib/LauncherTests.scala
@@ -1,5 +1,6 @@
 package mill.javalib
 
+import mill.api.opt.Opts
 import mill.api.{Discover, PathRef}
 import mill.testkit.{TestRootModule, UnitTester}
 import utest.*
@@ -11,7 +12,7 @@ object LauncherTests extends TestSuite {
   object HelloJava extends TestRootModule with JavaModule {
     def jvmId = s"temurin:$customJavaVersion"
 
-    def javacOptions = Seq("-target", "1.8", "-source", "1.8")
+    def javacOptions = Opts(JavacOptions.`-target`, "1.8", JavacOptions.`-source`, "1.8")
 
     lazy val millDiscover = Discover[this.type]
   }
diff --git a/libs/javalib/test/src/mill/javalib/errorprone/ErrorProneTests.scala b/libs/javalib/test/src/mill/javalib/errorprone/ErrorProneTests.scala
index d4811de2c8a5..b0904f117d56 100644
--- a/libs/javalib/test/src/mill/javalib/errorprone/ErrorProneTests.scala
+++ b/libs/javalib/test/src/mill/javalib/errorprone/ErrorProneTests.scala
@@ -2,11 +2,13 @@ package mill.javalib.errorprone
 
 import mill.{T, Task}
 import mill.api.Discover
+import mill.api.opt.*
 import mill.javalib.JavaModule
 import mill.testkit.{TestRootModule, UnitTester}
 import os.Path
 import utest.*
-import mill.util.TokenReaders._
+import mill.util.TokenReaders.*
+
 object ErrorProneTests extends TestSuite {
 
   object noErrorProne extends TestRootModule with JavaModule {
@@ -16,8 +18,8 @@ object ErrorProneTests extends TestSuite {
     lazy val millDiscover = Discover[this.type]
   }
   object errorProneCustom extends TestRootModule with JavaModule with ErrorProneModule {
-    override def errorProneOptions: T[Seq[String]] = Task {
-      Seq("-XepAllErrorsAsWarnings")
+    override def errorProneOptions: T[Opts] = Task {
+      Opts("-XepAllErrorsAsWarnings")
     }
     lazy val millDiscover = Discover[this.type]
   }
@@ -44,7 +46,7 @@ object ErrorProneTests extends TestSuite {
       test("compileWarn") {
         UnitTester(errorProneCustom, testModuleSourcesPath).scoped { eval =>
           val Right(opts) = eval(errorProneCustom.mandatoryJavacOptions): @unchecked
-          assert(opts.value.exists(_.contains("-XepAllErrorsAsWarnings")))
+          assert(opts.value.toStringSeq.exists(_.contains("-XepAllErrorsAsWarnings")))
           val res = eval(errorProneCustom.compile)
           assert(res.isRight)
         }
diff --git a/libs/kotlinlib/src/mill/kotlinlib/KotlinModule.scala b/libs/kotlinlib/src/mill/kotlinlib/KotlinModule.scala
index 62dd449500b7..b3da942df347 100644
--- a/libs/kotlinlib/src/mill/kotlinlib/KotlinModule.scala
+++ b/libs/kotlinlib/src/mill/kotlinlib/KotlinModule.scala
@@ -8,8 +8,8 @@ package kotlinlib
 
 import coursier.core.VariantSelector.VariantMatcher
 import coursier.params.ResolutionParams
-import mill.api.Result
-import mill.api.ModuleRef
+import mill.api.{ModuleRef, Result}
+import mill.api.opt.*
 import mill.kotlinlib.worker.api.KotlinWorkerTarget
 import mill.javalib.api.CompilationResult
 import mill.javalib.api.JvmWorkerApi as PublicJvmWorkerApi
@@ -187,7 +187,7 @@ trait KotlinModule extends JavaModule with KotlinModuleApi { outer =>
    * You might want to add additional arguments like `-X` to see extra help.
    */
   def kotlincHelp(args: String*): Command[Unit] = Task.Command {
-    kotlinCompileTask(Seq("-help") ++ args)()
+    kotlinCompileTask(Opts("-help") ++ Opts(args))()
     ()
   }
 
@@ -214,7 +214,7 @@ trait KotlinModule extends JavaModule with KotlinModuleApi { outer =>
 
       // TODO need to provide a dedicated source set for common sources in case of Multiplatform
       // platforms supported: jvm, js, wasm, native, common
-      val options = dokkaOptions() ++
+      val options = dokkaOptions().toStringSeq ++
         Seq("-outputDir", dokkaDir.toString()) ++
         pluginClasspathOption ++
         Seq(
@@ -258,7 +258,7 @@ trait KotlinModule extends JavaModule with KotlinModuleApi { outer =>
    * You should not set the `-outputDir` setting for specifying the target directory,
    * as that is done in the [[docJar]] target.
    */
-  def dokkaOptions: T[Seq[String]] = Task { Seq[String]() }
+  def dokkaOptions: T[Opts] = Task { Opts() }
 
   /**
    * Dokka version.
@@ -299,7 +299,7 @@ trait KotlinModule extends JavaModule with KotlinModuleApi { outer =>
   /**
    * The actual Kotlin compile task (used by [[compile]] and [[kotlincHelp]]).
    */
-  protected def kotlinCompileTask(extraKotlinArgs: Seq[String] = Seq()): Task[CompilationResult] =
+  protected def kotlinCompileTask(extraKotlinArgs: Opts = Opts()): Task[CompilationResult] =
     Task.Anon {
       val ctx = Task.ctx()
       val dest = ctx.dest
@@ -327,7 +327,7 @@ trait KotlinModule extends JavaModule with KotlinModuleApi { outer =>
           javaSourceFiles = javaSourceFiles,
           compileCp = compileCp,
           javaHome = javaHome().map(_.path),
-          javacOptions = javacOptions(),
+          javacOptions = javacOptions().toStringSeq,
           compileProblemReporter = ctx.reporter(hashCode),
           reportOldProblems = internalReportOldProblems()
         )
@@ -353,8 +353,8 @@ trait KotlinModule extends JavaModule with KotlinModuleApi { outer =>
           when(kotlinExplicitApi())(
             "-Xexplicit-api=strict"
           ),
-          allKotlincOptions(),
-          extraKotlinArgs
+          allKotlincOptions().toStringSeq,
+          extraKotlinArgs.toStringSeq
         ).flatten
 
         val workerResult =
@@ -391,7 +391,7 @@ trait KotlinModule extends JavaModule with KotlinModuleApi { outer =>
   /**
    * Additional Kotlin compiler options to be used by [[compile]].
    */
-  def kotlincOptions: T[Seq[String]] = Task { Seq.empty[String] }
+  def kotlincOptions: T[Opts] = Task { Opts() }
 
   /**
    * Enable use of new Kotlin Build API (Beta).
@@ -406,22 +406,24 @@ trait KotlinModule extends JavaModule with KotlinModuleApi { outer =>
    * Mandatory command-line options to pass to the Kotlin compiler
    * that shouldn't be removed by overriding `scalacOptions`
    */
-  protected def mandatoryKotlincOptions: T[Seq[String]] = Task {
+  protected def mandatoryKotlincOptions: T[Opts] = Task {
     val languageVersion = kotlinLanguageVersion()
     val kotlinkotlinApiVersion = kotlinApiVersion()
     val plugins = kotlincPluginJars().map(_.path)
 
-    Seq("-no-stdlib") ++
-      when(!languageVersion.isBlank)("-language-version", languageVersion) ++
-      when(!kotlinkotlinApiVersion.isBlank)("-api-version", kotlinkotlinApiVersion) ++
-      plugins.map(p => s"-Xplugin=$p")
+    Opts(
+      "-no-stdlib",
+      OptGroup.when(!languageVersion.isBlank)("-language-version", languageVersion),
+      OptGroup.when(!kotlinkotlinApiVersion.isBlank)("-api-version", kotlinkotlinApiVersion),
+      plugins.map(p => opt"-Xplugin=$p")
+    )
   }
 
   /**
    * Aggregation of all the options passed to the Kotlin compiler.
    * In most cases, instead of overriding this Target you want to override `kotlincOptions` instead.
    */
-  def allKotlincOptions: T[Seq[String]] = Task {
+  def allKotlincOptions: T[Opts] = Task {
     mandatoryKotlincOptions() ++ kotlincOptions()
   }
 
@@ -485,9 +487,9 @@ trait KotlinModule extends JavaModule with KotlinModuleApi { outer =>
     override def kotlincPluginMvnDeps: T[Seq[Dep]] =
       Task { outer.kotlincPluginMvnDeps() }
       // TODO: make Xfriend-path an explicit setting
-    override def kotlincOptions: T[Seq[String]] = Task {
-      outer.kotlincOptions().filterNot(_.startsWith("-Xcommon-sources")) ++
-        Seq(s"-Xfriend-paths=${outer.compile().classes.path.toString()}")
+    override def kotlincOptions: T[Opts] = Task {
+      outer.kotlincOptions().filterGroup(!_.head.startsWith(KotlincOptions.`-Xcommon-sources`)) ++
+        Opts(opt"${KotlincOptions.`-Xfriend-paths`}=${outer.compile().classes.path}")
     }
     override def kotlinUseEmbeddableCompiler: Task[Boolean] =
       Task.Anon { outer.kotlinUseEmbeddableCompiler() }
@@ -507,9 +509,9 @@ object KotlinModule {
     override def kotlincPluginMvnDeps: T[Seq[Dep]] =
       Task { outer.kotlincPluginMvnDeps() }
     // TODO: make Xfriend-path an explicit setting
-    override def kotlincOptions: T[Seq[String]] = Task {
-      outer.kotlincOptions().filterNot(_.startsWith("-Xcommon-sources")) ++
-        Seq(s"-Xfriend-paths=${outer.compile().classes.path.toString()}")
+    override def kotlincOptions: T[Opts] = Task {
+      outer.kotlincOptions().filterGroup(!_.head.startsWith(KotlincOptions.`-Xcommon-sources`)) ++
+        Opts(opt"${KotlincOptions.`-Xfriend-paths`}=${outer.compile().classes.path}")
     }
     override def kotlinUseEmbeddableCompiler: Task[Boolean] =
       Task.Anon { outer.kotlinUseEmbeddableCompiler() }
diff --git a/libs/kotlinlib/src/mill/kotlinlib/KotlincOptions.scala b/libs/kotlinlib/src/mill/kotlinlib/KotlincOptions.scala
new file mode 100644
index 000000000000..fc80ab030f16
--- /dev/null
+++ b/libs/kotlinlib/src/mill/kotlinlib/KotlincOptions.scala
@@ -0,0 +1,7 @@
+package mill.kotlinlib
+
+object KotlincOptions {
+  val `-Xcommon-sources` = "-Xcommon-sources"
+  val `-Xfriend-paths` = "-Xfriend-paths"
+  val friendPathSeparator = ","
+}
diff --git a/libs/kotlinlib/src/mill/kotlinlib/js/KotlinJsModule.scala b/libs/kotlinlib/src/mill/kotlinlib/js/KotlinJsModule.scala
index 88c721e855af..b71e26088bbb 100644
--- a/libs/kotlinlib/src/mill/kotlinlib/js/KotlinJsModule.scala
+++ b/libs/kotlinlib/src/mill/kotlinlib/js/KotlinJsModule.scala
@@ -3,12 +3,11 @@ package mill.kotlinlib.js
 import coursier.core.VariantSelector.VariantMatcher
 import coursier.params.ResolutionParams
 import mainargs.arg
-import mill.api.PathRef
-import mill.api.Result
+import mill.api.{PathRef, Result, Task}
+import mill.api.opt.*
 import mill.api.Task.Command
-import mill.api.Task
 import mill.kotlinlib.worker.api.{KotlinWorker, KotlinWorkerTarget}
-import mill.kotlinlib.{Dep, DepSyntax, KotlinModule, KotlinWorkerManager}
+import mill.kotlinlib.{Dep, DepSyntax, KotlinModule, KotlinWorkerManager, KotlincOptions}
 import mill.javalib.Lib
 import mill.javalib.api.CompilationResult
 import mill.util.Jvm
@@ -176,7 +175,7 @@ trait KotlinJsModule extends KotlinModule { outer =>
    * The actual Kotlin compile task (used by [[compile]] and [[kotlincHelp]]).
    */
   protected override def kotlinCompileTask(
-      extraKotlinArgs: Seq[String] = Seq.empty[String]
+      extraKotlinArgs: Opts = Opts()
   ): Task[CompilationResult] = Task.Anon {
     KotlinWorkerManager.kotlinWorker().withValue(kotlinCompilerClasspath()) {
       kotlinWorker =>
@@ -196,7 +195,7 @@ trait KotlinJsModule extends KotlinModule { outer =>
           destinationRoot = Task.dest,
           artifactId = artifactId(),
           explicitApi = kotlinExplicitApi(),
-          extraKotlinArgs = allKotlincOptions() ++ extraKotlinArgs,
+          extraKotlinArgs = (allKotlincOptions() ++ extraKotlinArgs).toStringSeq,
           worker = kotlinWorker,
           useBtApi = kotlincUseBtApi()
         )
@@ -227,7 +226,7 @@ trait KotlinJsModule extends KotlinModule { outer =>
           destinationRoot = Task.dest,
           artifactId = artifactId(),
           explicitApi = kotlinExplicitApi(),
-          extraKotlinArgs = allKotlincOptions(),
+          extraKotlinArgs = allKotlincOptions().toStringSeq,
           worker = kotlinWorker,
           useBtApi = kotlincUseBtApi()
         )
@@ -512,14 +511,12 @@ trait KotlinJsModule extends KotlinModule { outer =>
 
     // endregion
 
-    override def kotlincOptions: T[Seq[String]] = Task {
-      super.kotlincOptions().map { item =>
-        if (item.startsWith("-Xfriend-paths=")) {
-          // JVM -> JS option name
-          item.replace("-Xfriend-paths=", "-Xfriend-modules=")
-        } else {
-          item
-        }
+    override def kotlincOptions: T[Opts] = Task {
+      super.kotlincOptions().mapGroup { optGroup =>
+        // JVM -> JS option name
+        val head = optGroup.head.mapStartString(_.replace("-Xfriend-paths=", "-Xfriend-modules="))
+        val tail = optGroup.value.tail
+        OptGroup((head +: tail)*)
       }
     }
 
diff --git a/libs/kotlinlib/src/mill/kotlinlib/kover/KoverModule.scala b/libs/kotlinlib/src/mill/kotlinlib/kover/KoverModule.scala
index ce8a41d4f384..3411c61b260b 100644
--- a/libs/kotlinlib/src/mill/kotlinlib/kover/KoverModule.scala
+++ b/libs/kotlinlib/src/mill/kotlinlib/kover/KoverModule.scala
@@ -5,18 +5,15 @@
 package mill.kotlinlib.kover
 
 import mill.*
-import mill.api.{PathRef}
-import mill.api.{Result}
-import mill.api.{Discover, Evaluator, ExternalModule}
+import mill.api.{BuildCtx, Discover, Evaluator, ExternalModule, PathRef, Result, SelectMode}
+import mill.api.opt.*
 import ReportType.{Html, Xml}
 import mill.kotlinlib.{Dep, DepSyntax, KotlinModule, TestModule, Versions}
-import mill.api.SelectMode
 import mill.javalib.api.CompilationResult
 import mill.util.Jvm
 import os.Path
 
 import java.util.Locale
-import mill.api.BuildCtx
 
 /**
  * Adds targets to a [[mill.kotlinlib.KotlinModule]] to create test coverage reports.
@@ -101,15 +98,15 @@ trait KoverModule extends KotlinModule { outer =>
     /**
      * Add Kover specific javaagent options.
      */
-    override def forkArgs: T[Seq[String]] = Task {
+    override def forkArgs: T[Opts] = Task {
       val argsFile = koverDataDir().path / "kover-agent.args"
       val content = s"report.file=${koverBinaryReport().path}"
       BuildCtx.withFilesystemCheckerDisabled {
         os.write.over(argsFile, content)
       }
       super.forkArgs() ++
-        Seq(
-          s"-javaagent:${koverAgentJar().path}=file:$argsFile"
+        Opts(
+          opt"-javaagent:${koverAgentJar().path}=file:$argsFile"
         )
     }
   }
diff --git a/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala b/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala
index f96e66f40968..892f45fce36b 100644
--- a/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala
+++ b/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala
@@ -5,6 +5,7 @@ import coursier.params.ResolutionParams
 import mill.*
 import mill.api.daemon.MillURLClassLoader
 import mill.api.{ModuleRef, PathRef, Task}
+import mill.api.opt.*
 import mill.kotlinlib.ksp2.{KspWorker, KspWorkerArgs}
 import mill.kotlinlib.worker.api.KotlinWorkerTarget
 import mill.kotlinlib.{Dep, DepSyntax, KotlinModule, KotlinWorkerManager}
@@ -103,8 +104,8 @@ trait KspModule extends KotlinModule { outer =>
   /**
    * Processor options to be passed to KSP.
    */
-  def kspProcessorOptions: T[Map[String, String]] = Task {
-    Map.empty[String, String]
+  def kspProcessorOptions: T[Map[String, Opt]] = Task {
+    Map.empty[String, Opt]
   }
 
   /**
@@ -150,16 +151,15 @@ trait KspModule extends KotlinModule { outer =>
    *
    * @return
    */
-  def ksp1KotlincOptions: T[Seq[String]] = Task {
+  def ksp1KotlincOptions: T[Opts] = Task {
     if (!kspLanguageVersion().startsWith("1.")) {
       throw new RuntimeException("KSP needs a compatible language version <= 1.9 to be set!")
     }
-    kotlincOptions() ++ Seq(
+    kotlincOptions() ++ Opts(
       "-Xallow-unstable-dependencies",
       "-no-reflect",
       "-no-stdlib",
-      "-language-version",
-      kspLanguageVersion()
+      OptGroup("-language-version", kspLanguageVersion())
     )
   }
 
@@ -236,7 +236,7 @@ trait KspModule extends KotlinModule { outer =>
     ) ++ kspPluginParameters.map(p => s"$pluginOpt:$p")
 
     val kspCompilerArgs =
-      ksp1KotlincOptions() ++ Seq(xPluginArg) ++ Seq("-P", pluginConfigs.mkString(","))
+      ksp1KotlincOptions().toStringSeq ++ Seq(xPluginArg) ++ Seq("-P", pluginConfigs.mkString(","))
 
     Task.log.info(
       s"Running Kotlin Symbol Processing for ${sourceFiles.size} Kotlin sources to ${kspOutputDir} ..."
@@ -279,8 +279,8 @@ trait KspModule extends KotlinModule { outer =>
    *
    * For more info go to [[https://github.com/google/ksp/blob/main/docs/ksp2cmdline.md]]
    */
-  def ksp2Args: T[Seq[String]] = Task {
-    Seq.empty[String]
+  def ksp2Args: T[Opts] = Task {
+    Opts()
   }
 
   /**
@@ -341,7 +341,7 @@ trait KspModule extends KotlinModule { outer =>
     val kspCachesDir = Task.dest / "caches"
 
     val processorOptionsValue =
-      kspProcessorOptions().map((key, value) => s"$key=$value").toSeq.mkString(File.pathSeparator)
+      kspProcessorOptions().map((key, value) => s"$key=${value.toString}").toSeq.mkString(File.pathSeparator)
 
     val processorOptions = if (processorOptionsValue.isEmpty)
       ""
@@ -367,7 +367,7 @@ trait KspModule extends KotlinModule { outer =>
       s"-api-version=${kspApiVersion()}",
       processorOptions,
       s"-map-annotation-arguments-in-java=false"
-    ) ++ ksp2Args() :+ processorClasspath
+    ) ++ ksp2Args().toStringSeq :+ processorClasspath
 
     val kspJvmMainClasspath = ksp2ToolsDepsClasspath().map(_.path)
     val mainClass = "com.google.devtools.ksp.cmdline.KSPJvmMain"
@@ -448,7 +448,7 @@ trait KspModule extends KotlinModule { outer =>
     val kspCachesDir = Task.dest / "caches"
 
     val processorOptionsValue =
-      kspProcessorOptions().map((key, value) => s"$key=$value").toSeq.mkString(File.pathSeparator)
+      kspProcessorOptions().map((key, value) => s"$key=${value.toString()}").toSeq.mkString(File.pathSeparator)
 
     val processorOptions = if (processorOptionsValue.isEmpty)
       ""
@@ -474,7 +474,7 @@ trait KspModule extends KotlinModule { outer =>
       s"-api-version=${kspApiVersion()}",
       processorOptions,
       s"-map-annotation-arguments-in-java=false"
-    ) ++ ksp2Args()
+    ) ++ ksp2Args().toStringSeq
 
     val kspLogLevel = if (Task.log.debugEnabled)
       mill.kotlinlib.ksp2.LogLevel.Debug
diff --git a/libs/kotlinlib/test/src/mill/kotlinlib/contrib/kover/KoverModuleTests.scala b/libs/kotlinlib/test/src/mill/kotlinlib/contrib/kover/KoverModuleTests.scala
index 6013ce77cd8e..3117ec9825f5 100644
--- a/libs/kotlinlib/test/src/mill/kotlinlib/contrib/kover/KoverModuleTests.scala
+++ b/libs/kotlinlib/test/src/mill/kotlinlib/contrib/kover/KoverModuleTests.scala
@@ -7,6 +7,7 @@ import mill.kotlinlib.TestModule
 import mill.testkit.{TestRootModule, UnitTester}
 import mill.{T, Task, api}
 import utest.{TestSuite, Tests, assert, test}
+import mill.api.opt.*
 
 import scala.xml.{Node, XML}
 
@@ -19,8 +20,8 @@ object KoverModuleTests extends TestSuite {
   object module extends TestRootModule {
 
     trait KotestTestModule extends TestModule.Junit5 {
-      override def forkArgs: T[Seq[String]] = Task {
-        super.forkArgs() ++ Seq("-Dkotest.framework.classpath.scanning.autoscan.disable=true")
+      override def forkArgs: T[Opts] = Task {
+        super.forkArgs() ++ Opts("-Dkotest.framework.classpath.scanning.autoscan.disable=true")
 
       }
       override def mvnDeps = super.mvnDeps() ++ Seq(
diff --git a/libs/scalajslib/src/mill/scalajslib/ScalaJSModule.scala b/libs/scalajslib/src/mill/scalajslib/ScalaJSModule.scala
index 1f4941e97815..abeaca5a129d 100644
--- a/libs/scalajslib/src/mill/scalajslib/ScalaJSModule.scala
+++ b/libs/scalajslib/src/mill/scalajslib/ScalaJSModule.scala
@@ -9,6 +9,7 @@ import mill.api.CrossVersion
 import mill.scalalib.{Dep, DepSyntax, Lib, TestModule}
 import mill.javalib.api.JvmWorkerUtil
 import mill.scalajslib.api.*
+import mill.api.opt.*
 import mill.scalajslib.worker.{ScalaJSWorker, ScalaJSWorkerExternalModule}
 import mill.*
 import mill.javalib.testrunner.{TestResult, TestRunner, TestRunnerUtils}
@@ -211,15 +212,15 @@ trait ScalaJSModule extends scalalib.ScalaModule with ScalaJSModuleApi { outer =
     )
   }
 
-  override def mandatoryScalacOptions: T[Seq[String]] = Task {
+  override def mandatoryScalacOptions: T[Opts] = Task {
     // Don't add flag twice, e.g. if a test suite inherits it both directly
     // ScalaJSModule as well as from the enclosing non-test ScalaJSModule
     val scalajsFlag =
       if (
         JvmWorkerUtil.isScala3(scalaVersion()) &&
-        !super.mandatoryScalacOptions().contains("-scalajs")
-      ) Seq("-scalajs")
-      else Seq.empty
+        !super.mandatoryScalacOptions().toStringSeq.contains("-scalajs")
+      ) Opts("-scalajs")
+      else Opts()
 
     super.mandatoryScalacOptions() ++ scalajsFlag
   }
diff --git a/libs/scalajslib/test/src/mill/scalajslib/CompileLinkTests.scala b/libs/scalajslib/test/src/mill/scalajslib/CompileLinkTests.scala
index b5f577bc8aa3..efdeb186d643 100644
--- a/libs/scalajslib/test/src/mill/scalajslib/CompileLinkTests.scala
+++ b/libs/scalajslib/test/src/mill/scalajslib/CompileLinkTests.scala
@@ -1,13 +1,13 @@
 package mill.scalajslib
 
-import mill._
+import mill.*
 import mill.api.Discover
-import mill.scalalib.{DepSyntax, PublishModule, ScalaModule, TestModule}
+import mill.api.opt.*
+import mill.scalalib.{DepSyntax, PublishModule, ScalaModule, ScalacOptions, TestModule}
 import mill.scalalib.publish.{Developer, License, PomSettings, VersionControl}
 import mill.testkit.UnitTester
 import mill.testkit.TestRootModule
-import utest._
-
+import utest.*
 import mill.javalib.api.JvmWorkerUtil
 
 object CompileLinkTests extends TestSuite {
@@ -52,7 +52,7 @@ object CompileLinkTests extends TestSuite {
     }
     object inherited extends ScalaJSModule {
       val (scala, scalaJS) = matrix.head
-      def scalacOptions = Seq("-deprecation")
+      def scalacOptions = Opts(ScalacOptions.`-deprecation`)
       def scalaOrganization = "org.example"
       def scalaVersion = scala
       def scalaJSVersion = scalaJS
diff --git a/libs/scalalib/src/mill/scalalib/ScalaModule.scala b/libs/scalalib/src/mill/scalalib/ScalaModule.scala
index 242cb0920ebd..8bc5de47d850 100644
--- a/libs/scalalib/src/mill/scalalib/ScalaModule.scala
+++ b/libs/scalalib/src/mill/scalalib/ScalaModule.scala
@@ -3,6 +3,7 @@ package scalalib
 
 import mill.util.JarManifest
 import mill.api.{BuildCtx, DummyInputStream, ModuleRef, PathRef, Result, Task}
+import mill.api.opt.*
 import mill.util.BuildInfo
 import mill.util.Jvm
 import mill.javalib.api.{CompilationResult, JvmWorkerUtil, Versions}
@@ -30,8 +31,8 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
     override def scalaVersion: T[String] = outer.scalaVersion()
     override def scalacPluginMvnDeps: T[Seq[Dep]] = outer.scalacPluginMvnDeps()
     override def scalacPluginClasspath: T[Seq[PathRef]] = outer.scalacPluginClasspath()
-    override def scalacOptions: T[Seq[String]] = outer.scalacOptions()
-    override def mandatoryScalacOptions: T[Seq[String]] =
+    override def scalacOptions: T[Opts] = outer.scalacOptions()
+    override def mandatoryScalacOptions: T[Opts] =
       Task { super.mandatoryScalacOptions() }
   }
 
@@ -172,54 +173,56 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
    * Mandatory command-line options to pass to the Scala compiler
    * that shouldn't be removed by overriding `scalacOptions`
    */
-  protected def mandatoryScalacOptions: T[Seq[String]] = Task { Seq.empty[String] }
+  protected def mandatoryScalacOptions: T[Opts] = Task { Opts() }
 
   /**
    * Scalac options to activate the compiler plugins.
    */
-  private def enablePluginScalacOptions: T[Seq[String]] = Task {
+  private def enablePluginScalacOptions: T[Opts] = Task {
 
     val resolvedJars = defaultResolver().classpath(
       scalacPluginMvnDeps().map(_.exclude("*" -> "*"))
     )
-    resolvedJars.iterator.map(jar => s"-Xplugin:${jar.path}").toSeq
+    Opts(resolvedJars.map(jar => opt"-Xplugin:${jar.path}"))
   }
 
   /**
    * Scalac options to activate the compiler plugins for ScalaDoc generation.
    */
-  private def enableScalaDocPluginScalacOptions: T[Seq[String]] = Task {
+  private def enableScalaDocPluginScalacOptions: T[Opts] = Task {
     val resolvedJars = defaultResolver().classpath(
       scalaDocPluginMvnDeps().map(_.exclude("*" -> "*"))
     )
-    resolvedJars.iterator.map(jar => s"-Xplugin:${jar.path}").toSeq
+    Opts(
+      resolvedJars.map(jar => opt"-Xplugin:${jar.path}")
+    )
   }
 
   /**
    * Command-line options to pass to the Scala compiler defined by the user.
    * Consumers should use `allScalacOptions` to read them.
    */
-  override def scalacOptions: T[Seq[String]] = Task { Seq.empty[String] }
+  override def scalacOptions: T[Opts] = Task { Opts() }
 
   /**
    * Aggregation of all the options passed to the Scala compiler.
    * In most cases, instead of overriding this task you want to override `scalacOptions` instead.
    */
-  def allScalacOptions: T[Seq[String]] = Task {
+  def allScalacOptions: T[Opts] = Task {
     mandatoryScalacOptions() ++ enablePluginScalacOptions() ++ scalacOptions()
   }
 
   /**
    * Options to pass directly into Scaladoc.
    */
-  def scalaDocOptions: T[Seq[String]] = Task {
+  def scalaDocOptions: T[Opts] = Task {
     val defaults =
       if (JvmWorkerUtil.isDottyOrScala3(scalaVersion()))
-        Seq(
+        Opts(
           "-project",
           artifactName()
         )
-      else Seq()
+      else Opts()
     mandatoryScalacOptions() ++ enableScalaDocPluginScalacOptions() ++ scalacOptions() ++ defaults
   }
 
@@ -278,7 +281,7 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
         |For details, see: https://github.com/sbt/zinc/issues/1010""".stripMargin
     )
 
-    val jOpts = JavaCompilerOptions.split(javacOptions() ++ mandatoryJavacOptions())
+    val jOpts = JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
 
     val worker = jvmWorker().internalWorker()
 
@@ -290,7 +293,7 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
         javacOptions = jOpts.compiler,
         scalaVersion = sv,
         scalaOrganization = scalaOrganization(),
-        scalacOptions = allScalacOptions(),
+        scalacOptions = allScalacOptions().toStringSeq,
         compilerClasspath = scalaCompilerClasspath(),
         scalacPluginClasspath = scalacPluginClasspath(),
         incrementalCompilation = zincIncrementalCompilation(),
@@ -334,7 +337,7 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
             scalaOrganization(),
             scalaDocClasspath(),
             scalacPluginClasspath(),
-            options ++ compileCp ++ scalaDocOptions() ++ files.map(_.toString())
+            options ++ compileCp ++ scalaDocOptions().toStringSeq ++ files.map(_.toString())
           ),
           javaHome = javaHome().map(_.path)
         ) match {
@@ -422,8 +425,8 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
         classPath = runClasspath().map(_.path) ++ scalaCompilerClasspath().map(
           _.path
         ),
-        jvmArgs = forkArgs(),
-        env = allForkEnv(),
+        jvmArgs = forkArgs().toStringSeq,
+        env = allForkEnv().view.mapValues(_.toString()).toMap,
         mainArgs = Seq(useJavaCp) ++ consoleScalacOptions().filterNot(Set(useJavaCp)),
         cwd = forkWorkingDir(),
         stdin = os.Inherit,
@@ -493,8 +496,8 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
       Jvm.callProcess(
         mainClass = mainClass,
         classPath = ammoniteReplClasspath().map(_.path).toVector,
-        jvmArgs = forkArgs(),
-        env = allForkEnv(),
+        jvmArgs = forkArgs().toStringSeq,
+        env = allForkEnv().view.mapValues(_.toString()).toMap,
         mainArgs = replOptions,
         cwd = forkWorkingDir(),
         stdin = os.Inherit,
@@ -608,13 +611,13 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
       }
 
       val scalacOptions = (
-        allScalacOptions() ++
-          semanticDbEnablePluginScalacOptions() ++
+        allScalacOptions().toStringSeq ++
+          semanticDbEnablePluginScalacOptions().toStringSeq ++
           additionalScalacOptions
       )
         .filterNot(_ == "-Xfatal-warnings")
 
-      val javacOpts = SemanticDbJavaModule.javacOptionsTask(javacOptions(), semanticDbJavaVersion())
+      val javacOpts = SemanticDbJavaModule.javacOptionsTask(javacOptions(), semanticDbJavaVersion()).toStringSeq
 
       Task.log.debug(s"effective scalac options: ${scalacOptions}")
       Task.log.debug(s"effective javac options: ${javacOpts}")
@@ -674,8 +677,8 @@ object ScalaModule {
     override def scalaVersion: T[String] = outer.scalaVersion()
     override def scalacPluginMvnDeps: T[Seq[Dep]] = outer.scalacPluginMvnDeps()
     override def scalacPluginClasspath: T[Seq[PathRef]] = outer.scalacPluginClasspath()
-    override def scalacOptions: T[Seq[String]] = outer.scalacOptions()
-    override def mandatoryScalacOptions: T[Seq[String]] =
+    override def scalacOptions: T[Opts] = outer.scalacOptions()
+    override def mandatoryScalacOptions: T[Opts] =
       Task { super.mandatoryScalacOptions() }
   }
 }
diff --git a/libs/scalalib/src/mill/scalalib/ScalacOptions.scala b/libs/scalalib/src/mill/scalalib/ScalacOptions.scala
new file mode 100644
index 000000000000..6ed343a1a006
--- /dev/null
+++ b/libs/scalalib/src/mill/scalalib/ScalacOptions.scala
@@ -0,0 +1,9 @@
+package mill.scalalib
+
+object ScalacOptions {
+
+  val `-deprecation` = "-deprecation"
+  val `-Ywarn-unused` = "-Ywarn-unused"
+  val `-Xfatal-warnings` = "-Xfatal-warnings"
+  val `-Ypartial-unification` = "-Ypartial-unification"
+}
diff --git a/libs/scalalib/src/mill/scalalib/idea/GenIdeaModule.scala b/libs/scalalib/src/mill/scalalib/idea/GenIdeaModule.scala
index 657d60610d95..e3e693b0643c 100644
--- a/libs/scalalib/src/mill/scalalib/idea/GenIdeaModule.scala
+++ b/libs/scalalib/src/mill/scalalib/idea/GenIdeaModule.scala
@@ -4,6 +4,7 @@ import mill.scalalib.ScalaModule
 import mill.Task
 import mill.api.daemon.internal.internal
 import mill.api.ModuleCtx
+
 trait GenIdeaModule extends mill.javalib.idea.GenIdeaModule {
   def javaModuleRef: mill.api.ModuleRef[ScalaModule]
   override def scalaCompilerClasspath = Task.Anon(javaModuleRef().scalaCompilerClasspath())
diff --git a/libs/scalalib/test/src/mill/scalalib/CrossVersionTests.scala b/libs/scalalib/test/src/mill/scalalib/CrossVersionTests.scala
index f275de7b2078..6a4709a66e3e 100644
--- a/libs/scalalib/test/src/mill/scalalib/CrossVersionTests.scala
+++ b/libs/scalalib/test/src/mill/scalalib/CrossVersionTests.scala
@@ -1,6 +1,7 @@
 package mill.scalalib
 
 import mill.api.Discover
+import mill.api.opt.Opts
 import mill.util.TokenReaders.*
 import mill.testkit.UnitTester
 import mill.testkit.TestRootModule
@@ -103,7 +104,7 @@ object CrossVersionTests extends TestSuite {
     object sandwitch213 extends ScalaModule {
       override def scalaVersion = "2.13.6"
       override def moduleDeps = Seq(sandwitch3)
-      override def scalacOptions = Seq("-Ytasty-reader")
+      override def scalacOptions = Opts("-Ytasty-reader")
       val tree =
         """├─ sandwitch3
           |│  ├─ com.lihaoyi:upickle_3:1.4.0
diff --git a/libs/scalalib/test/src/mill/scalalib/HelloWorldTests.scala b/libs/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
index edfa402c6639..c5bd49c42dcc 100644
--- a/libs/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
+++ b/libs/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
@@ -8,6 +8,7 @@ import mill.*
 import mill.api.ExecResult
 import mill.api.Discover
 import mill.api.ExecutionPaths
+import mill.api.opt.*
 import mill.testkit.UnitTester
 import mill.testkit.TestRootModule
 import utest.*
@@ -75,7 +76,7 @@ object HelloWorldTests extends TestSuite {
 
   object HelloWorldFatalWarnings extends TestRootModule {
     object core extends HelloWorldModule {
-      override def scalacOptions = Task { Seq("-Ywarn-unused", "-Xfatal-warnings") }
+      override def scalacOptions = Task { Opts(ScalacOptions.`-Ywarn-unused`, ScalacOptions.`-Xfatal-warnings`) }
     }
     lazy val millDiscover = Discover[this.type]
   }
@@ -108,14 +109,14 @@ object HelloWorldTests extends TestSuite {
   }
 
   def compileClassfiles: Seq[os.RelPath] = Seq(
-    os.rel / "Main.class",
-    os.rel / "Main$.class",
-    os.rel / "Main0.class",
-    os.rel / "Main0$.class",
-    os.rel / "Main$delayedInit$body.class",
-    os.rel / "Person.class",
-    os.rel / "Person$.class"
-  )
+    "Main.class",
+    "Main$.class",
+    "Main0.class",
+    "Main0$.class",
+    "Main$delayedInit$body.class",
+    "Person.class",
+    "Person$.class"
+  ).map(os.rel / _)
 
   def tests: Tests = Tests {
     test("scalaVersion") {
@@ -151,7 +152,7 @@ object HelloWorldTests extends TestSuite {
         val Right(result) = eval.apply(HelloWorldFatalWarnings.core.scalacOptions): @unchecked
 
         assert(
-          result.value == Seq("-Ywarn-unused", "-Xfatal-warnings"),
+          result.value.toStringSeq == Seq("-Ywarn-unused", "-Xfatal-warnings"),
           result.evalCount > 0
         )
       }
diff --git a/libs/scalalib/test/src/mill/scalalib/ScalaColorOutputTests.scala b/libs/scalalib/test/src/mill/scalalib/ScalaColorOutputTests.scala
index c93b3ca16c6a..56c6526f6825 100644
--- a/libs/scalalib/test/src/mill/scalalib/ScalaColorOutputTests.scala
+++ b/libs/scalalib/test/src/mill/scalalib/ScalaColorOutputTests.scala
@@ -1,5 +1,6 @@
 package mill.scalalib
 
+import mill.api.opt.Opts
 import mill.api.{Discover, ExecResult}
 import mill.scalalib.HelloWorldTests.*
 import mill.testkit.{TestRootModule, UnitTester}
@@ -14,7 +15,7 @@ object ScalaColorOutputTests extends TestSuite {
     object core extends ScalaModule {
       def scalaVersion = scala213Version
 
-      override def scalacOptions = super.scalacOptions() ++ Seq(
+      override def scalacOptions = super.scalacOptions() ++ Opts(
         "-Vimplicits"
       )
     }
diff --git a/libs/scalalib/test/src/mill/scalalib/ScalaFlagsTests.scala b/libs/scalalib/test/src/mill/scalalib/ScalaFlagsTests.scala
index 4117796b1862..19e5d7c5808b 100644
--- a/libs/scalalib/test/src/mill/scalalib/ScalaFlagsTests.scala
+++ b/libs/scalalib/test/src/mill/scalalib/ScalaFlagsTests.scala
@@ -5,6 +5,7 @@ import mill.scalalib.HelloWorldTests.*
 import mill.testkit.{TestRootModule, UnitTester}
 import mill.util.TokenReaders.*
 import utest.*
+import mill.api.opt.*
 
 object ScalaFlagsTests extends TestSuite {
 
@@ -12,8 +13,8 @@ object ScalaFlagsTests extends TestSuite {
     object core extends ScalaModule {
       def scalaVersion = scala212Version
 
-      override def scalacOptions = super.scalacOptions() ++ Seq(
-        "-Ypartial-unification"
+      override def scalacOptions = super.scalacOptions() ++ Opts(
+        ScalacOptions.`-Ypartial-unification`
       )
     }
 
diff --git a/libs/scalalib/test/src/mill/scalalib/ScalaMacrosTests.scala b/libs/scalalib/test/src/mill/scalalib/ScalaMacrosTests.scala
index a071b68a67f1..fcba9bddffdd 100644
--- a/libs/scalalib/test/src/mill/scalalib/ScalaMacrosTests.scala
+++ b/libs/scalalib/test/src/mill/scalalib/ScalaMacrosTests.scala
@@ -3,6 +3,7 @@ package mill.scalalib
 import mill.*
 import mill.testkit.{TestRootModule, UnitTester}
 import utest.*
+import mill.api.opt.*
 
 import HelloWorldTests.*
 import mill.api.Discover
@@ -13,7 +14,7 @@ object ScalaMacrosTests extends TestSuite {
     object core extends ScalaModule {
       override def scalaVersion = scala213Version
       override def mvnDeps = Seq(mvn"com.github.julien-truffaut::monocle-macro::2.1.0")
-      override def scalacOptions = super.scalacOptions() ++ Seq("-Ymacro-annotations")
+      override def scalacOptions = super.scalacOptions() ++ Opts("-Ymacro-annotations")
     }
     lazy val millDiscover = Discover[this.type]
   }
diff --git a/libs/scalalib/test/src/mill/scalalib/ScalaScaladocTests.scala b/libs/scalalib/test/src/mill/scalalib/ScalaScaladocTests.scala
index 4e5e043728bb..f2054c18afd2 100644
--- a/libs/scalalib/test/src/mill/scalalib/ScalaScaladocTests.scala
+++ b/libs/scalalib/test/src/mill/scalalib/ScalaScaladocTests.scala
@@ -6,13 +6,14 @@ import mill.testkit.{TestRootModule, UnitTester}
 import utest.*
 import HelloWorldTests.*
 import mill.api.Discover
+import mill.api.opt.*
 
 object ScalaScaladocTests extends TestSuite {
 
   object HelloWorldWithDocVersion extends TestRootModule {
     object core extends HelloWorldModule {
-      override def scalacOptions = Task { Seq("-Ywarn-unused", "-Xfatal-warnings") }
-      override def scalaDocOptions = super.scalaDocOptions() ++ Seq("-doc-version", "1.2.3")
+      override def scalacOptions = Task { Opts(ScalacOptions.`-Ywarn-unused`, ScalacOptions.`-Xfatal-warnings`) }
+      override def scalaDocOptions = super.scalaDocOptions() ++ Opts("-doc-version", "1.2.3")
     }
 
     lazy val millDiscover = Discover[this.type]
@@ -20,8 +21,8 @@ object ScalaScaladocTests extends TestSuite {
 
   object HelloWorldOnlyDocVersion extends TestRootModule {
     object core extends HelloWorldModule {
-      override def scalacOptions = Task { Seq("-Ywarn-unused", "-Xfatal-warnings") }
-      override def scalaDocOptions = Task { Seq("-doc-version", "1.2.3") }
+      override def scalacOptions = Task { Opts(ScalacOptions.`-Ywarn-unused`, ScalacOptions.`-Xfatal-warnings`) }
+      override def scalaDocOptions = Task { Opts("-doc-version", "1.2.3") }
     }
 
     lazy val millDiscover = Discover[this.type]
@@ -30,7 +31,7 @@ object ScalaScaladocTests extends TestSuite {
 
   object HelloWorldDocTitle extends TestRootModule {
     object core extends HelloWorldModule {
-      override def scalaDocOptions = Task { Seq("-doc-title", "Hello World") }
+      override def scalaDocOptions = Task { Opts("-doc-title", "Hello World") }
     }
 
     lazy val millDiscover = Discover[this.type]
@@ -49,14 +50,14 @@ object ScalaScaladocTests extends TestSuite {
       test("override") - UnitTester(HelloWorldDocTitle, resourcePath).scoped { eval =>
         val Right(result) = eval.apply(HelloWorldDocTitle.core.scalaDocOptions): @unchecked
         assert(
-          result.value == Seq("-doc-title", "Hello World"),
+          result.value == Opts("-doc-title", "Hello World"),
           result.evalCount > 0
         )
       }
       test("extend") - UnitTester(HelloWorldWithDocVersion, resourcePath).scoped { eval =>
         val Right(result) = eval.apply(HelloWorldWithDocVersion.core.scalaDocOptions): @unchecked
         assert(
-          result.value == Seq("-Ywarn-unused", "-Xfatal-warnings", "-doc-version", "1.2.3"),
+          result.value == Opts("-Ywarn-unused", "-Xfatal-warnings", "-doc-version", "1.2.3"),
           result.evalCount > 0
         )
       }
diff --git a/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala b/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
index 256f6ba3f09e..5255b12c556b 100644
--- a/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
+++ b/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
@@ -358,8 +358,8 @@ trait ScalaNativeModule extends ScalaModule with ScalaNativeModuleApi { outer =>
     new NativeRunner(
       mainClassDefault = finalMainClassOpt(),
       nativeExe = nativeLink(),
-      forkArgsDefault = forkArgs(),
-      forkEnvDefault = allForkEnv(),
+      forkArgsDefault = forkArgs().toStringSeq,
+      forkEnvDefault = allForkEnv().view.mapValues(_.toString()).toMap,
       propagateEnvDefault = propagateEnv()
     )
   }
@@ -368,8 +368,8 @@ trait ScalaNativeModule extends ScalaModule with ScalaNativeModuleApi { outer =>
     new NativeRunner(
       mainClassDefault = Right(mainClass),
       nativeExe = nativeLinkOtherMain(mainClass)(),
-      forkArgsDefault = forkArgs(),
-      forkEnvDefault = allForkEnv(),
+      forkArgsDefault = forkArgs().toStringSeq,
+      forkEnvDefault = allForkEnv().view.mapValues(_.toString()).toMap,
       propagateEnvDefault = propagateEnv()
     )
   }
@@ -460,7 +460,7 @@ trait TestScalaNativeModule extends ScalaNativeModule with TestModule {
 
     val (close, framework) = withScalaNativeBridge.apply().apply(_.getFramework(
       nativeLink().path.toIO,
-      allForkEnv(),
+      allForkEnv().view.mapValues(_.toString()).toMap,
       toWorkerApi(logLevel()),
       testFramework()
     ))
diff --git a/libs/scalanativelib/test/src/mill/scalanativelib/CompileRunTests.scala b/libs/scalanativelib/test/src/mill/scalanativelib/CompileRunTests.scala
index 5e2987700a3d..2c2e9b5c18a3 100644
--- a/libs/scalanativelib/test/src/mill/scalanativelib/CompileRunTests.scala
+++ b/libs/scalanativelib/test/src/mill/scalanativelib/CompileRunTests.scala
@@ -3,6 +3,7 @@ package mill.scalanativelib
 import mill._
 import mill.api.Discover
 import mill.api.ExecutionPaths
+import mill.api.opt.*
 import mill.javalib.api.JvmWorkerUtil
 import mill.scalalib.{PublishModule, ScalaModule, TestModule}
 import mill.scalalib.publish.{Developer, License, PomSettings, VersionControl}
@@ -65,7 +66,7 @@ object CompileRunTests extends TestSuite {
 
     object inherited extends ScalaNativeModule {
       val (scala, scalaNative, _) = matrix.head
-      def scalacOptions = Seq("-deprecation")
+      def scalacOptions = Opts("-deprecation")
       def scalaOrganization = "org.example"
       def scalaVersion = scala
       def scalaNativeVersion = scalaNative
diff --git a/mill-itest-plugin.adoc b/mill-itest-plugin.adoc
new file mode 100644
index 000000000000..390ac551a577
--- /dev/null
+++ b/mill-itest-plugin.adoc
@@ -0,0 +1,55 @@
+= Draft for a new mill-itest plugin
+:version: 0.7.1
+:mill-version: 1.0.0-RC2
+
+
+.`build.mill`
+[,scala,subs="attributes"]
+----
+//| mill-version: {mill-version}
+//| mvnDeps:
+//| - de.tototec::de.tobiasroeser.mill.integrationtest::{version}
+package build
+
+import de.tobiasroeser.mill.integrationtest._
+
+object demo extends ScalaModule with PublishModule {
+  // ...
+}
+
+object itest extends Cross[ITestCross]("0.11.13", "0.12.14", "{mill-version}")
+trait ItestCross extends MillIntegrationTestCrossModule {
+  def pluginsUnderTest = Seq(demo)
+}
+----
+
+Plugin creates automatically a meta-build and loads the plugin under test into the classpath.
+
+You can write the test cases in the `/* Usage */` blocks
+
+.`itest/src/simple/build.mill`
+[,scala]
+----
+package build
+
+object simple extends Module {
+  def hello() = Task.Command {
+    println("hello")
+  }
+}
+
+
+
+/* Usage
+
+> mill simple
+hello
+
+ */
+----
+
+.`itest/src/simple/checks.itest.mill`
+----
+// Test file
+
+----
\ No newline at end of file
diff --git a/runner/bsp/worker/src/mill/bsp/worker/MillJavaBuildServer.scala b/runner/bsp/worker/src/mill/bsp/worker/MillJavaBuildServer.scala
index a96f39da832a..7bb27b9892a1 100644
--- a/runner/bsp/worker/src/mill/bsp/worker/MillJavaBuildServer.scala
+++ b/runner/bsp/worker/src/mill/bsp/worker/MillJavaBuildServer.scala
@@ -34,7 +34,7 @@ private trait MillJavaBuildServer extends JavaBuildServer { this: MillBuildServe
         val res = f(ev)
         new JavacOptionsItem(
           id,
-          res.javacOptions.asJava,
+          res.javacOptions.toStringSeq.asJava,
           res.classpath.asJava,
           sanitizeUri(res.classesPath)
         )
diff --git a/runner/bsp/worker/src/mill/bsp/worker/MillJvmBuildServer.scala b/runner/bsp/worker/src/mill/bsp/worker/MillJvmBuildServer.scala
index d214d7f215ca..b9357dad0977 100644
--- a/runner/bsp/worker/src/mill/bsp/worker/MillJvmBuildServer.scala
+++ b/runner/bsp/worker/src/mill/bsp/worker/MillJvmBuildServer.scala
@@ -57,23 +57,25 @@ private trait MillJvmBuildServer extends JvmBuildServer { this: MillBuildServer
             _,
             id,
             _,
-            (
-              _,
-              forkArgs,
-              forkWorkingDir,
-              forkEnv,
-              _,
-              Some(testEnvVars)
-            )
-          ) =>
+            res
+//            (
+//              _,
+//              forkArgs,
+//              forkWorkingDir,
+//              forkEnv,
+//              _,
+//              Some(testEnvVars)
+//            )
+          ) if res.testEnvVars.isDefined =>
+        val testEnvVars = res.testEnvVars.get
         val fullMainArgs: List[String] =
           List(testEnvVars.testRunnerClasspathArg, testEnvVars.argsFile)
         val item = new JvmEnvironmentItem(
           id,
           testEnvVars.classpath.map(sanitizeUri).asJava,
-          forkArgs.asJava,
-          forkWorkingDir.toString(),
-          forkEnv.asJava
+          res.forkArgs.toStringSeq.asJava,
+          res.forkWorkingDir.toString(),
+          res.forkEnv.view.mapValues(_.toString()).toMap.asJava
         )
         item.setMainClasses(List(testEnvVars.mainClass).map(new JvmMainClass(
           _,
@@ -107,9 +109,9 @@ private trait MillJvmBuildServer extends JvmBuildServer { this: MillBuildServer
         val item = new JvmEnvironmentItem(
           id,
           classpath.asJava,
-          res.forkArgs.asJava,
+          res.forkArgs.toStringSeq.asJava,
           res.forkWorkingDir.toString(),
-          res.forkEnv.asJava
+          res.forkEnv.view.mapValues(_.toString()).toMap.asJava
         )
 
         val classes = res.mainClass.toList ++ res.localMainClasses
diff --git a/runner/bsp/worker/src/mill/bsp/worker/MillScalaBuildServer.scala b/runner/bsp/worker/src/mill/bsp/worker/MillScalaBuildServer.scala
index ef2e0bdaaae0..94717de218c8 100644
--- a/runner/bsp/worker/src/mill/bsp/worker/MillScalaBuildServer.scala
+++ b/runner/bsp/worker/src/mill/bsp/worker/MillScalaBuildServer.scala
@@ -41,13 +41,13 @@ private trait MillScalaBuildServer extends ScalaBuildServer { this: MillBuildSer
             _,
             id,
             _,
-            (allScalacOptions, compileClasspath, classesPathTask)
+            res
           ) =>
         new ScalacOptionsItem(
           id,
-          allScalacOptions.asJava,
-          compileClasspath(ev).asJava,
-          sanitizeUri(classesPathTask(ev))
+          res.scalacOptionsTask.toStringSeq.asJava,
+          res.compileClasspathTask(ev).asJava,
+          sanitizeUri(res.classPathTask(ev))
         )
 
     } { values =>
@@ -67,7 +67,7 @@ private trait MillScalaBuildServer extends ScalaBuildServer { this: MillBuildSer
         val mainClasses = res.classes
         // val mainMain = m.mainClass().orElse(if(mainClasses.size == 1) mainClasses.headOption else None)
         val items = mainClasses.map { mc =>
-          val scalaMc = new ScalaMainClass(mc, Seq().asJava, res.forkArgs.asJava)
+          val scalaMc = new ScalaMainClass(mc, Seq().asJava, res.forkArgs.toStringSeq.asJava)
           scalaMc.setEnvironmentVariables(res.forkEnv.map(e => s"${e._1}=${e._2}").toSeq.asJava)
           scalaMc
         }
diff --git a/runner/meta/src/mill/meta/MillBuildRootModule.scala b/runner/meta/src/mill/meta/MillBuildRootModule.scala
index c88b8373e306..183dbb055b81 100644
--- a/runner/meta/src/mill/meta/MillBuildRootModule.scala
+++ b/runner/meta/src/mill/meta/MillBuildRootModule.scala
@@ -1,13 +1,12 @@
 package mill.meta
 
 import java.nio.file.Path
-import mill.api.BuildCtx
+import mill.api.{BuildCtx, Discover, PathRef, Result, Task}
 import mill.*
-import mill.api.Result
+import mill.api.opt.*
 import mill.api.daemon.internal.internal
 import mill.constants.CodeGenConstants.buildFileExtensions
 import mill.constants.OutFiles.*
-import mill.api.{Discover, PathRef, Task}
 import mill.api.internal.RootModule
 import mill.scalalib.{Dep, DepSyntax, Lib, ScalaModule}
 import mill.javalib.api.{CompilationResult, Versions}
@@ -255,11 +254,11 @@ trait MillBuildRootModule()(using
       .exclude("com.lihaoyi" -> "sourcecode_3")
   )
 
-  override def scalacOptions: T[Seq[String]] = Task {
+  override def scalacOptions: T[Opts] = Task {
     super.scalacOptions() ++
       // This warning comes up for package names with dashes in them like "package build.`foo-bar`",
       // but Mill generally handles these fine, so no need to warn the user
-      Seq("-deprecation", "-Wconf:msg=will be encoded on the classpath:silent")
+      Opts("-deprecation", "-Wconf:msg=will be encoded on the classpath:silent")
   }
 
   /** Used in BSP IntelliJ, which can only work with directories */
@@ -288,7 +287,7 @@ trait MillBuildRootModule()(using
     }
 
     // copied from `ScalaModule`
-    val jOpts = JavaCompilerOptions.split(javacOptions() ++ mandatoryJavacOptions())
+    val jOpts = JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
     val worker = jvmWorker().internalWorker()
     worker.apply(
       ZincCompileMixed(
@@ -298,7 +297,7 @@ trait MillBuildRootModule()(using
         javacOptions = jOpts.compiler,
         scalaVersion = scalaVersion(),
         scalaOrganization = scalaOrganization(),
-        scalacOptions = allScalacOptions(),
+        scalacOptions = allScalacOptions().toStringSeq,
         compilerClasspath = scalaCompilerClasspath(),
         scalacPluginClasspath = scalacPluginClasspath(),
         incrementalCompilation = zincIncrementalCompilation(),

From 84c9c7e9ec12c63e4edeec3879445f1a50b14d28 Mon Sep 17 00:00:00 2001
From: "autofix-ci[bot]" <114827586+autofix-ci[bot]@users.noreply.github.com>
Date: Wed, 5 Nov 2025 17:20:42 +0000
Subject: [PATCH 04/67] [autofix.ci] apply automated fixes

---
 .../mill/api/daemon/internal/OptsApi.scala    |  3 +-
 core/api/src/mill/api/opt/Opt.scala           |  1 -
 core/api/src/mill/api/opt/OptGroup.scala      |  1 -
 .../test/src/mill/api/ApplicativeTests.scala  | 64 ++++++++++++++-----
 .../mill/androidlib/AndroidKotlinModule.scala |  7 +-
 .../src/mill/androidlib/AndroidModule.scala   |  6 +-
 .../javalib/src/mill/javalib/JavaModule.scala | 19 +++++-
 .../src/mill/javalib/JavacOptions.scala       |  2 +-
 .../mill/javalib/SemanticDbJavaModule.scala   | 10 +--
 .../javalib/errorprone/ErrorProneModule.scala |  1 -
 .../mill/kotlinlib/js/KotlinJsModule.scala    |  2 +-
 .../src/mill/kotlinlib/ksp/KspModule.scala    |  8 ++-
 .../src/mill/scalalib/ScalaModule.scala       |  6 +-
 .../src/mill/scalalib/HelloWorldTests.scala   |  3 +-
 .../mill/scalalib/ScalaScaladocTests.scala    |  6 +-
 .../src/mill/meta/MillBuildRootModule.scala   |  3 +-
 16 files changed, 97 insertions(+), 45 deletions(-)

diff --git a/core/api/daemon/src/mill/api/daemon/internal/OptsApi.scala b/core/api/daemon/src/mill/api/daemon/internal/OptsApi.scala
index 37700ac9450e..4070fafb541a 100644
--- a/core/api/daemon/src/mill/api/daemon/internal/OptsApi.scala
+++ b/core/api/daemon/src/mill/api/daemon/internal/OptsApi.scala
@@ -5,8 +5,7 @@ trait OptsApi {
   def value: Seq[OptGroupApi]
 }
 
-trait OptGroupApi {
-}
+trait OptGroupApi {}
 
 trait OptApi {
   def toString(): String
diff --git a/core/api/src/mill/api/opt/Opt.scala b/core/api/src/mill/api/opt/Opt.scala
index e3f9d7305a63..7e6396823d93 100644
--- a/core/api/src/mill/api/opt/Opt.scala
+++ b/core/api/src/mill/api/opt/Opt.scala
@@ -83,7 +83,6 @@ object Opt {
 //  implicit def IterableToOpt[T](s: Iterable[T])(using f: T => Opt): Opt =
 //    Opt(s.toSeq.flatMap(f(_).value))
 
-
   implicit def StringToOpt(s: String): Opt = Opt(s)
 
   implicit def OsPathToOpt(p: os.Path): Opt = Opt(p)
diff --git a/core/api/src/mill/api/opt/OptGroup.scala b/core/api/src/mill/api/opt/OptGroup.scala
index 83fefd77b59f..2d2e3c484527 100644
--- a/core/api/src/mill/api/opt/OptGroup.scala
+++ b/core/api/src/mill/api/opt/OptGroup.scala
@@ -50,5 +50,4 @@ object OptGroup {
 //  implicit def ArrayToOptGroup[T](s: Array[T])(using f: T => OptGroup): OptGroup =
 //    OptGroup(s.flatMap(f(_).value))
 
-
 }
diff --git a/core/api/test/src/mill/api/ApplicativeTests.scala b/core/api/test/src/mill/api/ApplicativeTests.scala
index f85db1e5e378..5e5e617b763b 100644
--- a/core/api/test/src/mill/api/ApplicativeTests.scala
+++ b/core/api/test/src/mill/api/ApplicativeTests.scala
@@ -24,7 +24,9 @@ object ApplicativeTests extends TestSuite {
     test("selfContained") {
 
       test("simple") - assert(TestOpt("lol " + 1) == TestOpt.some("lol 1"))
-      test("singleSome") - assert(TestOpt("lol " + TestOpt.some("hello")()) == TestOpt.some("lol hello"))
+      test("singleSome") - assert(
+        TestOpt("lol " + TestOpt.some("hello")()) == TestOpt.some("lol hello")
+      )
       test("twoSomes") - assert(
         TestOpt(TestOpt.some("lol ")() + TestOpt.some("hello")()) == TestOpt.some("lol hello")
       )
@@ -35,27 +37,55 @@ object ApplicativeTests extends TestSuite {
           TestOpt(
             "lol " +
               TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() +
-              TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.some(" world")() +
+              TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.some(
+                " world"
+              )() +
               TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() +
               TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() +
-              TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.some(" moo")()
+              TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.none() + TestOpt.some(
+                " moo"
+              )()
           ) == TestOpt.none
         )
         assert(
           TestOpt(
             "lol " +
-              TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() +
-              TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() +
-              TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() +
-              TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some("a")() +
-              TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some("e")() +
-              TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() +
-              TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() +
-              TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() +
-              TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some("a")() +
-              TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some("e")() +
-              TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() +
-              TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() +
+              TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some(
+                "d"
+              )() +
+              TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some(
+                "c"
+              )() +
+              TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some(
+                "b"
+              )() +
+              TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some(
+                "a"
+              )() +
+              TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some(
+                "e"
+              )() +
+              TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some(
+                "d"
+              )() +
+              TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some(
+                "c"
+              )() +
+              TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some(
+                "b"
+              )() +
+              TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some("e")() + TestOpt.some(
+                "a"
+              )() +
+              TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some("d")() + TestOpt.some(
+                "e"
+              )() +
+              TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some("c")() + TestOpt.some(
+                "d"
+              )() +
+              TestOpt.some("e")() + TestOpt.some("a")() + TestOpt.some("b")() + TestOpt.some(
+                "c"
+              )() +
               TestOpt.some("d")() + TestOpt.some("e")()
           ) == TestOpt.some("lol abcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcdeabcde")
         )
@@ -68,7 +98,9 @@ object ApplicativeTests extends TestSuite {
       val lol = "lol "
       def hell(o: String) = "hell" + o
       test("simple") - assert(TestOpt(lol + 1) == TestOpt.some("lol 1"))
-      test("singleSome") - assert(TestOpt(lol + TestOpt.some(hell("o"))()) == TestOpt.some("lol hello"))
+      test("singleSome") - assert(
+        TestOpt(lol + TestOpt.some(hell("o"))()) == TestOpt.some("lol hello")
+      )
       test("twoSomes") - assert(
         TestOpt(TestOpt.some(lol)() + TestOpt.some(hell("o"))()) == TestOpt.some("lol hello")
       )
diff --git a/libs/androidlib/src/mill/androidlib/AndroidKotlinModule.scala b/libs/androidlib/src/mill/androidlib/AndroidKotlinModule.scala
index 0c23379dbe77..b4b503599470 100644
--- a/libs/androidlib/src/mill/androidlib/AndroidKotlinModule.scala
+++ b/libs/androidlib/src/mill/androidlib/AndroidKotlinModule.scala
@@ -6,7 +6,12 @@ import mill.api.opt.*
 import mill.javalib.{CoursierModule, Dep}
 import mill.kotlinlib.{Dep, DepSyntax, KotlinModule, KotlincOptions}
 import mill.{T, Task}
-import mill.androidlib.databinding.{AndroidDataBindingWorker, AndroidDataBindingWorkerModule, GenerateBindingSourcesArgs, ProcessResourcesArgs}
+import mill.androidlib.databinding.{
+  AndroidDataBindingWorker,
+  AndroidDataBindingWorkerModule,
+  GenerateBindingSourcesArgs,
+  ProcessResourcesArgs
+}
 import mill.util.Jvm
 
 // TODO expose Compose configuration options
diff --git a/libs/androidlib/src/mill/androidlib/AndroidModule.scala b/libs/androidlib/src/mill/androidlib/AndroidModule.scala
index 0e93003dd768..9ea29657588c 100644
--- a/libs/androidlib/src/mill/androidlib/AndroidModule.scala
+++ b/libs/androidlib/src/mill/androidlib/AndroidModule.scala
@@ -516,7 +516,8 @@ trait AndroidModule extends JavaModule { outer =>
    * The Java compiled classes of [[androidResources]]
    */
   def androidCompiledRClasses: T[CompilationResult] = Task(persistent = true) {
-    val jOpts = JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
+    val jOpts =
+      JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
     val worker = jvmWorker().internalWorker()
     worker.apply(
       ZincCompileJava(
@@ -728,7 +729,8 @@ trait AndroidModule extends JavaModule { outer =>
 
     val rJar = Task.dest / "R.jar"
 
-    val jOpts = JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
+    val jOpts =
+      JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
     val worker = jvmWorker().internalWorker()
     val classesDest = worker
       .apply(
diff --git a/libs/javalib/src/mill/javalib/JavaModule.scala b/libs/javalib/src/mill/javalib/JavaModule.scala
index bde994b4975a..7dcdb646ee9f 100644
--- a/libs/javalib/src/mill/javalib/JavaModule.scala
+++ b/libs/javalib/src/mill/javalib/JavaModule.scala
@@ -7,10 +7,25 @@ import coursier.params.ResolutionParams
 import coursier.parse.{JavaOrScalaModule, ModuleParser}
 import coursier.util.{EitherT, ModuleMatcher, Monad}
 import mainargs.Flag
-import mill.api.{DefaultTaskModule, MillException, ModuleRef, PathRef, Result, Segment, Task, TaskCtx}
+import mill.api.{
+  DefaultTaskModule,
+  MillException,
+  ModuleRef,
+  PathRef,
+  Result,
+  Segment,
+  Task,
+  TaskCtx
+}
 import mill.api.opt.*
 import mill.api.daemon.internal.{EvaluatorApi, JavaModuleApi, internal}
-import mill.api.daemon.internal.bsp.{BspBuildTarget, BspJavaModuleApi, BspModuleApi, BspUri, JvmBuildTarget}
+import mill.api.daemon.internal.bsp.{
+  BspBuildTarget,
+  BspJavaModuleApi,
+  BspModuleApi,
+  BspUri,
+  JvmBuildTarget
+}
 import mill.api.daemon.internal.eclipse.GenEclipseInternalApi
 import mill.javalib.*
 import mill.api.daemon.internal.idea.GenIdeaInternalApi
diff --git a/libs/javalib/src/mill/javalib/JavacOptions.scala b/libs/javalib/src/mill/javalib/JavacOptions.scala
index 89d5d13aef40..afc89a73b616 100644
--- a/libs/javalib/src/mill/javalib/JavacOptions.scala
+++ b/libs/javalib/src/mill/javalib/JavacOptions.scala
@@ -5,5 +5,5 @@ object JavacOptions {
   val `-target` = "-target"
   val `-source` = "-source"
   val `-deprecation` = "-deprecation"
-  
+
 }
diff --git a/libs/javalib/src/mill/javalib/SemanticDbJavaModule.scala b/libs/javalib/src/mill/javalib/SemanticDbJavaModule.scala
index 1b593a3d7575..7d41e82652d3 100644
--- a/libs/javalib/src/mill/javalib/SemanticDbJavaModule.scala
+++ b/libs/javalib/src/mill/javalib/SemanticDbJavaModule.scala
@@ -1,14 +1,6 @@
 package mill.javalib
 
-import mill.api.{
-  BuildCtx,
-  Discover,
-  ExternalModule,
-  ModuleRef,
-  PathRef,
-  Result,
-  experimental
-}
+import mill.api.{BuildCtx, Discover, ExternalModule, ModuleRef, PathRef, Result, experimental}
 import mill.api.daemon.internal.SemanticDbJavaModuleApi
 import mill.api.opt.*
 import mill.constants.CodeGenConstants
diff --git a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
index 6053471d1adf..911a60a01549 100644
--- a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
+++ b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
@@ -5,7 +5,6 @@ import mill.api.opt.*
 import mill.javalib.{Dep, DepSyntax, JavaModule}
 import mill.{T, Task}
 
-import java.io.File
 
 /**
  * Integrated Error Prone into a [[JavaModule]].
diff --git a/libs/kotlinlib/src/mill/kotlinlib/js/KotlinJsModule.scala b/libs/kotlinlib/src/mill/kotlinlib/js/KotlinJsModule.scala
index b71e26088bbb..67eb4c9dcb31 100644
--- a/libs/kotlinlib/src/mill/kotlinlib/js/KotlinJsModule.scala
+++ b/libs/kotlinlib/src/mill/kotlinlib/js/KotlinJsModule.scala
@@ -7,7 +7,7 @@ import mill.api.{PathRef, Result, Task}
 import mill.api.opt.*
 import mill.api.Task.Command
 import mill.kotlinlib.worker.api.{KotlinWorker, KotlinWorkerTarget}
-import mill.kotlinlib.{Dep, DepSyntax, KotlinModule, KotlinWorkerManager, KotlincOptions}
+import mill.kotlinlib.{Dep, DepSyntax, KotlinModule, KotlinWorkerManager}
 import mill.javalib.Lib
 import mill.javalib.api.CompilationResult
 import mill.util.Jvm
diff --git a/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala b/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala
index 892f45fce36b..8952fa1dedf9 100644
--- a/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala
+++ b/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala
@@ -341,7 +341,9 @@ trait KspModule extends KotlinModule { outer =>
     val kspCachesDir = Task.dest / "caches"
 
     val processorOptionsValue =
-      kspProcessorOptions().map((key, value) => s"$key=${value.toString}").toSeq.mkString(File.pathSeparator)
+      kspProcessorOptions().map((key, value) => s"$key=${value.toString}").toSeq.mkString(
+        File.pathSeparator
+      )
 
     val processorOptions = if (processorOptionsValue.isEmpty)
       ""
@@ -448,7 +450,9 @@ trait KspModule extends KotlinModule { outer =>
     val kspCachesDir = Task.dest / "caches"
 
     val processorOptionsValue =
-      kspProcessorOptions().map((key, value) => s"$key=${value.toString()}").toSeq.mkString(File.pathSeparator)
+      kspProcessorOptions().map((key, value) => s"$key=${value.toString()}").toSeq.mkString(
+        File.pathSeparator
+      )
 
     val processorOptions = if (processorOptionsValue.isEmpty)
       ""
diff --git a/libs/scalalib/src/mill/scalalib/ScalaModule.scala b/libs/scalalib/src/mill/scalalib/ScalaModule.scala
index 8bc5de47d850..01a56e497d04 100644
--- a/libs/scalalib/src/mill/scalalib/ScalaModule.scala
+++ b/libs/scalalib/src/mill/scalalib/ScalaModule.scala
@@ -281,7 +281,8 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
         |For details, see: https://github.com/sbt/zinc/issues/1010""".stripMargin
     )
 
-    val jOpts = JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
+    val jOpts =
+      JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
 
     val worker = jvmWorker().internalWorker()
 
@@ -617,7 +618,8 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
       )
         .filterNot(_ == "-Xfatal-warnings")
 
-      val javacOpts = SemanticDbJavaModule.javacOptionsTask(javacOptions(), semanticDbJavaVersion()).toStringSeq
+      val javacOpts =
+        SemanticDbJavaModule.javacOptionsTask(javacOptions(), semanticDbJavaVersion()).toStringSeq
 
       Task.log.debug(s"effective scalac options: ${scalacOptions}")
       Task.log.debug(s"effective javac options: ${javacOpts}")
diff --git a/libs/scalalib/test/src/mill/scalalib/HelloWorldTests.scala b/libs/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
index c5bd49c42dcc..8862ce444b09 100644
--- a/libs/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
+++ b/libs/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
@@ -76,7 +76,8 @@ object HelloWorldTests extends TestSuite {
 
   object HelloWorldFatalWarnings extends TestRootModule {
     object core extends HelloWorldModule {
-      override def scalacOptions = Task { Opts(ScalacOptions.`-Ywarn-unused`, ScalacOptions.`-Xfatal-warnings`) }
+      override def scalacOptions =
+        Task { Opts(ScalacOptions.`-Ywarn-unused`, ScalacOptions.`-Xfatal-warnings`) }
     }
     lazy val millDiscover = Discover[this.type]
   }
diff --git a/libs/scalalib/test/src/mill/scalalib/ScalaScaladocTests.scala b/libs/scalalib/test/src/mill/scalalib/ScalaScaladocTests.scala
index f2054c18afd2..283fabd5eab8 100644
--- a/libs/scalalib/test/src/mill/scalalib/ScalaScaladocTests.scala
+++ b/libs/scalalib/test/src/mill/scalalib/ScalaScaladocTests.scala
@@ -12,7 +12,8 @@ object ScalaScaladocTests extends TestSuite {
 
   object HelloWorldWithDocVersion extends TestRootModule {
     object core extends HelloWorldModule {
-      override def scalacOptions = Task { Opts(ScalacOptions.`-Ywarn-unused`, ScalacOptions.`-Xfatal-warnings`) }
+      override def scalacOptions =
+        Task { Opts(ScalacOptions.`-Ywarn-unused`, ScalacOptions.`-Xfatal-warnings`) }
       override def scalaDocOptions = super.scalaDocOptions() ++ Opts("-doc-version", "1.2.3")
     }
 
@@ -21,7 +22,8 @@ object ScalaScaladocTests extends TestSuite {
 
   object HelloWorldOnlyDocVersion extends TestRootModule {
     object core extends HelloWorldModule {
-      override def scalacOptions = Task { Opts(ScalacOptions.`-Ywarn-unused`, ScalacOptions.`-Xfatal-warnings`) }
+      override def scalacOptions =
+        Task { Opts(ScalacOptions.`-Ywarn-unused`, ScalacOptions.`-Xfatal-warnings`) }
       override def scalaDocOptions = Task { Opts("-doc-version", "1.2.3") }
     }
 
diff --git a/runner/meta/src/mill/meta/MillBuildRootModule.scala b/runner/meta/src/mill/meta/MillBuildRootModule.scala
index 183dbb055b81..739724f445a2 100644
--- a/runner/meta/src/mill/meta/MillBuildRootModule.scala
+++ b/runner/meta/src/mill/meta/MillBuildRootModule.scala
@@ -287,7 +287,8 @@ trait MillBuildRootModule()(using
     }
 
     // copied from `ScalaModule`
-    val jOpts = JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
+    val jOpts =
+      JavaCompilerOptions.split(javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq)
     val worker = jvmWorker().internalWorker()
     worker.apply(
       ZincCompileMixed(

From 57af960ac836d5ad10c3cd27f440225766fe7949 Mon Sep 17 00:00:00 2001
From: "autofix-ci[bot]" <114827586+autofix-ci[bot]@users.noreply.github.com>
Date: Wed, 5 Nov 2025 17:37:58 +0000
Subject: [PATCH 05/67] [autofix.ci] apply automated fixes (attempt 2/3)

---
 libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala | 1 -
 1 file changed, 1 deletion(-)

diff --git a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
index 911a60a01549..707f7c19e4bb 100644
--- a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
+++ b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
@@ -5,7 +5,6 @@ import mill.api.opt.*
 import mill.javalib.{Dep, DepSyntax, JavaModule}
 import mill.{T, Task}
 
-
 /**
  * Integrated Error Prone into a [[JavaModule]].
  *

From 3bdb2e91ed1f311afa611e632db67d5de304e8ce Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 5 Nov 2025 21:28:38 +0100
Subject: [PATCH 06/67] Don't use Opts for errorProneOptions

---
 .../src/mill/javalib/errorprone/ErrorProneModule.scala       | 4 ++--
 .../test/src/mill/javalib/errorprone/ErrorProneTests.scala   | 5 ++---
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
index 707f7c19e4bb..e5f0cebb062e 100644
--- a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
+++ b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
@@ -43,7 +43,7 @@ trait ErrorProneModule extends JavaModule {
       "-XDcompilePolicy=simple",
       "-processorpath",
       processorPath,
-      (Seq("-Xplugin:ErrorProne") ++ errorProneOptions().toStringSeq).mkString(" ")
+      (Seq("-Xplugin:ErrorProne") ++ errorProneOptions()).mkString(" ")
     )
     val java17Options: Seq[String] = Option.when(scala.util.Properties.isJavaAtLeast(16))(Seq(
       "--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED",
@@ -66,7 +66,7 @@ trait ErrorProneModule extends JavaModule {
    *
    * Those are documented as "flags" at https://errorprone.info/docs/flags
    */
-  def errorProneOptions: T[Opts] = Task { Opts() }
+  def errorProneOptions: T[Seq[String]] = Task { Seq() }
 
   /**
    * Appends the [[errorProneJavacEnableOptions]] to the Java compiler options.
diff --git a/libs/javalib/test/src/mill/javalib/errorprone/ErrorProneTests.scala b/libs/javalib/test/src/mill/javalib/errorprone/ErrorProneTests.scala
index b0904f117d56..b30046ff9e8d 100644
--- a/libs/javalib/test/src/mill/javalib/errorprone/ErrorProneTests.scala
+++ b/libs/javalib/test/src/mill/javalib/errorprone/ErrorProneTests.scala
@@ -2,7 +2,6 @@ package mill.javalib.errorprone
 
 import mill.{T, Task}
 import mill.api.Discover
-import mill.api.opt.*
 import mill.javalib.JavaModule
 import mill.testkit.{TestRootModule, UnitTester}
 import os.Path
@@ -18,8 +17,8 @@ object ErrorProneTests extends TestSuite {
     lazy val millDiscover = Discover[this.type]
   }
   object errorProneCustom extends TestRootModule with JavaModule with ErrorProneModule {
-    override def errorProneOptions: T[Opts] = Task {
-      Opts("-XepAllErrorsAsWarnings")
+    override def errorProneOptions: T[Seq[String]] = Task {
+      Seq("-XepAllErrorsAsWarnings")
     }
     lazy val millDiscover = Discover[this.type]
   }

From 3e40f64b3914c40bffed0ab60fc7c6667517f475 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 5 Nov 2025 22:12:30 +0100
Subject: [PATCH 07/67] Fix ErrorProneModule

---
 .../src/mill/javalib/errorprone/ErrorProneModule.scala      | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
index e5f0cebb062e..83f8871a60cb 100644
--- a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
+++ b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
@@ -37,12 +37,12 @@ trait ErrorProneModule extends JavaModule {
    * Options used to enable and configure the `error-prone` plugin in the Java compiler.
    */
   def errorProneJavacEnableOptions: T[Opts] = Task {
-    val processorPath: Seq[os.Path] = errorProneClasspath().map(_.path)
+    val processorPath: Opt = Opt.mkPath(errorProneClasspath().map(_.path), sep = java.io.File.pathSeparator)
 
     val enableOpts: Opts = Opts(
       "-XDcompilePolicy=simple",
-      "-processorpath",
-      processorPath,
+      OptGroup("-processorpath", processorPath),
+      // ErrorProne options are given as a single argument containing spaces
       (Seq("-Xplugin:ErrorProne") ++ errorProneOptions()).mkString(" ")
     )
     val java17Options: Seq[String] = Option.when(scala.util.Properties.isJavaAtLeast(16))(Seq(

From 033832aea6675c10cb825701ebd253dc9c8b3e50 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 5 Nov 2025 22:13:48 +0100
Subject: [PATCH 08/67] Experiemnt with OptGroup.apply

---
 core/api/src/mill/api/opt/Opt.scala            | 14 ++++++++++----
 core/api/src/mill/api/opt/OptGroup.scala       | 15 ++++++++++++---
 core/api/test/src/mill/api/opt/OptsTests.scala |  2 +-
 3 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/core/api/src/mill/api/opt/Opt.scala b/core/api/src/mill/api/opt/Opt.scala
index 7e6396823d93..ce8e6c9bba84 100644
--- a/core/api/src/mill/api/opt/Opt.scala
+++ b/core/api/src/mill/api/opt/Opt.scala
@@ -83,10 +83,16 @@ object Opt {
 //  implicit def IterableToOpt[T](s: Iterable[T])(using f: T => Opt): Opt =
 //    Opt(s.toSeq.flatMap(f(_).value))
 
-  implicit def StringToOpt(s: String): Opt = Opt(s)
-
-  implicit def OsPathToOpt(p: os.Path): Opt = Opt(p)
+  implicit def AllToOpt(o: String | os.Path | Opt): Opt = o match {
+    case s: String => Opt(s)
+    case p: os.Path => Opt(p)
+    case o: Opt => o
+  }
 
-  implicit def OptToOpt(o: Opt): Opt = o
+//  implicit def StringToOpt(s: String): Opt = Opt(s)
+//
+//  implicit def OsPathToOpt(p: os.Path): Opt = Opt(p)
+//
+//  implicit def OptToOpt(o: Opt): Opt = o
 
 }
diff --git a/core/api/src/mill/api/opt/OptGroup.scala b/core/api/src/mill/api/opt/OptGroup.scala
index 2d2e3c484527..68a5c7502102 100644
--- a/core/api/src/mill/api/opt/OptGroup.scala
+++ b/core/api/src/mill/api/opt/OptGroup.scala
@@ -28,9 +28,18 @@ case class OptGroup private (value: Seq[Opt]) extends OptGroupApi
 
 object OptGroup {
   @targetName("applyVarAar")
-  def apply(opts: Opt*): OptGroup = new OptGroup(opts)
-  @targetName("applyIterable")
-  def apply[T](opts: T*)(using f: T => Opt): OptGroup = new OptGroup(opts.map(f(_)))
+  def apply(opts: (String | os.Path | Opt | Seq[(String | os.Path | Opt)])*): OptGroup = new OptGroup(opts.flatMap {
+    case s: String => Seq(Opt(s))
+    case p: os.Path => Seq(Opt(p))
+    case o: Opt => Seq(o)
+    case o: Seq[(String | os.Path | Opt)] => o.map {
+      case s: String => Opt(s)
+      case p: os.Path => Opt(p)
+      case o: Opt => o
+    }
+  })
+//  @targetName("applyIterable")
+//  def apply[T](opts: T*)(using f: T => Opt): OptGroup = new OptGroup(opts.map(f(_)))
 
   def when(cond: Boolean)(value: Opt*): OptGroup = if (cond) OptGroup(value*) else OptGroup()
 
diff --git a/core/api/test/src/mill/api/opt/OptsTests.scala b/core/api/test/src/mill/api/opt/OptsTests.scala
index b015bd867f40..afb663ab932e 100644
--- a/core/api/test/src/mill/api/opt/OptsTests.scala
+++ b/core/api/test/src/mill/api/opt/OptsTests.scala
@@ -35,7 +35,7 @@ class OptsTests extends TestSuite {
     // some files as ArgGroup
     OptGroup(sources2*),
     // Mixed ArgGroup
-    OptGroup(opt"--extra", opt"-Xplugin=${plugin1}") ++ OptGroup(sources1*)
+    OptGroup("--extra", opt"-Xplugin=${plugin1}") ++ OptGroup(sources1*)
   )
 
   val expectedOpts1 = Opts(

From ef8a23beefd662c687b95d77065198696ba6b580 Mon Sep 17 00:00:00 2001
From: "autofix-ci[bot]" <114827586+autofix-ci[bot]@users.noreply.github.com>
Date: Wed, 5 Nov 2025 21:57:50 +0000
Subject: [PATCH 09/67] [autofix.ci] apply automated fixes

---
 core/api/src/mill/api/opt/OptGroup.scala      | 21 ++++++++++---------
 .../javalib/errorprone/ErrorProneModule.scala |  3 ++-
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/core/api/src/mill/api/opt/OptGroup.scala b/core/api/src/mill/api/opt/OptGroup.scala
index 68a5c7502102..e35a747ce526 100644
--- a/core/api/src/mill/api/opt/OptGroup.scala
+++ b/core/api/src/mill/api/opt/OptGroup.scala
@@ -28,16 +28,17 @@ case class OptGroup private (value: Seq[Opt]) extends OptGroupApi
 
 object OptGroup {
   @targetName("applyVarAar")
-  def apply(opts: (String | os.Path | Opt | Seq[(String | os.Path | Opt)])*): OptGroup = new OptGroup(opts.flatMap {
-    case s: String => Seq(Opt(s))
-    case p: os.Path => Seq(Opt(p))
-    case o: Opt => Seq(o)
-    case o: Seq[(String | os.Path | Opt)] => o.map {
-      case s: String => Opt(s)
-      case p: os.Path => Opt(p)
-      case o: Opt => o
-    }
-  })
+  def apply(opts: (String | os.Path | Opt | Seq[(String | os.Path | Opt)])*): OptGroup =
+    new OptGroup(opts.flatMap {
+      case s: String => Seq(Opt(s))
+      case p: os.Path => Seq(Opt(p))
+      case o: Opt => Seq(o)
+      case o: Seq[(String | os.Path | Opt)] => o.map {
+          case s: String => Opt(s)
+          case p: os.Path => Opt(p)
+          case o: Opt => o
+        }
+    })
 //  @targetName("applyIterable")
 //  def apply[T](opts: T*)(using f: T => Opt): OptGroup = new OptGroup(opts.map(f(_)))
 
diff --git a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
index 83f8871a60cb..e8f4f6869309 100644
--- a/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
+++ b/libs/javalib/src/mill/javalib/errorprone/ErrorProneModule.scala
@@ -37,7 +37,8 @@ trait ErrorProneModule extends JavaModule {
    * Options used to enable and configure the `error-prone` plugin in the Java compiler.
    */
   def errorProneJavacEnableOptions: T[Opts] = Task {
-    val processorPath: Opt = Opt.mkPath(errorProneClasspath().map(_.path), sep = java.io.File.pathSeparator)
+    val processorPath: Opt =
+      Opt.mkPath(errorProneClasspath().map(_.path), sep = java.io.File.pathSeparator)
 
     val enableOpts: Opts = Opts(
       "-XDcompilePolicy=simple",

From b8c7b7667c12ef7f7de7e7aa3256b8612544e6f9 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 08:30:57 +0100
Subject: [PATCH 10/67] make test path non-user specific

---
 core/api/test/src/mill/api/opt/OptsTests.scala | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/core/api/test/src/mill/api/opt/OptsTests.scala b/core/api/test/src/mill/api/opt/OptsTests.scala
index afb663ab932e..d35a6e1bc077 100644
--- a/core/api/test/src/mill/api/opt/OptsTests.scala
+++ b/core/api/test/src/mill/api/opt/OptsTests.scala
@@ -5,7 +5,7 @@ import mill.api.opt.*
 
 class OptsTests extends TestSuite {
 
-  val homeDir = os.home
+  val homeDir = os.root / "tmp/opts-test"
   val workDir = homeDir / "work"
   val outDir = workDir / "out"
 
@@ -91,7 +91,7 @@ class OptsTests extends TestSuite {
         val json = upickle.write(opts1)
         assertGoldenLiteral(
           json,
-          "{\"value\":[{\"value\":[[[\"-deprecation\",null]],[[\"-verbose\",null]],[[\"--release\",null]],[[\"17\",null]],[[\"-Xplugin=\",null],[null,\"/home/lefou/.cache/plugin1\"]],[[\"-Xplugin:\",null],[null,\"/home/lefou/.cache/plugin1\"]],[[null,\"/home/lefou/work/src1\"]],[[null,\"/home/lefou/work/src2\"]],[[null,\"/home/lefou/work/src3\"]],[[null,\"/home/lefou/work/src4\"]],[[\"--extra\",null]],[[\"-Xplugin=\",null],[null,\"/home/lefou/.cache/plugin1\"]],[[null,\"/home/lefou/work/src1\"]],[[null,\"/home/lefou/work/src2\"]]]}]}"
+          "{\"value\":[{\"value\":[[[\"-deprecation\",null]],[[\"-verbose\",null]],[[\"--release\",null]],[[\"17\",null]],[[\"-Xplugin=\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]],[[\"-Xplugin:\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]],[[null,\"/tmp/opts-test/work/src1\"]],[[null,\"/tmp/opts-test/work/src2\"]],[[null,\"/tmp/opts-test/work/src3\"]],[[null,\"/tmp/opts-test/work/src4\"]],[[\"--extra\",null]],[[\"-Xplugin=\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]],[[null,\"/tmp/opts-test/work/src1\"]],[[null,\"/tmp/opts-test/work/src2\"]]]}]}"
         )
         assert(json.split("\\Q$HOME\\E").size == 1)
         val back = upickle.read[Opts](json)

From 9810fb8b0536690bd3ba39b45d76859035aee9ec Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 09:06:46 +0100
Subject: [PATCH 11/67] More Opts construction tweaks

---
 core/api/src/mill/api/opt/OptGroup.scala      | 16 ++++++------
 core/api/src/mill/api/opt/Opts.scala          | 26 ++++++++++++++-----
 .../api/test/src/mill/api/opt/OptsTests.scala | 19 +++++++++++---
 3 files changed, 43 insertions(+), 18 deletions(-)

diff --git a/core/api/src/mill/api/opt/OptGroup.scala b/core/api/src/mill/api/opt/OptGroup.scala
index e35a747ce526..f65015469fba 100644
--- a/core/api/src/mill/api/opt/OptGroup.scala
+++ b/core/api/src/mill/api/opt/OptGroup.scala
@@ -48,14 +48,14 @@ object OptGroup {
 //    (tuple: (OptTypes, OptTypes)) =>
 //      OptGroup(Opt(tuple._1), Opt(tuple._2))
 
-  implicit def StringToOptGroup(s: String): OptGroup = OptGroup(Seq(Opt(s)))
-
-  implicit def OsPathToOptGroup(p: os.Path): OptGroup = OptGroup(Seq(Opt(p)))
-
-  implicit def OptToOptGroup(o: Opt): OptGroup = OptGroup(Seq(o))
-
-  implicit def IterableToOptGroup[T](s: Iterable[T])(using f: T => OptGroup): OptGroup =
-    OptGroup(s.toSeq.flatMap(f(_).value))
+//  implicit def StringToOptGroup(s: String): OptGroup = OptGroup(Seq(Opt(s)))
+//
+//  implicit def OsPathToOptGroup(p: os.Path): OptGroup = OptGroup(Seq(Opt(p)))
+//
+//  implicit def OptToOptGroup(o: Opt): OptGroup = OptGroup(Seq(o))
+//
+//  implicit def IterableToOptGroup[T](s: Iterable[T])(using f: T => OptGroup): OptGroup =
+//    OptGroup(s.toSeq.flatMap(f(_).value))
 
 //  implicit def ArrayToOptGroup[T](s: Array[T])(using f: T => OptGroup): OptGroup =
 //    OptGroup(s.flatMap(f(_).value))
diff --git a/core/api/src/mill/api/opt/Opts.scala b/core/api/src/mill/api/opt/Opts.scala
index 6d5958c09821..1e1407e405fa 100644
--- a/core/api/src/mill/api/opt/Opts.scala
+++ b/core/api/src/mill/api/opt/Opts.scala
@@ -2,9 +2,10 @@ package mill.api.opt
 
 import mill.api.daemon.internal.OptsApi
 
+import scala.annotation.targetName
 import scala.language.implicitConversions
 
-case class Opts private (override val value: OptGroup*) extends OptsApi
+case class Opts private (override val value: Seq[OptGroup]) extends OptsApi
     derives upickle.ReadWriter {
 
   def toStringSeq: Seq[String] = value.flatMap(_.toStringSeq)
@@ -16,17 +17,28 @@ case class Opts private (override val value: OptGroup*) extends OptsApi
   def containsPaths: Boolean = value.exists(_.containsPaths)
 
   def isEmpty: Boolean = value.isEmpty
-  def filterGroup(pred: OptGroup => Boolean): Opts = Opts(value.filter(pred)*)
-  def mapGroup(f: OptGroup => OptGroup): Opts = Opts(value.map(f)*)
-  def flatMap(f: OptGroup => Seq[OptGroup]): Opts = Opts(value.flatMap(f)*)
+  def filterGroup(pred: OptGroup => Boolean): Opts = Opts.apply(value.filter(pred)*)
+  def mapGroup(f: OptGroup => OptGroup): Opts = Opts.apply(value.map(f)*)
+  def flatMap(f: OptGroup => Seq[OptGroup]): Opts = Opts.apply(value.flatMap(f)*)
 }
 
 object Opts {
-  def apply(value: OptGroup*): Opts = new Opts(value.filter(!_.isEmpty))
-//  @targetName("applyUnion")
-//  def apply(value: (Opt | OptGroup | Seq[Opt])*): Opts = Opts(value.flatMap {
+  @targetName("applyVarArgUnion")
+//  def apply(value: OptGroup*): Opts = new Opts(value.filter(!_.isEmpty))
+//  def apply(value: (String | os.Path | Opt | OptGroup | Seq[Opt])*): Opts = Opts(value.flatMap {
 //    case a: Opt => Seq(OptGroup(a))
 //    case a: OptGroup => Seq(a)
 //    case s: Iterable[Opt] => s.map(OptGroup(_))
 //  })
+
+  def apply(opts: (String | os.Path | Opt | Seq[(String | os.Path | Opt)] | OptGroup | Opts)*): Opts =
+    new Opts(opts.flatMap {
+      // Seq of OptGroup
+      case s: String => Seq(OptGroup(s))
+      case p: os.Path => Seq(OptGroup(p))
+      case o: Opt => Seq(OptGroup(o))
+      case o: Seq[(String | os.Path | Opt)] => o.map(OptGroup(_))
+      case o: OptGroup => Seq(o)
+      case o: Opts => o.value
+    })
 }
diff --git a/core/api/test/src/mill/api/opt/OptsTests.scala b/core/api/test/src/mill/api/opt/OptsTests.scala
index d35a6e1bc077..57b438654215 100644
--- a/core/api/test/src/mill/api/opt/OptsTests.scala
+++ b/core/api/test/src/mill/api/opt/OptsTests.scala
@@ -49,8 +49,7 @@ class OptsTests extends TestSuite {
     Opt("-Xplugin:", plugin1),
     Opt(srcDir1),
     Opt(srcDir2),
-    Opt(srcDir3),
-    Opt(srcDir4),
+    OptGroup(Opt(srcDir3), Opt(srcDir4)),
     OptGroup(
       Opt("--extra"),
       Opt("-Xplugin=", plugin1),
@@ -79,7 +78,18 @@ class OptsTests extends TestSuite {
     sources1.map(_.toString())
 
   override def tests: Tests = Tests {
+    test("OptGroup.isEmpty") {
+      val emptyGroup = OptGroup()
+      assert(emptyGroup.isEmpty)
+      assert(emptyGroup.toStringSeq.isEmpty)
+    }
+    test("Opts.isEmpty") {
+      val empty = Opts()
+      assert(empty.isEmpty)
+      assert(empty.toStringSeq.isEmpty)
+    }
     test("structure") {
+      assert(Opts("arg1").toStringSeq == Seq("arg1"))
       assert(opts1 == expectedOpts1)
     }
     test("toStringSeq") {
@@ -91,8 +101,11 @@ class OptsTests extends TestSuite {
         val json = upickle.write(opts1)
         assertGoldenLiteral(
           json,
-          "{\"value\":[{\"value\":[[[\"-deprecation\",null]],[[\"-verbose\",null]],[[\"--release\",null]],[[\"17\",null]],[[\"-Xplugin=\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]],[[\"-Xplugin:\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]],[[null,\"/tmp/opts-test/work/src1\"]],[[null,\"/tmp/opts-test/work/src2\"]],[[null,\"/tmp/opts-test/work/src3\"]],[[null,\"/tmp/opts-test/work/src4\"]],[[\"--extra\",null]],[[\"-Xplugin=\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]],[[null,\"/tmp/opts-test/work/src1\"]],[[null,\"/tmp/opts-test/work/src2\"]]]}]}"
+          "{\"value\":[{\"value\":[[[\"-deprecation\",null]]]},{\"value\":[[[\"-verbose\",null]]]},{\"value\":[[[\"--release\",null]],[[\"17\",null]]]},{\"value\":[[[\"-Xplugin=\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]]]},{\"value\":[[[\"-Xplugin:\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]]]},{\"value\":[[[null,\"/tmp/opts-test/work/src1\"]]]},{\"value\":[[[null,\"/tmp/opts-test/work/src2\"]]]},{\"value\":[[[null,\"/tmp/opts-test/work/src3\"]],[[null,\"/tmp/opts-test/work/src4\"]]]},{\"value\":[[[\"--extra\",null]],[[\"-Xplugin=\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]],[[null,\"/tmp/opts-test/work/src1\"]],[[null,\"/tmp/opts-test/work/src2\"]]]}]}"
         )
+//        pprint.log(upickle.write(opts1, indent = 4))
+        pprint.log(opts1)
+        pprint.log(opts1.toString)
         assert(json.split("\\Q$HOME\\E").size == 1)
         val back = upickle.read[Opts](json)
         assert(opts1 == back)

From 8af03cd8f5478955c3a8eb78a887546544c5708e Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 09:33:45 +0100
Subject: [PATCH 12/67] WIP

---
 core/api/src/mill/api/opt/Opt.scala                |  8 ++++----
 core/api/src/mill/api/opt/OptGroup.scala           | 14 +++++++++-----
 core/api/src/mill/api/opt/Opts.scala               | 11 +++++++----
 .../src/mill/scalanativelib/TestingTests.scala     |  5 +++--
 4 files changed, 23 insertions(+), 15 deletions(-)

diff --git a/core/api/src/mill/api/opt/Opt.scala b/core/api/src/mill/api/opt/Opt.scala
index ce8e6c9bba84..c3c284922ac2 100644
--- a/core/api/src/mill/api/opt/Opt.scala
+++ b/core/api/src/mill/api/opt/Opt.scala
@@ -11,14 +11,14 @@ case class Opt private (value: Seq[Opt.OptTypes]) extends OptApi {
 
   def map(conv: Opt.OptTypes => Opt.OptTypes): Opt = Opt(value.map(conv)*)
 
-  private def startStrings: Seq[String] =
-    value.takeWhile(_.isInstanceOf[String]).collect { case s: String => s }
+  private def startString: String =
+    value.takeWhile(_.isInstanceOf[String]).collect { case s: String => s }.mkString("")
 
-  def startsWith(prefix: String): Boolean = startStrings.mkString("").startsWith(prefix)
+  def startsWith(prefix: String): Boolean = startString.startsWith(prefix)
 
   def mapStartString(rep: String => String): Opt = {
     val rest = value.dropWhile(_.isInstanceOf[String])
-    Opt((rep(startStrings.mkString("")) +: rest)*)
+    Opt.apply((rep(startString) +: rest)*)
   }
 
   def containsPaths: Boolean = value.exists {
diff --git a/core/api/src/mill/api/opt/OptGroup.scala b/core/api/src/mill/api/opt/OptGroup.scala
index f65015469fba..5c1e578a8644 100644
--- a/core/api/src/mill/api/opt/OptGroup.scala
+++ b/core/api/src/mill/api/opt/OptGroup.scala
@@ -17,6 +17,7 @@ case class OptGroup private (value: Seq[Opt]) extends OptGroupApi
   def containsPaths: Boolean = value.exists(_.containsPaths)
 
   def head: Opt = value.head
+  def headOption: Option[Opt] = value.headOption
 
   def toStringSeq: Seq[String] = value.map(_.toString())
 
@@ -28,18 +29,21 @@ case class OptGroup private (value: Seq[Opt]) extends OptGroupApi
 
 object OptGroup {
   @targetName("applyVarAar")
-  def apply(opts: (String | os.Path | Opt | Seq[(String | os.Path | Opt)])*): OptGroup =
-    new OptGroup(opts.flatMap {
+  def apply(opts: (String | os.Path | Opt | Seq[(String | os.Path | Opt)])*): OptGroup = {
+    val opts0 = opts.flatMap {
       case s: String => Seq(Opt(s))
       case p: os.Path => Seq(Opt(p))
       case o: Opt => Seq(o)
-      case o: Seq[(String | os.Path | Opt)] => o.map {
+      case o: Seq[(String | os.Path | Opt)] =>
+        o.map {
           case s: String => Opt(s)
           case p: os.Path => Opt(p)
           case o: Opt => o
         }
-    })
-//  @targetName("applyIterable")
+    }
+    new OptGroup(opts0)
+  }
+  //  @targetName("applyIterable")
 //  def apply[T](opts: T*)(using f: T => Opt): OptGroup = new OptGroup(opts.map(f(_)))
 
   def when(cond: Boolean)(value: Opt*): OptGroup = if (cond) OptGroup(value*) else OptGroup()
diff --git a/core/api/src/mill/api/opt/Opts.scala b/core/api/src/mill/api/opt/Opts.scala
index 1e1407e405fa..a9c209434e72 100644
--- a/core/api/src/mill/api/opt/Opts.scala
+++ b/core/api/src/mill/api/opt/Opts.scala
@@ -7,6 +7,7 @@ import scala.language.implicitConversions
 
 case class Opts private (override val value: Seq[OptGroup]) extends OptsApi
     derives upickle.ReadWriter {
+  require(value.forall(!_.isEmpty))
 
   def toStringSeq: Seq[String] = value.flatMap(_.toStringSeq)
   override def toString(): String = value.mkString("Opts(", ", ", ")")
@@ -31,14 +32,16 @@ object Opts {
 //    case s: Iterable[Opt] => s.map(OptGroup(_))
 //  })
 
-  def apply(opts: (String | os.Path | Opt | Seq[(String | os.Path | Opt)] | OptGroup | Opts)*): Opts =
-    new Opts(opts.flatMap {
+  def apply(opts: (String | os.Path | Opt | IterableOnce[(String | os.Path | Opt)] | OptGroup | Opts)*): Opts = {
+    val groups = opts.flatMap {
       // Seq of OptGroup
       case s: String => Seq(OptGroup(s))
       case p: os.Path => Seq(OptGroup(p))
       case o: Opt => Seq(OptGroup(o))
-      case o: Seq[(String | os.Path | Opt)] => o.map(OptGroup(_))
+      case o: IterableOnce[(String | os.Path | Opt)] => Seq.from(o).map(OptGroup(_))
       case o: OptGroup => Seq(o)
       case o: Opts => o.value
-    })
+    }
+    new Opts(groups.filter(!_.isEmpty))
+  }
 }
diff --git a/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala b/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala
index 4a5062c93c0f..69901ec5c325 100644
--- a/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala
+++ b/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala
@@ -2,6 +2,7 @@ package mill.scalanativelib
 
 import mill.*
 import mill.api.ExecResult
+import mill.api.opt.Opts
 import mill.javalib.testrunner.TestResult
 import mill.scalanativelib.api.*
 import mill.testkit.UnitTester
@@ -65,10 +66,10 @@ object TestingTests extends TestSuite {
         assert(testResult.value == expected)
       }
     test("test-scalacOptions") {
-      checkInheritedTasks(_.scalacOptions, Seq("-deprecation"))
+      checkInheritedTasks(_.scalacOptions, Opts("-deprecation"))
     }
     test("test-scalaOrganization") {
-      checkInheritedTasks(_.scalaOrganization, "org.example")
+      checkInheritedTasks(_.scalaOrganization, Opts("org.example"))
     }
   }
 

From ce9c8a54e4bcaf9f52369d78db4fbe67aca08efa Mon Sep 17 00:00:00 2001
From: "autofix-ci[bot]" <114827586+autofix-ci[bot]@users.noreply.github.com>
Date: Thu, 6 Nov 2025 08:42:43 +0000
Subject: [PATCH 13/67] [autofix.ci] apply automated fixes

---
 core/api/src/mill/api/opt/Opts.scala | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/core/api/src/mill/api/opt/Opts.scala b/core/api/src/mill/api/opt/Opts.scala
index a9c209434e72..09d8f689ace8 100644
--- a/core/api/src/mill/api/opt/Opts.scala
+++ b/core/api/src/mill/api/opt/Opts.scala
@@ -32,7 +32,9 @@ object Opts {
 //    case s: Iterable[Opt] => s.map(OptGroup(_))
 //  })
 
-  def apply(opts: (String | os.Path | Opt | IterableOnce[(String | os.Path | Opt)] | OptGroup | Opts)*): Opts = {
+  def apply(
+      opts: (String | os.Path | Opt | IterableOnce[(String | os.Path | Opt)] | OptGroup | Opts)*
+  ): Opts = {
     val groups = opts.flatMap {
       // Seq of OptGroup
       case s: String => Seq(OptGroup(s))

From c0e15f6d90cf80cda5f5d0f381c8852e0bda71bb Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 10:24:59 +0100
Subject: [PATCH 14/67] Fix test

---
 .../test/src/mill/scalanativelib/TestingTests.scala             | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala b/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala
index 69901ec5c325..23c9ac8efae0 100644
--- a/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala
+++ b/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala
@@ -69,7 +69,7 @@ object TestingTests extends TestSuite {
       checkInheritedTasks(_.scalacOptions, Opts("-deprecation"))
     }
     test("test-scalaOrganization") {
-      checkInheritedTasks(_.scalaOrganization, Opts("org.example"))
+      checkInheritedTasks(_.scalaOrganization, Seq("org.example"))
     }
   }
 

From a72cdc0f47d9e63bc6c44ce897333f2c50544483 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 10:28:24 +0100
Subject: [PATCH 15/67] fix test

---
 libs/scalajslib/test/src/mill/scalajslib/UtestTests.scala | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/libs/scalajslib/test/src/mill/scalajslib/UtestTests.scala b/libs/scalajslib/test/src/mill/scalajslib/UtestTests.scala
index 0eb0a9c2f201..a467f7aff15e 100644
--- a/libs/scalajslib/test/src/mill/scalajslib/UtestTests.scala
+++ b/libs/scalajslib/test/src/mill/scalajslib/UtestTests.scala
@@ -1,10 +1,11 @@
 package mill.scalajslib
 
-import mill._
+import mill.*
 import mill.api.ExecResult
+import mill.api.opt.Opts
 import mill.javalib.testrunner.TestResult
 import mill.testkit.UnitTester
-import utest._
+import utest.*
 
 object UtestTests extends TestSuite {
   import CompileLinkTests._
@@ -76,7 +77,7 @@ object UtestTests extends TestSuite {
         assert(testResult.value == expected)
       }
     test("test-scalacOptions") {
-      checkInheritedTasks(_.scalacOptions, Seq("-deprecation"))
+      checkInheritedTasks(_.scalacOptions, Opts("-deprecation"))
     }
     test("test-scalaOrganization") {
       checkInheritedTasks(_.scalaOrganization, "org.example")

From 1159e4ac535f4021481a9d38ede5a53238874b48 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 14:29:37 +0100
Subject: [PATCH 16/67] Use a more compact JSON format

---
 core/api/src/mill/api/opt/Opt.scala           | 41 ++++++++++++++-----
 core/api/src/mill/api/opt/OptGroup.scala      | 12 +++++-
 core/api/src/mill/api/opt/Opts.scala          |  9 +++-
 .../api/test/src/mill/api/opt/OptsTests.scala | 21 +++++++---
 4 files changed, 63 insertions(+), 20 deletions(-)

diff --git a/core/api/src/mill/api/opt/Opt.scala b/core/api/src/mill/api/opt/Opt.scala
index c3c284922ac2..ae1c4f886b4f 100644
--- a/core/api/src/mill/api/opt/Opt.scala
+++ b/core/api/src/mill/api/opt/Opt.scala
@@ -64,20 +64,39 @@ object Opt {
 
   def mkPlatformPath(paths: Seq[os.Path]): Opt = mkPath(paths, sep = java.io.File.pathSeparator)
 
+//  given jsonReadWriter: upickle.ReadWriter[Opt] =
+//    upickle.readwriter[Seq[(Option[String], Option[os.Path])]].bimap(
+//      _.value.map {
+//        case path: os.Path => (None, Some(path))
+//        case str: String => (Some(str), None)
+//      },
+//      seq =>
+//        Opt(seq.map {
+//          case (Some(str), _) => str
+//          case (_, Some(path)) => path
+//        }*)
+//    )
+
   given jsonReadWriter: upickle.ReadWriter[Opt] =
-    upickle.readwriter[Seq[(Option[String], Option[os.Path])]].bimap(
-      _.value.map {
-        case path: os.Path => (None, Some(path))
-        case str: String => (Some(str), None)
-      },
-      seq =>
-        Opt(seq.map {
-          case (Some(str), _) => str
-          case (_, Some(path)) => path
-        }*)
+    upickle.readwriter[ujson.Value].bimap(
+      opt =>
+        if (!opt.containsPaths) ujson.Str(opt.toString())
+        else opt.value.map {
+          case str: String => ujson.Str(str)
+          case path: os.Path => ujson.Obj("path" -> upickle.transform(path).to[ujson.Value])
+        },
+      {
+        case ujson.Str(opt) => Opt(opt)
+        case arr: ujson.Arr =>
+          val elems = arr.value.map {
+            case ujson.Str(opt) => opt
+            case ujson.Obj(map) => upickle.read[os.Path](map("path"))
+          }
+          Opt(elems.toSeq*)
+      }
     )
 
-//  given stringToOpt: Conversion[String, Opt] = (value: String) => Opt(value)
+  //  given stringToOpt: Conversion[String, Opt] = (value: String) => Opt(value)
 //  given osPathToOpt: Conversion[os.Path, Opt] = (value: os.Path) => Opt(value)
 
 //  implicit def IterableToOpt[T](s: Iterable[T])(using f: T => Opt): Opt =
diff --git a/core/api/src/mill/api/opt/OptGroup.scala b/core/api/src/mill/api/opt/OptGroup.scala
index 5c1e578a8644..c624e5a7d7d2 100644
--- a/core/api/src/mill/api/opt/OptGroup.scala
+++ b/core/api/src/mill/api/opt/OptGroup.scala
@@ -8,12 +8,14 @@ import scala.language.implicitConversions
 /**
  * A set of options, which are used together
  */
-case class OptGroup private (value: Seq[Opt]) extends OptGroupApi
-    derives upickle.ReadWriter {
+case class OptGroup private (value: Seq[Opt]) extends OptGroupApi {
+
   override def toString(): String = value.mkString("(", ", ", ")")
 
   def isEmpty: Boolean = value.isEmpty
 
+  def size: Int = value.size
+
   def containsPaths: Boolean = value.exists(_.containsPaths)
 
   def head: Opt = value.head
@@ -64,4 +66,10 @@ object OptGroup {
 //  implicit def ArrayToOptGroup[T](s: Array[T])(using f: T => OptGroup): OptGroup =
 //    OptGroup(s.flatMap(f(_).value))
 
+  given jsonReadWriter: upickle.ReadWriter[OptGroup] =
+    upickle.readwriter[Seq[Opt]].bimap(
+      _.value,
+      OptGroup(_*)
+    )
+
 }
diff --git a/core/api/src/mill/api/opt/Opts.scala b/core/api/src/mill/api/opt/Opts.scala
index 09d8f689ace8..e3d79e1d9399 100644
--- a/core/api/src/mill/api/opt/Opts.scala
+++ b/core/api/src/mill/api/opt/Opts.scala
@@ -5,8 +5,7 @@ import mill.api.daemon.internal.OptsApi
 import scala.annotation.targetName
 import scala.language.implicitConversions
 
-case class Opts private (override val value: Seq[OptGroup]) extends OptsApi
-    derives upickle.ReadWriter {
+case class Opts private (override val value: Seq[OptGroup]) extends OptsApi {
   require(value.forall(!_.isEmpty))
 
   def toStringSeq: Seq[String] = value.flatMap(_.toStringSeq)
@@ -46,4 +45,10 @@ object Opts {
     }
     new Opts(groups.filter(!_.isEmpty))
   }
+
+  given jsonReadWriter: upickle.ReadWriter[Opts] =
+    upickle.readwriter[Seq[OptGroup]].bimap(
+      _.value,
+      Opts(_*)
+    )
 }
diff --git a/core/api/test/src/mill/api/opt/OptsTests.scala b/core/api/test/src/mill/api/opt/OptsTests.scala
index 57b438654215..bb811966b22a 100644
--- a/core/api/test/src/mill/api/opt/OptsTests.scala
+++ b/core/api/test/src/mill/api/opt/OptsTests.scala
@@ -92,6 +92,14 @@ class OptsTests extends TestSuite {
       assert(Opts("arg1").toStringSeq == Seq("arg1"))
       assert(opts1 == expectedOpts1)
     }
+    test("toString") {
+      val a = Opts(Opt("-arg", "1")).toString()
+      assertGoldenLiteral(a, "Opts((-arg1))")
+      val b = Opts(OptGroup("-arg", "1")).toString()
+      assertGoldenLiteral(b, "Opts((-arg, 1))")
+
+      assert(a != b)
+    }
     test("toStringSeq") {
       val str = opts1.toStringSeq
       assert(str == expectedSeq1)
@@ -99,14 +107,17 @@ class OptsTests extends TestSuite {
     test("jsonify") {
       test("without-mapping") {
         val json = upickle.write(opts1)
+
+//        pprint.log(upickle.write(opts1, indent = 4))
+//        pprint.log(opts1)
+//        pprint.log(opts1.toString)
+
         assertGoldenLiteral(
           json,
-          "{\"value\":[{\"value\":[[[\"-deprecation\",null]]]},{\"value\":[[[\"-verbose\",null]]]},{\"value\":[[[\"--release\",null]],[[\"17\",null]]]},{\"value\":[[[\"-Xplugin=\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]]]},{\"value\":[[[\"-Xplugin:\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]]]},{\"value\":[[[null,\"/tmp/opts-test/work/src1\"]]]},{\"value\":[[[null,\"/tmp/opts-test/work/src2\"]]]},{\"value\":[[[null,\"/tmp/opts-test/work/src3\"]],[[null,\"/tmp/opts-test/work/src4\"]]]},{\"value\":[[[\"--extra\",null]],[[\"-Xplugin=\",null],[null,\"/tmp/opts-test/.cache/plugin1\"]],[[null,\"/tmp/opts-test/work/src1\"]],[[null,\"/tmp/opts-test/work/src2\"]]]}]}"
+          "[[\"-deprecation\"],[\"-verbose\"],[\"--release\",\"17\"],[[\"-Xplugin=\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}]],[[\"-Xplugin:\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}]],[[{\"path\":\"/tmp/opts-test/work/src1\"}]],[[{\"path\":\"/tmp/opts-test/work/src2\"}]],[[{\"path\":\"/tmp/opts-test/work/src3\"}],[{\"path\":\"/tmp/opts-test/work/src4\"}]],[\"--extra\",[\"-Xplugin=\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}],[{\"path\":\"/tmp/opts-test/work/src1\"}],[{\"path\":\"/tmp/opts-test/work/src2\"}]]]"
         )
-//        pprint.log(upickle.write(opts1, indent = 4))
-        pprint.log(opts1)
-        pprint.log(opts1.toString)
-        assert(json.split("\\Q$HOME\\E").size == 1)
+
+        assert(json.split("\\Q$HOME\\E").size == 1 + 0)
         val back = upickle.read[Opts](json)
         assert(opts1 == back)
       }

From dd67e4d833f9cae4be7a162fb35d4dc06d6c4259 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 15:34:51 +0100
Subject: [PATCH 17/67] adapt examples

---
 example/javalib/config/1-common-config/build.mill.yaml |  4 ++--
 example/javalib/module/15-jni/build.mill               |  6 ++++--
 example/javalib/module/2-common-config/build.mill      |  6 ++++--
 example/javalib/module/3-custom-tasks/build.mill       |  4 +++-
 .../module/5-compilation-execution-flags/build.mill    |  8 +++++---
 example/javalib/module/7-resources/build.mill          |  4 +++-
 example/javalib/module/9-docjar/build.mill             |  4 +++-
 example/javalib/web/4-hello-micronaut/build.mill       | 10 +++++++---
 example/javalib/web/5-todo-micronaut/build.mill        | 10 +++++++---
 .../kotlinlib/config/4-native-image/build.mill.yaml    |  2 +-
 example/kotlinlib/linting/4-kover/build.mill           |  5 +++--
 .../module/10-dependency-injection/build.mill          |  7 ++++---
 example/kotlinlib/module/15-jni/build.mill             |  6 ++++--
 example/kotlinlib/module/2-common-config/build.mill    |  6 ++++--
 example/kotlinlib/module/3-custom-tasks/build.mill     |  4 +++-
 .../module/5-compilation-execution-flags/build.mill    |  6 ++++--
 example/kotlinlib/module/7-resources/build.mill        |  6 ++++--
 .../module/8-kotlin-compiler-plugins/build.mill        |  5 +++--
 example/kotlinlib/testing/1-test-suite/build.mill      |  6 ++++--
 example/kotlinlib/web/8-hello-micronaut/build.mill     |  7 ++++---
 example/scalalib/module/2-common-config/build.mill     |  8 +++++---
 example/scalalib/module/3-custom-tasks/build.mill      |  3 ++-
 .../module/5-compilation-execution-flags/build.mill    |  8 +++++---
 example/scalalib/module/7-resources/build.mill         |  4 +++-
 .../module/8-scala-compiler-plugins/build.mill         |  4 +++-
 example/scalalib/module/9-docjar/build.mill            |  4 +++-
 example/scalalib/spark/1-hello-spark/build.mill        |  4 +++-
 example/scalalib/spark/3-semi-realistic/build.mill     |  4 +++-
 example/springboot/kotlin/1-web-initializr/build.mill  |  3 ++-
 example/thirdparty/mockito/build.mill                  |  9 +++++----
 30 files changed, 110 insertions(+), 57 deletions(-)

diff --git a/example/javalib/config/1-common-config/build.mill.yaml b/example/javalib/config/1-common-config/build.mill.yaml
index 54e57b339ab3..ea09b3436f62 100644
--- a/example/javalib/config/1-common-config/build.mill.yaml
+++ b/example/javalib/config/1-common-config/build.mill.yaml
@@ -13,6 +13,6 @@ sources: ["./src", "./custom-src"]
 resources: ["./resources", "./custom-resources"]
 
 # Configure java compiler and runtime options and env vars
-javacOptions: ["-deprecation"]
-forkArgs: ["-Dmy.custom.property=my-prop-value"]
+javacOptions: [["-deprecation"]]
+forkArgs: [["-Dmy.custom.property=my-prop-value"]]
 forkEnv: { "MY_CUSTOM_ENV": "my-env-value" }
\ No newline at end of file
diff --git a/example/javalib/module/15-jni/build.mill b/example/javalib/module/15-jni/build.mill
index 084412e0e44e..1c23a4c66b22 100644
--- a/example/javalib/module/15-jni/build.mill
+++ b/example/javalib/module/15-jni/build.mill
@@ -2,7 +2,9 @@
 // code using JNI.
 
 package build
+
 import mill.*, javalib.*, util.Jvm
+import mill.api.opt.*
 
 object `package` extends JavaModule {
   // Additional source folder to put C sources
@@ -46,10 +48,10 @@ object `package` extends JavaModule {
     PathRef(Task.dest / output)
   }
 
-  def forkEnv = Map("HELLO_WORLD_BINARY" -> nativeCompiled().path.toString)
+  def forkEnv = Map("HELLO_WORLD_BINARY" -> Opt(nativeCompiled().path))
 
   object test extends JavaTests, TestModule.Junit4 {
-    def forkEnv = Map("HELLO_WORLD_BINARY" -> nativeCompiled().path.toString)
+    def forkEnv = Map("HELLO_WORLD_BINARY" -> Opt(nativeCompiled().path))
   }
 }
 
diff --git a/example/javalib/module/2-common-config/build.mill b/example/javalib/module/2-common-config/build.mill
index 4ed007eb8ffc..a3d205840d2c 100644
--- a/example/javalib/module/2-common-config/build.mill
+++ b/example/javalib/module/2-common-config/build.mill
@@ -1,6 +1,8 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, javalib.*
+import mill.api.opt.*
 
 object `package` extends JavaModule {
   // You can have arbitrary numbers of third-party dependencies
@@ -36,11 +38,11 @@ object `package` extends JavaModule {
 
   // Pass additional JVM flags when `.run` is called or in the executable
   // generated by `.assembly`
-  def forkArgs: T[Seq[String]] = Seq("-Dmy.custom.property=my-prop-value")
+  def forkArgs: T[Opts] = Opts("-Dmy.custom.property=my-prop-value")
 
   // Pass additional environmental variables when `.run` is called. Note that
   // this does not apply to running externally via `.assembly
-  def forkEnv: T[Map[String, String]] = Map("MY_CUSTOM_ENV" -> "my-env-value")
+  def forkEnv: T[Map[String, Opt]] = Map("MY_CUSTOM_ENV" -> opt"my-env-value")
 }
 // If you want to better understand how the various upstream tasks feed into
 // a task of interest, such as `run`, you can visualize their relationships via
diff --git a/example/javalib/module/3-custom-tasks/build.mill b/example/javalib/module/3-custom-tasks/build.mill
index b5de6fb781cb..29836b7d1afb 100644
--- a/example/javalib/module/3-custom-tasks/build.mill
+++ b/example/javalib/module/3-custom-tasks/build.mill
@@ -1,6 +1,8 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, javalib.*
+import mill.api.opt.*
 
 object `package` extends JavaModule {
   def mvnDeps = Seq(mvn"net.sourceforge.argparse4j:argparse4j:0.9.0")
@@ -35,7 +37,7 @@ object `package` extends JavaModule {
       .sum
   }
 
-  def forkArgs: T[Seq[String]] = Seq(s"-Dmy.line.count=${lineCount()}")
+  def forkArgs: T[Opts] = Opts(s"-Dmy.line.count=${lineCount()}")
 
   def printLineCount() = Task.Command { println(lineCount()) }
 }
diff --git a/example/javalib/module/5-compilation-execution-flags/build.mill b/example/javalib/module/5-compilation-execution-flags/build.mill
index 45d0da9f4fab..6b1d459907cd 100644
--- a/example/javalib/module/5-compilation-execution-flags/build.mill
+++ b/example/javalib/module/5-compilation-execution-flags/build.mill
@@ -1,11 +1,13 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, javalib.*
+import mill.api.opt.*
 
 object `package` extends JavaModule {
-  def forkArgs = Seq("-Xmx4g", "-Dmy.jvm.property=hello")
-  def forkEnv = Map("MY_ENV_VAR" -> "WORLD")
-  def javacOptions = Seq("-deprecation")
+  def forkArgs = Opts("-Xmx4g", "-Dmy.jvm.property=hello")
+  def forkEnv = Map("MY_ENV_VAR" -> opt"WORLD")
+  def javacOptions = Opts("-deprecation")
 }
 
 // You can pass flags to the Java compiler via `javacOptions`.
diff --git a/example/javalib/module/7-resources/build.mill b/example/javalib/module/7-resources/build.mill
index 31588e6a4fde..170208b34da7 100644
--- a/example/javalib/module/7-resources/build.mill
+++ b/example/javalib/module/7-resources/build.mill
@@ -1,13 +1,15 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, javalib.*
+import mill.api.opt.*
 
 object foo extends JavaModule {
   object test extends JavaTests, TestModule.Junit4 {
     def otherFiles = Task.Source("other-files")
 
     def forkEnv = super.forkEnv() ++ Map(
-      "OTHER_FILES_DIR" -> otherFiles().path.toString
+      "OTHER_FILES_DIR" -> Opt(otherFiles().path)
     )
   }
 }
diff --git a/example/javalib/module/9-docjar/build.mill b/example/javalib/module/9-docjar/build.mill
index 92c282337fb6..bc29555b2aa7 100644
--- a/example/javalib/module/9-docjar/build.mill
+++ b/example/javalib/module/9-docjar/build.mill
@@ -1,9 +1,11 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, javalib.*
+import mill.api.opt.*
 
 object foo extends JavaModule {
-  def javadocOptions = Seq("-quiet")
+  def javadocOptions = Opts("-quiet")
 }
 
 /** Usage
diff --git a/example/javalib/web/4-hello-micronaut/build.mill b/example/javalib/web/4-hello-micronaut/build.mill
index f8ca939fca1e..4992d227241c 100644
--- a/example/javalib/web/4-hello-micronaut/build.mill
+++ b/example/javalib/web/4-hello-micronaut/build.mill
@@ -1,5 +1,7 @@
 package build
+
 import mill.*, javalib.*
+import mill.api.opt.*
 
 object `package` extends MicronautModule {
   def micronautVersion = "4.5.3"
@@ -37,9 +39,11 @@ trait MicronautModule extends MavenModule {
     )
   }
 
-  def javacOptions = Seq(
-    "-processorpath",
-    processors().map(_.path).mkString(":"),
+  def javacOptions = Opts(
+    OptGroup(
+      "-processorpath",
+      Opt.mkPath(processors().map(_.path), sep = ":")
+    ),
     "-parameters",
     "-Amicronaut.processing.incremental=true",
     "-Amicronaut.processing.group=example.micronaut",
diff --git a/example/javalib/web/5-todo-micronaut/build.mill b/example/javalib/web/5-todo-micronaut/build.mill
index fb4eb08019e0..ecf307a75b8f 100644
--- a/example/javalib/web/5-todo-micronaut/build.mill
+++ b/example/javalib/web/5-todo-micronaut/build.mill
@@ -1,5 +1,7 @@
 package build
+
 import mill.*, javalib.*
+import mill.api.opt.*
 
 object `package` extends MicronautModule {
   def micronautVersion = "4.4.3"
@@ -50,9 +52,11 @@ trait MicronautModule extends MavenModule {
     )
   }
 
-  def javacOptions = Seq(
-    "-processorpath",
-    processors().map(_.path).mkString(":"),
+  def javacOptions = Opts(
+    OptGroup(
+      "-processorpath",
+      Opt.mkPath(processors().map(_.path), ":")
+    ),
     "-parameters",
     "-Amicronaut.processing.incremental=true",
     "-Amicronaut.processing.group=example.micronaut",
diff --git a/example/kotlinlib/config/4-native-image/build.mill.yaml b/example/kotlinlib/config/4-native-image/build.mill.yaml
index bf13ed3d14cc..d8c61d63f3de 100644
--- a/example/kotlinlib/config/4-native-image/build.mill.yaml
+++ b/example/kotlinlib/config/4-native-image/build.mill.yaml
@@ -1,4 +1,4 @@
 extends: [mill.kotlinlib.KotlinModule, mill.kotlinlib.NativeImageModule]
 kotlinVersion: 2.0.20
 jvmId: "graalvm-community:24"
-nativeImageOptions: ["--no-fallback"]
+nativeImageOptions: [["--no-fallback"]]
diff --git a/example/kotlinlib/linting/4-kover/build.mill b/example/kotlinlib/linting/4-kover/build.mill
index 37a393ff13cd..d6e7c4e6d31f 100644
--- a/example/kotlinlib/linting/4-kover/build.mill
+++ b/example/kotlinlib/linting/4-kover/build.mill
@@ -2,12 +2,13 @@ package build
 
 import mill.*, kotlinlib.*
 import kotlinlib.kover.KoverModule
+import mill.api.opt.*
 
 object `package` extends KotlinModule, KoverModule {
 
   trait KotestTests extends TestModule.Junit5 {
-    override def forkArgs: T[Seq[String]] = Task {
-      super.forkArgs() ++ Seq("-Dkotest.framework.classpath.scanning.autoscan.disable=true")
+    override def forkArgs: T[Opts] = Task {
+      super.forkArgs() ++ Opts("-Dkotest.framework.classpath.scanning.autoscan.disable=true")
 
     }
     override def mvnDeps = super.mvnDeps() ++ Seq(
diff --git a/example/kotlinlib/module/10-dependency-injection/build.mill b/example/kotlinlib/module/10-dependency-injection/build.mill
index ed47d4eafa03..dc636171f9d1 100644
--- a/example/kotlinlib/module/10-dependency-injection/build.mill
+++ b/example/kotlinlib/module/10-dependency-injection/build.mill
@@ -1,8 +1,9 @@
 //// SNIPPET:BUILD
-
 package build
+
 import mill.*, kotlinlib.*
 import kotlinlib.ksp.KspModule
+import mill.api.opt.*
 
 object dagger extends KspModule {
 
@@ -12,7 +13,7 @@ object dagger extends KspModule {
 
   def mainClass = Some("com.example.dagger.MainKt")
 
-  def kotlincOptions = super.kotlincOptions() ++ Seq("-no-reflect", "-verbose")
+  def kotlincOptions = super.kotlincOptions() ++ Opts("-no-reflect", "-verbose")
 
 //  override def kotlincUseBtApi = false
 
@@ -26,7 +27,7 @@ object dagger extends KspModule {
 
   object test extends KspTests, TestModule.Junit5 {
 
-    def kotlincOptions = super.kotlincOptions() ++ Seq("-no-reflect", "-verbose")
+    def kotlincOptions = super.kotlincOptions() ++ Opts("-no-reflect", "-verbose")
 
     def mvnDeps = super.mvnDeps() ++ Seq(
       mvn"com.google.dagger:dagger-compiler:2.57",
diff --git a/example/kotlinlib/module/15-jni/build.mill b/example/kotlinlib/module/15-jni/build.mill
index b09c8e0a83ed..c6b8489413a2 100644
--- a/example/kotlinlib/module/15-jni/build.mill
+++ b/example/kotlinlib/module/15-jni/build.mill
@@ -1,5 +1,7 @@
 package build
+
 import mill.*, kotlinlib.*, util.Jvm
+import mill.api.opt.*
 
 object `package` extends KotlinModule {
 
@@ -32,13 +34,13 @@ object `package` extends KotlinModule {
     PathRef(Task.dest / output)
   }
 
-  def forkEnv = Map("HELLO_WORLD_BINARY" -> nativeCompiled().path.toString)
+  def forkEnv = Map("HELLO_WORLD_BINARY" -> Opt(nativeCompiled().path))
 
   object test extends KotlinTests, TestModule.Junit5 {
     def mvnDeps = super.mvnDeps() ++ Seq(
       mvn"io.kotest:kotest-runner-junit5:5.9.1"
     )
-    def forkEnv = Map("HELLO_WORLD_BINARY" -> nativeCompiled().path.toString)
+    def forkEnv = Map("HELLO_WORLD_BINARY" -> Opt(nativeCompiled().path))
   }
 }
 
diff --git a/example/kotlinlib/module/2-common-config/build.mill b/example/kotlinlib/module/2-common-config/build.mill
index 6f4bfb14f6e3..5c8b89bfbf66 100644
--- a/example/kotlinlib/module/2-common-config/build.mill
+++ b/example/kotlinlib/module/2-common-config/build.mill
@@ -1,6 +1,8 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, kotlinlib.*
+import mill.api.opt.*
 
 object `package` extends KotlinModule {
   // You can have arbitrary numbers of third-party dependencies
@@ -39,11 +41,11 @@ object `package` extends KotlinModule {
 
   // Pass additional JVM flags when `.run` is called or in the executable
   // generated by `.assembly`
-  def forkArgs: T[Seq[String]] = Seq("-Dmy.custom.property=my-prop-value")
+  def forkArgs: T[Opts] = Opts("-Dmy.custom.property=my-prop-value")
 
   // Pass additional environmental variables when `.run` is called. Note that
   // this does not apply to running externally via `.assembly
-  def forkEnv: T[Map[String, String]] = Map("MY_CUSTOM_ENV" -> "my-env-value")
+  def forkEnv: T[Map[String, Opt]] = Map("MY_CUSTOM_ENV" -> opt"my-env-value")
 }
 // If you want to better understand how the various upstream tasks feed into
 // a task of interest, such as `run`, you can visualize their relationships via
diff --git a/example/kotlinlib/module/3-custom-tasks/build.mill b/example/kotlinlib/module/3-custom-tasks/build.mill
index 686fb66d034e..0765d33d6024 100644
--- a/example/kotlinlib/module/3-custom-tasks/build.mill
+++ b/example/kotlinlib/module/3-custom-tasks/build.mill
@@ -1,6 +1,8 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, kotlinlib.*
+import mill.api.opt.*
 
 object `package` extends KotlinModule {
 
@@ -38,7 +40,7 @@ object `package` extends KotlinModule {
       .sum
   }
 
-  def forkArgs: T[Seq[String]] = Seq(s"-Dmy.line.count=${lineCount()}")
+  def forkArgs: T[Opts] = Opts(s"-Dmy.line.count=${lineCount()}")
 
   def printLineCount() = Task.Command { println(lineCount()) }
 }
diff --git a/example/kotlinlib/module/5-compilation-execution-flags/build.mill b/example/kotlinlib/module/5-compilation-execution-flags/build.mill
index 3745f3acda58..59a6341be8c3 100644
--- a/example/kotlinlib/module/5-compilation-execution-flags/build.mill
+++ b/example/kotlinlib/module/5-compilation-execution-flags/build.mill
@@ -1,13 +1,15 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, kotlinlib.*
+import mill.api.opt.*
 
 object `package` extends KotlinModule {
 
   def kotlinVersion = "1.9.24"
 
-  def forkArgs = Seq("-Xmx4g", "-Dmy.jvm.property=hello")
-  def forkEnv = Map("MY_ENV_VAR" -> "WORLD")
+  def forkArgs = Opts("-Xmx4g", "-Dmy.jvm.property=hello")
+  def forkEnv = Map("MY_ENV_VAR" -> opt"WORLD")
 
   def kotlincOptions = super.kotlincOptions() ++ Seq("-Werror")
 }
diff --git a/example/kotlinlib/module/7-resources/build.mill b/example/kotlinlib/module/7-resources/build.mill
index fc2fc355f2b7..cb2f7d5f5b55 100644
--- a/example/kotlinlib/module/7-resources/build.mill
+++ b/example/kotlinlib/module/7-resources/build.mill
@@ -1,6 +1,8 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, kotlinlib.*
+import mill.api.opt.*
 
 object foo extends KotlinModule {
 
@@ -9,8 +11,8 @@ object foo extends KotlinModule {
   object test extends KotlinTests, TestModule.Junit5 {
     def otherFiles = Task.Source("other-files")
 
-    def forkEnv = super.forkEnv() ++ Map(
-      "OTHER_FILES_DIR" -> otherFiles().path.toString
+    def forkEnv = super.forkEnv() ++ Opt(
+      "OTHER_FILES_DIR" -> Opt(otherFiles().path)
     )
 
     def mvnDeps = super.mvnDeps() ++ Seq(
diff --git a/example/kotlinlib/module/8-kotlin-compiler-plugins/build.mill b/example/kotlinlib/module/8-kotlin-compiler-plugins/build.mill
index 93f4e665006a..78b80db0b1dc 100644
--- a/example/kotlinlib/module/8-kotlin-compiler-plugins/build.mill
+++ b/example/kotlinlib/module/8-kotlin-compiler-plugins/build.mill
@@ -6,6 +6,7 @@ package build
 
 import mill.*, kotlinlib.*
 import java.io.File
+import mill.api.opt.*
 
 object foo extends KotlinModule {
 
@@ -23,8 +24,8 @@ object foo extends KotlinModule {
     )
   }
 
-  def kotlincOptions = super.kotlincOptions() ++ Seq(
-    s"-Xplugin=${processors().head.path}"
+  def kotlincOptions = super.kotlincOptions() ++ Opts(
+    opt"-Xplugin=${processors().head.path}"
   )
 
   object test extends KotlinTests, TestModule.Junit5 {
diff --git a/example/kotlinlib/testing/1-test-suite/build.mill b/example/kotlinlib/testing/1-test-suite/build.mill
index dc4d323ec68e..bbf664b3acc4 100644
--- a/example/kotlinlib/testing/1-test-suite/build.mill
+++ b/example/kotlinlib/testing/1-test-suite/build.mill
@@ -1,6 +1,8 @@
 //// SNIPPET:BUILD1
 package build
+
 import mill.*, kotlinlib.*
+import mill.api.opt.*
 
 object foo extends KotlinModule {
 
@@ -15,7 +17,7 @@ object foo extends KotlinModule {
     )
 
     // This is needed because of the "mockito-kotlin"
-    def kotlincOptions = super.kotlincOptions() ++ Seq("-jvm-target", "11")
+    def kotlincOptions = super.kotlincOptions() ++ Opts("-jvm-target", "11")
   }
 }
 
@@ -40,7 +42,7 @@ object bar extends KotlinModule {
     )
 
     // This is needed because of the "mockito-kotlin"
-    def kotlincOptions = super.kotlincOptions() ++ Seq("-jvm-target", "11")
+    def kotlincOptions = super.kotlincOptions() ++ Opts("-jvm-target", "11")
   }
 }
 
diff --git a/example/kotlinlib/web/8-hello-micronaut/build.mill b/example/kotlinlib/web/8-hello-micronaut/build.mill
index 89e859a29704..17156bd7446c 100644
--- a/example/kotlinlib/web/8-hello-micronaut/build.mill
+++ b/example/kotlinlib/web/8-hello-micronaut/build.mill
@@ -5,6 +5,7 @@ import coursier.{MavenRepository, Repository}
 import mill.*
 import kotlinlib.*
 import kotlinlib.ksp.KspModule
+import mill.api.opt.*
 
 object micronaut extends MicronautModule {
 
@@ -42,10 +43,10 @@ trait MicronautModule extends KspModule { outer =>
     mvn"io.micronaut:micronaut-inject-kotlin:4.9.9"
   )
 
-  override def kspProcessorOptions: T[Map[String, String]] = Task {
+  override def kspProcessorOptions: T[Map[String, Opt]] = Task {
     super.kspProcessorOptions() ++ Map(
-      "micronaut.processing.module" -> moduleSegments.render,
-      "micronaut.processing.incremental" -> "true"
+      "micronaut.processing.module" -> Opt(moduleSegments.render),
+      "micronaut.processing.incremental" -> opt"true"
     )
   }
 
diff --git a/example/scalalib/module/2-common-config/build.mill b/example/scalalib/module/2-common-config/build.mill
index 6103b6ff4cea..f8c96618f494 100644
--- a/example/scalalib/module/2-common-config/build.mill
+++ b/example/scalalib/module/2-common-config/build.mill
@@ -7,7 +7,9 @@
 
 //// SNIPPET:BUILD
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
 
 object `package` extends ScalaModule {
   def scalaVersion = "2.13.16"
@@ -46,14 +48,14 @@ object `package` extends ScalaModule {
 
   // Pass additional JVM flags when `.run` is called or in the executable
   // generated by `.assembly`
-  def forkArgs: T[Seq[String]] = Seq("-Dmy.custom.property=my-prop-value")
+  def forkArgs: T[Opts] = Opts("-Dmy.custom.property=my-prop-value")
 
   // Pass additional environmental variables when `.run` is called. Note that
   // this does not apply to running externally via `.assembly
-  def forkEnv: T[Map[String, String]] = Map("MY_CUSTOM_ENV" -> "my-env-value")
+  def forkEnv: T[Map[String, Opt]] = Map("MY_CUSTOM_ENV" -> opt"my-env-value")
 
   // Additional Scala compiler options, e.g. to turn warnings into errors
-  def scalacOptions: T[Seq[String]] = Seq("-deprecation", "-Xfatal-warnings")
+  def scalacOptions: T[Opts] = Opts("-deprecation", "-Xfatal-warnings")
 }
 // If you want to better understand how the various upstream tasks feed into
 // a task of interest, such as `run`, you can visualize their relationships via
diff --git a/example/scalalib/module/3-custom-tasks/build.mill b/example/scalalib/module/3-custom-tasks/build.mill
index 79c177635e38..d3e4d5310133 100644
--- a/example/scalalib/module/3-custom-tasks/build.mill
+++ b/example/scalalib/module/3-custom-tasks/build.mill
@@ -11,6 +11,7 @@
 //// SNIPPET:BUILD
 package build
 import mill.*, scalalib.*
+import mill.api.opt.*
 
 object `package` extends ScalaModule {
   def scalaVersion = "3.7.1"
@@ -46,7 +47,7 @@ object `package` extends ScalaModule {
       .sum
   }
 
-  def forkArgs: T[Seq[String]] = Seq(s"-Dmy.line.count=${lineCount()}")
+  def forkArgs: T[Opts] = Opts(s"-Dmy.line.count=${lineCount()}")
 
   def printLineCount() = Task.Command { println(lineCount()) }
 }
diff --git a/example/scalalib/module/5-compilation-execution-flags/build.mill b/example/scalalib/module/5-compilation-execution-flags/build.mill
index ca81ce863857..ec19296c6713 100644
--- a/example/scalalib/module/5-compilation-execution-flags/build.mill
+++ b/example/scalalib/module/5-compilation-execution-flags/build.mill
@@ -1,12 +1,14 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
 
 object `package` extends ScalaModule {
   def scalaVersion = "3.7.1"
-  def scalacOptions = Seq("-Ydelambdafy:inline")
-  def forkArgs = Seq("-Xmx4g", "-Dmy.jvm.property=hello")
-  def forkEnv = Map("MY_ENV_VAR" -> "WORLD")
+  def scalacOptions = Opts("-Ydelambdafy:inline")
+  def forkArgs = Opts("-Xmx4g", "-Dmy.jvm.property=hello")
+  def forkEnv = Map("MY_ENV_VAR" -> opt"WORLD")
 }
 
 // You can pass flags to the Scala compiler via `scalacOptions`.
diff --git a/example/scalalib/module/7-resources/build.mill b/example/scalalib/module/7-resources/build.mill
index 9ca927227db7..6667df7e06ef 100644
--- a/example/scalalib/module/7-resources/build.mill
+++ b/example/scalalib/module/7-resources/build.mill
@@ -1,6 +1,8 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
 
 object foo extends ScalaModule {
   def scalaVersion = "3.7.1"
@@ -15,7 +17,7 @@ object foo extends ScalaModule {
     def otherFiles = Task.Source("other-files")
 
     def forkEnv = super.forkEnv() ++ Map(
-      "OTHER_FILES_DIR" -> otherFiles().path.toString
+      "OTHER_FILES_DIR" -> Opt(otherFiles().path)
     )
   }
 }
diff --git a/example/scalalib/module/8-scala-compiler-plugins/build.mill b/example/scalalib/module/8-scala-compiler-plugins/build.mill
index 4c2eb952effa..d90dc8ecafa1 100644
--- a/example/scalalib/module/8-scala-compiler-plugins/build.mill
+++ b/example/scalalib/module/8-scala-compiler-plugins/build.mill
@@ -1,11 +1,13 @@
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
 
 object `package` extends ScalaModule {
   def scalaVersion = "2.13.16"
 
   def compileMvnDeps = Seq(mvn"com.lihaoyi:::acyclic:0.3.18")
-  def scalacOptions = Seq("-P:acyclic:force")
+  def scalacOptions = Opts("-P:acyclic:force")
   def scalacPluginMvnDeps = Seq(mvn"com.lihaoyi:::acyclic:0.3.18")
 }
 
diff --git a/example/scalalib/module/9-docjar/build.mill b/example/scalalib/module/9-docjar/build.mill
index ac78d9447890..47a0911eec5d 100644
--- a/example/scalalib/module/9-docjar/build.mill
+++ b/example/scalalib/module/9-docjar/build.mill
@@ -3,12 +3,14 @@
 
 //// SNIPPET:BUILD
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
 
 object foo extends ScalaModule {
   def scalaVersion = "3.3.6"
 
-  def scalaDocOptions = Seq("-siteroot", "mydocs", "-no-link-warnings")
+  def scalaDocOptions = Opts("-siteroot", "mydocs", "-no-link-warnings")
 }
 
 /** Usage
diff --git a/example/scalalib/spark/1-hello-spark/build.mill b/example/scalalib/spark/1-hello-spark/build.mill
index b4a625ce8de5..854fed482c3e 100644
--- a/example/scalalib/spark/1-hello-spark/build.mill
+++ b/example/scalalib/spark/1-hello-spark/build.mill
@@ -1,5 +1,7 @@
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
 
 object foo extends ScalaModule {
   def scalaVersion = "2.12.20"
@@ -14,7 +16,7 @@ object foo extends ScalaModule {
     def mvnDeps = Seq(mvn"com.lihaoyi::utest:0.9.1")
     def testFramework = "utest.runner.Framework"
 
-    def forkArgs = Seq("--add-opens", "java.base/sun.nio.ch=ALL-UNNAMED")
+    def forkArgs = Opts("--add-opens", "java.base/sun.nio.ch=ALL-UNNAMED")
   }
 
 }
diff --git a/example/scalalib/spark/3-semi-realistic/build.mill b/example/scalalib/spark/3-semi-realistic/build.mill
index 73668b29a008..a2cf1d6c54af 100644
--- a/example/scalalib/spark/3-semi-realistic/build.mill
+++ b/example/scalalib/spark/3-semi-realistic/build.mill
@@ -1,5 +1,7 @@
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
 
 object `package` extends ScalaModule {
   def scalaVersion = "2.12.20"
@@ -8,7 +10,7 @@ object `package` extends ScalaModule {
     mvn"org.apache.spark::spark-sql:3.5.6"
   )
 
-  def forkArgs = Seq("--add-opens", "java.base/sun.nio.ch=ALL-UNNAMED")
+  def forkArgs = Opts("--add-opens", "java.base/sun.nio.ch=ALL-UNNAMED")
 
   def prependShellScript = ""
 
diff --git a/example/springboot/kotlin/1-web-initializr/build.mill b/example/springboot/kotlin/1-web-initializr/build.mill
index 62785513d86e..7063c49c6ba1 100644
--- a/example/springboot/kotlin/1-web-initializr/build.mill
+++ b/example/springboot/kotlin/1-web-initializr/build.mill
@@ -3,12 +3,13 @@ package build
 import mill.*
 import mill.javalib.*
 import mill.kotlinlib.KotlinMavenModule
+import mill.api.opt.*
 
 object `package` extends KotlinMavenModule {
 
   def kotlinVersion = "1.9.25"
 
-  override def kotlincOptions: T[Seq[String]] = super.kotlincOptions() ++ Seq("-jvm-target", "17")
+  override def kotlincOptions: T[Opts] = super.kotlincOptions() ++ Opts("-jvm-target", "17")
 
   def bomMvnDeps = Seq(
     mvn"org.springframework.boot:spring-boot-dependencies:3.5.7"
diff --git a/example/thirdparty/mockito/build.mill b/example/thirdparty/mockito/build.mill
index 5b226f6019d4..90484c256c73 100644
--- a/example/thirdparty/mockito/build.mill
+++ b/example/thirdparty/mockito/build.mill
@@ -1,6 +1,7 @@
 package build
 import mill.*, javalib.*
 import mill.api.ModuleRef
+import mill.api.opt.*
 
 object libraries {
 
@@ -44,7 +45,7 @@ trait MockitoModule extends MavenModule {
   def testMvnDeps: T[Seq[Dep]] = Seq()
   def testRuntimeMvnDeps: T[Seq[Dep]] = Seq()
   def testFramework = "com.novocode.junit.JUnitFramework"
-  def testForkArgs: T[Seq[String]] = Seq()
+  def testForkArgs: T[Opts] = Seq()
 
   def testFilteredSources: T[Seq[os.Path]] = Task { Seq() }
 
@@ -118,7 +119,7 @@ object `package` extends MockitoModule {
       def mvnDeps = Seq(libraries.errorprone)
       def testMvnDeps = Seq(libraries.errorproneTestApi)
 
-      def forkArgs = Seq(
+      def forkArgs = Opts(
         //      "-processorpath", libraries.autoservice,
         "-Xbootclasspath/a:${configurations.errorproneJavac.asPath}",
         "--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED",
@@ -132,7 +133,7 @@ object `package` extends MockitoModule {
         "--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED"
       )
 
-      def javacOptions = Seq(
+      def javacOptions = Opts(
         "--add-exports=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED",
         "--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED",
         "--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED"
@@ -214,7 +215,7 @@ object `package` extends MockitoModule {
     object `memory-test` extends MockitoModule {
       def testModuleDeps = Seq(build)
       def testMvnDeps = Seq(libraries.assertj)
-      def testForkArgs = Seq("-Xmx128m")
+      def testForkArgs = Opts("-Xmx128m")
     }
 
 //    object `osgi-test` extends MockitoModule {

From c9e11a4484121ff8f3f2d7bd58a624064d9686b4 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 15:35:04 +0100
Subject: [PATCH 18/67] Adapt BuildWriter

---
 .../src/mill/main/buildgen/BuildWriter.scala  | 36 +++++++++++++++++--
 1 file changed, 33 insertions(+), 3 deletions(-)

diff --git a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
index 3ee93e876059..7d71eeab4fa5 100644
--- a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
+++ b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
@@ -236,7 +236,7 @@ class BuildWriter(build: BuildSpec, renderCrossValueInTask: String = "crossValue
         renderModuleDep
       ),
       renderMemberAsSeq("runModuleDeps", runModuleDeps, cross(_.runModuleDeps), renderModuleDep),
-      renderTaskAsSeq("javacOptions", javacOptions, cross(_.javacOptions), literalize(_)),
+      renderTaskAsOpts("javacOptions", javacOptions, cross(_.javacOptions), literalize(_)),
       renderTask("artifactName", artifactName, cross(_.artifactName), literalize(_))
     )
   }
@@ -287,7 +287,7 @@ class BuildWriter(build: BuildSpec, renderCrossValueInTask: String = "crossValue
         cross(_.errorProneOptions),
         literalize(_)
       ),
-      renderTaskAsSeq(
+      renderTaskAsOpts(
         "errorProneJavacEnableOptions",
         errorProneJavacEnableOptions,
         cross(_.errorProneJavacEnableOptions),
@@ -302,7 +302,7 @@ class BuildWriter(build: BuildSpec, renderCrossValueInTask: String = "crossValue
     import config.*
     renderBlocks(
       renderTask("scalaVersion", scalaVersion, cross(_.scalaVersion), literalize(_)),
-      renderTaskAsSeq("scalacOptions", scalacOptions, cross(_.scalacOptions), literalize(_)),
+      renderTaskAsOpts("scalacOptions", scalacOptions, cross(_.scalacOptions), literalize(_)),
       renderTaskAsSeq(
         "scalacPluginMvnDeps",
         scalacPluginMvnDeps,
@@ -604,4 +604,34 @@ class BuildWriter(build: BuildSpec, renderCrossValueInTask: String = "crossValue
       }
     }
   }
+
+  private def renderTaskAsOpts[A](
+      name: String,
+      value: Seq[A],
+      crossValues: Seq[(String, Seq[A])],
+      renderValue: A => String
+  ) = {
+    val crossValues0 = crossValues.filter(_._2.nonEmpty)
+    if (value.isEmpty && crossValues0.isEmpty) ""
+    else {
+      def renderSeq(value: Seq[A]) = value.map(renderValue).mkString("Opts(", ", ", ")")
+
+      s"def $name = super.$name()" + {
+        if (value.isEmpty) "" else " ++ " + renderSeq(value)
+      } + {
+        if (crossValues0.isEmpty) ""
+        else {
+          val crossCases = crossValues0.groupMap(_._2)(_._1).toSeq.sortBy(_._2).map {
+            (value, crosses) =>
+              val matchValues = crosses.sorted.map(literalize(_)).mkString(" | ")
+              s"case $matchValues => ${renderSeq(value)}"
+          }
+          s""" ++ ($renderCrossValueInTask match {
+             |  ${renderLines(crossCases)}
+             |  case _ => Nil
+             |})""".stripMargin
+        }
+      }
+    }
+  }
 }

From bc0595362c02a2e5e7193621e5095fe480582c16 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 17:32:54 +0100
Subject: [PATCH 19/67] Update expected

---
 .../test/resources/expected/gradle-6-0/build.mill    |  2 +-
 .../mill-build/src/ProjectBaseModule.scala           |  2 +-
 .../resources/expected/gradle-8-0/list/package.mill  |  4 ++--
 .../mill-build/src/ProjectBaseModule.scala           |  2 +-
 .../expected/gradle-8-0/utilities/package.mill       |  2 +-
 .../mill-build/src/ProjectBaseModule.scala           |  2 +-
 .../expected/with-args/gradle-8-0/build.mill         | 12 ++++++------
 .../expected/maven-samples/multi-module/package.mill |  2 +-
 .../maven-samples/multi-module/server/package.mill   |  2 +-
 .../maven-samples/multi-module/webapp/package.mill   |  2 +-
 .../maven-samples/single-module/package.mill         |  2 +-
 .../test/resources/expected/quickstart/build.mill    |  7 ++++---
 .../expected/with-args/maven-samples/build.mill      |  8 ++++----
 .../expected/with-args/quickstart/build.mill         |  7 ++++---
 .../test/resources/expected/cross-version/build.mill |  2 +-
 .../sbt-multi-project-example/common/package.mill    |  2 +-
 .../src/SbtMultiProjectExampleBaseModule.scala       |  2 +-
 .../sbt-multi-project-example/multi1/package.mill    |  2 +-
 .../nested/nested/package.mill                       |  2 +-
 19 files changed, 34 insertions(+), 32 deletions(-)

diff --git a/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill b/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill
index b696d391b9d5..a968b085285e 100644
--- a/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill
@@ -9,7 +9,7 @@ object `package` extends MavenModule {
   def mvnDeps = super.mvnDeps() ++ Seq(Deps.guava)
 
   def javacOptions = super.javacOptions() ++
-    Seq("-source", "11", "-target", "11")
+    Opts("-source", "11", "-target", "11")
 
   def jvmId = "zulu:11"
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala b/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala
index 83a2088d2eea..fbd4d4ae2d11 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala
@@ -5,7 +5,7 @@ import mill.javalib.*
 trait ProjectBaseModule extends MavenModule {
 
   def javacOptions = super.javacOptions() ++
-    Seq("-source", "11", "-target", "11")
+    Opts("-source", "11", "-target", "11")
 
   def jvmId = "zulu:11"
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill b/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill
index 7f447fc382a6..438c8c414452 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill
@@ -9,7 +9,7 @@ object `package` extends ErrorProneModule with ProjectBaseModule {
   def errorProneVersion = "2.28.0"
 
   def errorProneJavacEnableOptions = super.errorProneJavacEnableOptions() ++
-    Seq("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
+    Opts("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
 
   object test extends MavenTests with ErrorProneModule with TestModule.Junit5 {
 
@@ -21,7 +21,7 @@ object `package` extends ErrorProneModule with ProjectBaseModule {
       Seq("-XepCompilingTestOnlyCode")
 
     def errorProneJavacEnableOptions = super.errorProneJavacEnableOptions() ++
-      Seq("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
+      Opts("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
 
     def testParallelism = false
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala b/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala
index 83a2088d2eea..fbd4d4ae2d11 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala
@@ -5,7 +5,7 @@ import mill.javalib.*
 trait ProjectBaseModule extends MavenModule {
 
   def javacOptions = super.javacOptions() ++
-    Seq("-source", "11", "-target", "11")
+    Opts("-source", "11", "-target", "11")
 
   def jvmId = "zulu:11"
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill b/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill
index f2e7c18dada1..a6390faa5df1 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill
@@ -11,6 +11,6 @@ object `package` extends ErrorProneModule with ProjectBaseModule {
   def errorProneVersion = "2.28.0"
 
   def errorProneJavacEnableOptions = super.errorProneJavacEnableOptions() ++
-    Seq("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
+    Opts("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
 
 }
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala b/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala
index 220ca6eb4a0c..a7048f70dc4c 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala
@@ -5,7 +5,7 @@ import mill.javalib.*
 trait ProjectBaseModule extends MavenModule {
 
   def javacOptions = super.javacOptions() ++
-    Seq("-source", "21", "-target", "21")
+    Opts("-source", "21", "-target", "21")
 
   def jvmId = "zulu:21"
 
diff --git a/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill b/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill
index dd6d1bad9ca3..f6add7c26b2e 100644
--- a/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill
@@ -15,7 +15,7 @@ object `package` extends Module {
     def moduleDeps = super.moduleDeps ++ Seq(build.utilities)
 
     def javacOptions = super.javacOptions() ++
-      Seq("-source", "17", "-target", "17")
+      Opts("-source", "17", "-target", "17")
 
     def jvmId = "zulu:17"
 
@@ -35,12 +35,12 @@ object `package` extends Module {
   object list extends MavenModule with ErrorProneModule {
 
     def javacOptions = super.javacOptions() ++
-      Seq("-source", "17", "-target", "17")
+      Opts("-source", "17", "-target", "17")
 
     def errorProneVersion = "2.28.0"
 
     def errorProneJavacEnableOptions = super.errorProneJavacEnableOptions() ++
-      Seq("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
+      Opts("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
 
     def jvmId = "zulu:17"
 
@@ -56,7 +56,7 @@ object `package` extends Module {
         Seq("-XepCompilingTestOnlyCode")
 
       def errorProneJavacEnableOptions = super.errorProneJavacEnableOptions() ++
-        Seq("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
+        Opts("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
 
       def testParallelism = false
 
@@ -71,12 +71,12 @@ object `package` extends Module {
     def moduleDeps = super.moduleDeps ++ Seq(build.list)
 
     def javacOptions = super.javacOptions() ++
-      Seq("-source", "17", "-target", "17")
+      Opts("-source", "17", "-target", "17")
 
     def errorProneVersion = "2.28.0"
 
     def errorProneJavacEnableOptions = super.errorProneJavacEnableOptions() ++
-      Seq("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
+      Opts("-XDshould-stop.ifError=FLOW", "-XDshouldStopPolicyIfError=FLOW")
 
     def jvmId = "zulu:17"
 
diff --git a/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill b/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill
index fa2cb300e2e8..4f8e0ba07dcc 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill
@@ -7,7 +7,7 @@ import millbuild.*
 object `package` extends MavenSamplesBaseModule {
 
   def javacOptions = super.javacOptions() ++
-    Seq("-source", "1.6", "-target", "1.6")
+    Opts("-source", "1.6", "-target", "1.6")
 
   def artifactName = "multi-module-parent"
 
diff --git a/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill b/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill
index 74071cee72f3..ad9f6bf86255 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill
@@ -7,7 +7,7 @@ import millbuild.*
 object `package` extends MavenSamplesBaseModule {
 
   def javacOptions = super.javacOptions() ++
-    Seq("-source", "1.6", "-target", "1.6")
+    Opts("-source", "1.6", "-target", "1.6")
 
   def artifactName = "server"
 
diff --git a/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill b/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill
index 04d63535f5bf..4468b1517c96 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill
@@ -12,7 +12,7 @@ object `package` extends MavenSamplesBaseModule {
   def moduleDeps = super.moduleDeps ++ Seq(build.`multi-module`.server)
 
   def javacOptions = super.javacOptions() ++
-    Seq("-source", "1.6", "-target", "1.6")
+    Opts("-source", "1.6", "-target", "1.6")
 
   def artifactName = "webapp"
 
diff --git a/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill b/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill
index 42deca2fc205..cd9615ecf95f 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill
@@ -9,7 +9,7 @@ object `package` extends MavenSamplesBaseModule {
   def mvnDeps = super.mvnDeps() ++ Seq(Deps.servletApi, Deps.jspApi)
 
   def javacOptions = super.javacOptions() ++
-    Seq("-source", "1.6", "-target", "1.6")
+    Opts("-source", "1.6", "-target", "1.6")
 
   def artifactName = "single-module-project"
 
diff --git a/libs/init/maven/test/resources/expected/quickstart/build.mill b/libs/init/maven/test/resources/expected/quickstart/build.mill
index 73ca114dcf8a..8376bf2db5e5 100644
--- a/libs/init/maven/test/resources/expected/quickstart/build.mill
+++ b/libs/init/maven/test/resources/expected/quickstart/build.mill
@@ -8,12 +8,13 @@ import millbuild.*
 
 object `package` extends MavenModule with ErrorProneModule with PublishModule {
 
-  def javacOptions = super.javacOptions() ++ Seq("-source", "8", "-target", "8")
+  def javacOptions = super.javacOptions() ++
+    Opts("-source", "8", "-target", "8")
 
   def artifactName = "maven-3-9-11"
 
   def errorProneJavacEnableOptions = super.errorProneJavacEnableOptions() ++
-    Seq("-XDshould-stop=ifError=FLOW")
+    Opts("-XDshould-stop=ifError=FLOW")
 
   def jvmId = "zulu:11"
 
@@ -34,7 +35,7 @@ object `package` extends MavenModule with ErrorProneModule with PublishModule {
       Seq(Deps.junitJupiterApi, Deps.junitJupiterParams)
 
     def errorProneJavacEnableOptions = super.errorProneJavacEnableOptions() ++
-      Seq("-XDshould-stop=ifError=FLOW")
+      Opts("-XDshould-stop=ifError=FLOW")
 
     def forkWorkingDir = moduleDir
 
diff --git a/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill b/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill
index 77032e4ae11d..4c93e79ec69c 100644
--- a/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill
+++ b/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill
@@ -24,7 +24,7 @@ object `package` extends MavenModule with PublishModule {
   object `multi-module` extends MavenModule with PublishModule {
 
     def javacOptions = super.javacOptions() ++
-      Seq("-source", "1.6", "-target", "1.6")
+      Opts("-source", "1.6", "-target", "1.6")
 
     def artifactName = "multi-module-parent"
 
@@ -56,7 +56,7 @@ object `package` extends MavenModule with PublishModule {
     object server extends MavenModule with PublishModule {
 
       def javacOptions = super.javacOptions() ++
-        Seq("-source", "1.6", "-target", "1.6")
+        Opts("-source", "1.6", "-target", "1.6")
 
       def artifactName = "server"
 
@@ -108,7 +108,7 @@ object `package` extends MavenModule with PublishModule {
       def moduleDeps = super.moduleDeps ++ Seq(build.`multi-module`.server)
 
       def javacOptions = super.javacOptions() ++
-        Seq("-source", "1.6", "-target", "1.6")
+        Opts("-source", "1.6", "-target", "1.6")
 
       def artifactName = "webapp"
 
@@ -145,7 +145,7 @@ object `package` extends MavenModule with PublishModule {
     )
 
     def javacOptions = super.javacOptions() ++
-      Seq("-source", "1.6", "-target", "1.6")
+      Opts("-source", "1.6", "-target", "1.6")
 
     def artifactName = "single-module-project"
 
diff --git a/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill b/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill
index 458044a36b60..8d4c0769a37b 100644
--- a/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill
+++ b/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill
@@ -7,12 +7,13 @@ import mill.javalib.publish.*
 
 object `package` extends MavenModule with ErrorProneModule with PublishModule {
 
-  def javacOptions = super.javacOptions() ++ Seq("-source", "8", "-target", "8")
+  def javacOptions = super.javacOptions() ++
+    Opts("-source", "8", "-target", "8")
 
   def artifactName = "maven-3-9-11"
 
   def errorProneJavacEnableOptions = super.errorProneJavacEnableOptions() ++
-    Seq("-XDshould-stop=ifError=FLOW")
+    Opts("-XDshould-stop=ifError=FLOW")
 
   def jvmId = "zulu:11"
 
@@ -40,7 +41,7 @@ object `package` extends MavenModule with ErrorProneModule with PublishModule {
     )
 
     def errorProneJavacEnableOptions = super.errorProneJavacEnableOptions() ++
-      Seq("-XDshould-stop=ifError=FLOW")
+      Opts("-XDshould-stop=ifError=FLOW")
 
     def forkWorkingDir = moduleDir
 
diff --git a/libs/init/sbt/test/resources/expected/cross-version/build.mill b/libs/init/sbt/test/resources/expected/cross-version/build.mill
index 241a215be4db..c075a3671cf5 100644
--- a/libs/init/sbt/test/resources/expected/cross-version/build.mill
+++ b/libs/init/sbt/test/resources/expected/cross-version/build.mill
@@ -12,7 +12,7 @@ object `package`
     extends Cross[CrossVersionModule]("2.12.18", "2.13.12", "3.3.1")
 trait CrossVersionModule extends PublishModule with CrossSbtModule {
 
-  def javacOptions = super.javacOptions() ++ Seq("--release", "8")
+  def javacOptions = super.javacOptions() ++ Opts("--release", "8")
 
   def artifactName = "my-cross-project"
 
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill
index d83ddd075969..5006c9f3afd9 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill
@@ -18,7 +18,7 @@ object `package` extends SbtMultiProjectExampleBaseModule {
 
   def artifactName = "common"
 
-  def scalacOptions = super.scalacOptions() ++ Seq("-encoding", "utf8")
+  def scalacOptions = super.scalacOptions() ++ Opts("-encoding", "utf8")
 
   def pomSettings = PomSettings(
     "This is the common module.",
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala
index af1b7f85c54d..9506ab29dc27 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala
@@ -8,7 +8,7 @@ trait SbtMultiProjectExampleBaseModule extends PublishModule with SbtModule {
 
   def scalaVersion = "2.12.3"
 
-  def scalacOptions = super.scalacOptions() ++ Seq(
+  def scalacOptions = super.scalacOptions() ++ Opts(
     "-unchecked",
     "-feature",
     "-language:existentials",
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill
index 87e7ced59315..5d791d852b3b 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill
@@ -22,7 +22,7 @@ object `package` extends SbtMultiProjectExampleBaseModule {
 
   def artifactName = "multi1"
 
-  def scalacOptions = super.scalacOptions() ++ Seq("-encoding", "utf8", "-V")
+  def scalacOptions = super.scalacOptions() ++ Opts("-encoding", "utf8", "-V")
 
   def pomSettings = PomSettings(
     "This is an sbt sample project for testing Mill's init command.",
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill
index 4d12b13f7f96..46377a0fb8e7 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill
@@ -11,7 +11,7 @@ object `package` extends SbtMultiProjectExampleBaseModule {
 
   def artifactName = "nested"
 
-  def scalacOptions = super.scalacOptions() ++ Seq("-encoding", "utf8")
+  def scalacOptions = super.scalacOptions() ++ Opts("-encoding", "utf8")
 
   def pomSettings = PomSettings(
     "This is an sbt sample project for testing Mill's init command.",

From e37054a3c3a0cb2e30499df85e49407900e645ce Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 17:49:01 +0100
Subject: [PATCH 20/67] Avoid mutating parameters

---
 .../src/mill/main/buildgen/BuildWriter.scala  | 22 +++++++++----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
index 7d71eeab4fa5..70740671a45e 100644
--- a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
+++ b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
@@ -101,7 +101,7 @@ class BuildWriter(build: BuildSpec, renderCrossValueInTask: String = "crossValue
   private def renderBaseModuleImports(baseModule: ModuleSpec) = {
     val wildcards = mutable.SortedSet.empty[String]
     import baseModule.*
-    configs.foreach(addImports(wildcards, _))
+    wildcards ++= configs.flatMap(imports)
     renderLines(wildcards.map(s => s"import $s.*"))
   }
 
@@ -111,22 +111,22 @@ class BuildWriter(build: BuildSpec, renderCrossValueInTask: String = "crossValue
       import spec.*
       if (supertypes.isEmpty || crossConfigs.nonEmpty) wildcards += "mill"
       if (configs.nonEmpty) wildcards ++= build.metaBuild.map(_.rootModuleName)
-      configs.foreach(addImports(wildcards, _))
+      wildcards ++= configs.flatMap(imports)
     }
     renderLines(wildcards.map(s => s"import $s.*"))
   }
 
-  private def addImports(wildcards: mutable.SortedSet[String], config: ModuleConfig) =
+  private def imports(config: ModuleConfig): Set[String] = {
     config match {
-      case _: PublishModule => wildcards += "mill.javalib" += "mill.javalib.publish"
-      case _: ErrorProneModule => wildcards += "mill.javalib" += "mill.javalib.errorprone"
-      case _: ScalaModule => wildcards += "mill.scalalib"
-      case _: ScalaJSModule =>
-        wildcards += "mill.scalalib" += "mill.scalajslib" += "mill.scalajslib.api"
-      case _: ScalaNativeModule => wildcards += "mill.scalalib" += "mill.scalanativelib"
-      case _: SbtPlatformModule => wildcards += "mill.scalalib"
-      case _ => wildcards += "mill.javalib"
+      case _: PublishModule => Set("mill.javalib", "mill.javalib.publish")
+      case _: ErrorProneModule => Set("mill.javalib", "mill.javalib.errorprone")
+      case _: ScalaModule => Set("mill.scalalib")
+      case _: ScalaJSModule => Set("mill.scalalib", "mill.scalajslib", "mill.scalajslib.api")
+      case _: ScalaNativeModule => Set("mill.scalalib", "mill.scalanativelib")
+      case _: SbtPlatformModule => Set("mill.scalalib")
+      case _ => Set("mill.javalib")
     }
+  }
 
   private def renderModule(module: ModuleSpec, isPackageRoot: Boolean = false): String = {
     import module.*

From 34a4320e76cb79184b2a72ce95b1ffe8df4a0c6f Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 17:49:21 +0100
Subject: [PATCH 21/67] Add imports

---
 libs/init/buildgen/api/src/mill/main/buildgen/ModuleConfig.scala | 1 +
 libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala      | 1 +
 2 files changed, 2 insertions(+)

diff --git a/libs/init/buildgen/api/src/mill/main/buildgen/ModuleConfig.scala b/libs/init/buildgen/api/src/mill/main/buildgen/ModuleConfig.scala
index 00f87d016834..bc4abcf77e9c 100644
--- a/libs/init/buildgen/api/src/mill/main/buildgen/ModuleConfig.scala
+++ b/libs/init/buildgen/api/src/mill/main/buildgen/ModuleConfig.scala
@@ -10,6 +10,7 @@ import upickle.default.{ReadWriter, macroRW}
  * to configure tasks/members like `mvnDeps`/`moduleDeps`.
  */
 sealed trait ModuleConfig
+
 object ModuleConfig {
   case class Artifact(group: String, id: String, version: String)
   object Artifact {
diff --git a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
index 70740671a45e..d55d81148f83 100644
--- a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
+++ b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
@@ -101,6 +101,7 @@ class BuildWriter(build: BuildSpec, renderCrossValueInTask: String = "crossValue
   private def renderBaseModuleImports(baseModule: ModuleSpec) = {
     val wildcards = mutable.SortedSet.empty[String]
     import baseModule.*
+    wildcards += "mill.api" += "mill.api.opts"
     wildcards ++= configs.flatMap(imports)
     renderLines(wildcards.map(s => s"import $s.*"))
   }

From 39838caf05fa3f2341f5bd9a65f0f4bd8b142dfb Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 18:01:33 +0100
Subject: [PATCH 22/67] Add imports

---
 .../expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala  | 2 ++
 .../expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala  | 2 ++
 .../gradle-9-0-0/mill-build/src/ProjectBaseModule.scala         | 2 ++
 .../gradle-9-0-0/mill-build/src/ProjectPublishModule.scala      | 2 ++
 .../maven-samples/mill-build/src/MavenSamplesBaseModule.scala   | 2 ++
 .../mill-build/src/SbtMultiProjectExampleBaseModule.scala       | 2 ++
 6 files changed, 12 insertions(+)

diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala b/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala
index fbd4d4ae2d11..cf5cf15e8f6c 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala
@@ -1,5 +1,7 @@
 package millbuild
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 
 trait ProjectBaseModule extends MavenModule {
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala b/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala
index fbd4d4ae2d11..cf5cf15e8f6c 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala
@@ -1,5 +1,7 @@
 package millbuild
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 
 trait ProjectBaseModule extends MavenModule {
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala b/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala
index a7048f70dc4c..61164d62d78b 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala
@@ -1,5 +1,7 @@
 package millbuild
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 
 trait ProjectBaseModule extends MavenModule {
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectPublishModule.scala b/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectPublishModule.scala
index 4dda0873edb0..7bfc0715ca4a 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectPublishModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectPublishModule.scala
@@ -1,5 +1,7 @@
 package millbuild
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 
diff --git a/libs/init/maven/test/resources/expected/maven-samples/mill-build/src/MavenSamplesBaseModule.scala b/libs/init/maven/test/resources/expected/maven-samples/mill-build/src/MavenSamplesBaseModule.scala
index 48f9326cb9e0..e4cf622b96ab 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/mill-build/src/MavenSamplesBaseModule.scala
+++ b/libs/init/maven/test/resources/expected/maven-samples/mill-build/src/MavenSamplesBaseModule.scala
@@ -1,5 +1,7 @@
 package millbuild
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala
index 9506ab29dc27..a37140cd775b 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala
@@ -1,5 +1,7 @@
 package millbuild
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*

From e05881f00e3ac1ded3ea7ecc20a5b286af7834d7 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Thu, 6 Nov 2025 18:17:53 +0100
Subject: [PATCH 23/67] Add imports

---
 libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala     | 1 +
 libs/init/gradle/test/resources/expected/gradle-6-0/build.mill  | 2 ++
 .../gradle/test/resources/expected/gradle-7-0/app/package.mill  | 2 ++
 libs/init/gradle/test/resources/expected/gradle-7-0/build.mill  | 2 ++
 .../gradle/test/resources/expected/gradle-7-0/list/package.mill | 2 ++
 .../test/resources/expected/gradle-7-0/utilities/package.mill   | 2 ++
 .../gradle/test/resources/expected/gradle-8-0/app/package.mill  | 2 ++
 libs/init/gradle/test/resources/expected/gradle-8-0/build.mill  | 2 ++
 .../gradle/test/resources/expected/gradle-8-0/list/package.mill | 2 ++
 .../test/resources/expected/gradle-8-0/utilities/package.mill   | 2 ++
 .../test/resources/expected/gradle-9-0-0/app/package.mill       | 2 ++
 .../init/gradle/test/resources/expected/gradle-9-0-0/build.mill | 2 ++
 .../test/resources/expected/gradle-9-0-0/list/package.mill      | 2 ++
 .../test/resources/expected/gradle-9-0-0/utilities/package.mill | 2 ++
 .../test/resources/expected/with-args/gradle-8-0/build.mill     | 2 ++
 .../init/maven/test/resources/expected/maven-samples/build.mill | 2 ++
 .../resources/expected/maven-samples/multi-module/package.mill  | 2 ++
 .../expected/maven-samples/multi-module/server/package.mill     | 2 ++
 .../expected/maven-samples/multi-module/webapp/package.mill     | 2 ++
 .../resources/expected/maven-samples/single-module/package.mill | 2 ++
 libs/init/maven/test/resources/expected/quickstart/build.mill   | 2 ++
 libs/init/maven/test/resources/expected/spring-start/build.mill | 2 ++
 .../test/resources/expected/with-args/maven-samples/build.mill  | 2 ++
 .../test/resources/expected/with-args/quickstart/build.mill     | 2 ++
 libs/init/sbt/test/resources/expected/cross-version/build.mill  | 2 ++
 .../resources/expected/sbt-multi-project-example/build.mill     | 2 ++
 .../expected/sbt-multi-project-example/common/package.mill      | 2 ++
 .../expected/sbt-multi-project-example/multi1/package.mill      | 2 ++
 .../expected/sbt-multi-project-example/multi2/package.mill      | 2 ++
 .../sbt-multi-project-example/nested/nested/package.mill        | 2 ++
 .../expected/sbt-multi-project-example/nested/package.mill      | 2 ++
 .../sbt/test/resources/expected/scala-seed-project/build.mill   | 2 ++
 32 files changed, 63 insertions(+)

diff --git a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
index d55d81148f83..35120b18ad9a 100644
--- a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
+++ b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
@@ -108,6 +108,7 @@ class BuildWriter(build: BuildSpec, renderCrossValueInTask: String = "crossValue
 
   private def renderModuleImports(module: ModuleSpec) = {
     val wildcards = mutable.SortedSet.empty[String]
+    wildcards += "mill.api" += "mill.api.opts"
     for spec <- module.sequence do {
       import spec.*
       if (supertypes.isEmpty || crossConfigs.nonEmpty) wildcards += "mill"
diff --git a/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill b/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill
index a968b085285e..8316a7b8af8c 100644
--- a/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill
@@ -1,6 +1,8 @@
 //| mill-version: SNAPSHOT
 package build
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/app/package.mill b/libs/init/gradle/test/resources/expected/gradle-7-0/app/package.mill
index b854d0883228..4c98e7e89b8f 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/app/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/app/package.mill
@@ -1,5 +1,7 @@
 package build.app
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/build.mill b/libs/init/gradle/test/resources/expected/gradle-7-0/build.mill
index 8958ea4ccb06..7dbc474fa239 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/build.mill
@@ -2,5 +2,7 @@
 package build
 
 import mill.*
+import mill.api.*
+import mill.api.opts.*
 
 object `package` extends Module {}
diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/list/package.mill b/libs/init/gradle/test/resources/expected/gradle-7-0/list/package.mill
index 911aa4e895bb..8f4d7e36d5b1 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/list/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/list/package.mill
@@ -1,5 +1,7 @@
 package build.list
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/utilities/package.mill b/libs/init/gradle/test/resources/expected/gradle-7-0/utilities/package.mill
index ed2fbd197899..649cd0be4c57 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/utilities/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/utilities/package.mill
@@ -1,5 +1,7 @@
 package build.utilities
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/app/package.mill b/libs/init/gradle/test/resources/expected/gradle-8-0/app/package.mill
index 7617c99dfe83..7a3c062e24b5 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/app/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/app/package.mill
@@ -1,5 +1,7 @@
 package build.app
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/build.mill b/libs/init/gradle/test/resources/expected/gradle-8-0/build.mill
index 8958ea4ccb06..7dbc474fa239 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/build.mill
@@ -2,5 +2,7 @@
 package build
 
 import mill.*
+import mill.api.*
+import mill.api.opts.*
 
 object `package` extends Module {}
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill b/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill
index 438c8c414452..b2ebb0a17310 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill
@@ -1,5 +1,7 @@
 package build.list
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.errorprone.*
 import millbuild.*
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill b/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill
index a6390faa5df1..444b7517dfbd 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill
@@ -1,5 +1,7 @@
 package build.utilities
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.errorprone.*
 import millbuild.*
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/app/package.mill b/libs/init/gradle/test/resources/expected/gradle-9-0-0/app/package.mill
index dc8cc4b94a1f..7d79151c9bdc 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/app/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/app/package.mill
@@ -1,5 +1,7 @@
 package build.app
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/build.mill b/libs/init/gradle/test/resources/expected/gradle-9-0-0/build.mill
index 890787f10447..13a2db102d16 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/build.mill
@@ -3,5 +3,7 @@
 package build
 
 import mill.*
+import mill.api.*
+import mill.api.opts.*
 
 object `package` extends Module {}
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/list/package.mill b/libs/init/gradle/test/resources/expected/gradle-9-0-0/list/package.mill
index 38af5a7dea86..f255608b29e8 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/list/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/list/package.mill
@@ -1,5 +1,7 @@
 package build.list
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/utilities/package.mill b/libs/init/gradle/test/resources/expected/gradle-9-0-0/utilities/package.mill
index f05b04a567f9..058c8926a0f9 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/utilities/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/utilities/package.mill
@@ -1,5 +1,7 @@
 package build.utilities
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill b/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill
index f6add7c26b2e..4ffd8d096d8a 100644
--- a/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill
@@ -2,6 +2,8 @@
 package build
 
 import mill.*
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.errorprone.*
 
diff --git a/libs/init/maven/test/resources/expected/maven-samples/build.mill b/libs/init/maven/test/resources/expected/maven-samples/build.mill
index c3eebeadbc6e..a937819422ac 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/build.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/build.mill
@@ -1,6 +1,8 @@
 //| mill-version: SNAPSHOT
 package build
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill b/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill
index 4f8e0ba07dcc..41eb89d68f24 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill
@@ -1,5 +1,7 @@
 package build.`multi-module`
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill b/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill
index ad9f6bf86255..957a2a20f442 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill
@@ -1,5 +1,7 @@
 package build.`multi-module`.server
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill b/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill
index 4468b1517c96..38151735a17d 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill
@@ -1,5 +1,7 @@
 package build.`multi-module`.webapp
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill b/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill
index cd9615ecf95f..ab5ddd26937d 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill
@@ -1,5 +1,7 @@
 package build.`single-module`
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/quickstart/build.mill b/libs/init/maven/test/resources/expected/quickstart/build.mill
index 8376bf2db5e5..62ce7136f3a9 100644
--- a/libs/init/maven/test/resources/expected/quickstart/build.mill
+++ b/libs/init/maven/test/resources/expected/quickstart/build.mill
@@ -1,6 +1,8 @@
 //| mill-version: SNAPSHOT
 package build
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.errorprone.*
 import mill.javalib.publish.*
diff --git a/libs/init/maven/test/resources/expected/spring-start/build.mill b/libs/init/maven/test/resources/expected/spring-start/build.mill
index 097f12c07bea..421b11487658 100644
--- a/libs/init/maven/test/resources/expected/spring-start/build.mill
+++ b/libs/init/maven/test/resources/expected/spring-start/build.mill
@@ -1,6 +1,8 @@
 //| mill-version: SNAPSHOT
 package build
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill b/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill
index 4c93e79ec69c..a25a11501ea7 100644
--- a/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill
+++ b/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill
@@ -1,6 +1,8 @@
 //| mill-version: SNAPSHOT
 package build
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 
diff --git a/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill b/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill
index 8d4c0769a37b..cc05bdfae586 100644
--- a/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill
+++ b/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill
@@ -1,6 +1,8 @@
 //| mill-version: SNAPSHOT
 package build
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.errorprone.*
 import mill.javalib.publish.*
diff --git a/libs/init/sbt/test/resources/expected/cross-version/build.mill b/libs/init/sbt/test/resources/expected/cross-version/build.mill
index c075a3671cf5..32dd2a646f82 100644
--- a/libs/init/sbt/test/resources/expected/cross-version/build.mill
+++ b/libs/init/sbt/test/resources/expected/cross-version/build.mill
@@ -3,6 +3,8 @@
 package build
 
 import mill.*
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/build.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/build.mill
index 8958ea4ccb06..7dbc474fa239 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/build.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/build.mill
@@ -2,5 +2,7 @@
 package build
 
 import mill.*
+import mill.api.*
+import mill.api.opts.*
 
 object `package` extends Module {}
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill
index 5006c9f3afd9..70b067d0d497 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill
@@ -1,5 +1,7 @@
 package build.common
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill
index 5d791d852b3b..9ff351267bcf 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill
@@ -1,5 +1,7 @@
 package build.multi1
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi2/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi2/package.mill
index d6b4fba2276e..646a226255b9 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi2/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi2/package.mill
@@ -1,5 +1,7 @@
 package build.multi2
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill
index 46377a0fb8e7..c7380175478a 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill
@@ -1,5 +1,7 @@
 package build.nested.nested
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/package.mill
index 2ba917ae0300..3aa5969e613d 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/package.mill
@@ -1,5 +1,7 @@
 package build.nested
 
 import mill.*
+import mill.api.*
+import mill.api.opts.*
 
 object `package` extends Module {}
diff --git a/libs/init/sbt/test/resources/expected/scala-seed-project/build.mill b/libs/init/sbt/test/resources/expected/scala-seed-project/build.mill
index d2c8210c1848..5fcef3a10116 100644
--- a/libs/init/sbt/test/resources/expected/scala-seed-project/build.mill
+++ b/libs/init/sbt/test/resources/expected/scala-seed-project/build.mill
@@ -1,6 +1,8 @@
 //| mill-version: SNAPSHOT
 package build
 
+import mill.api.*
+import mill.api.opts.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*

From 37846fd5f121fb7bbe8036d1e256245a9fac4424 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Fri, 7 Nov 2025 14:14:40 +0100
Subject: [PATCH 24/67] Fix test

---
 .../test/src/mill/scalanativelib/TestingTests.scala             | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala b/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala
index 23c9ac8efae0..01e113604d0a 100644
--- a/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala
+++ b/libs/scalanativelib/test/src/mill/scalanativelib/TestingTests.scala
@@ -69,7 +69,7 @@ object TestingTests extends TestSuite {
       checkInheritedTasks(_.scalacOptions, Opts("-deprecation"))
     }
     test("test-scalaOrganization") {
-      checkInheritedTasks(_.scalaOrganization, Seq("org.example"))
+      checkInheritedTasks(_.scalaOrganization, "org.example")
     }
   }
 

From c8cc3bee7aefe1448a95cbfd797a3bf0e71e4dda Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Fri, 7 Nov 2025 14:37:13 +0100
Subject: [PATCH 25/67] More Seq -> Opts fixes

---
 .../javalib/module/8-annotation-processors/build.mill    | 6 ++++--
 example/javalib/publishing/7-native-image/build.mill     | 4 +++-
 .../javalib/publishing/8-native-image-libs/build.mill    | 4 +++-
 .../module/5-compilation-execution-flags/build.mill      | 2 +-
 .../kotlinlib/publishing/8-native-image-libs/build.mill  | 4 +++-
 example/scalalib/linting/3-acyclic/build.mill            | 5 ++++-
 example/scalalib/native/4-common-config/build.mill       | 4 +++-
 example/scalalib/publishing/7-native-image/build.mill    | 4 +++-
 .../scalalib/publishing/8-native-image-libs/build.mill   | 4 +++-
 example/thirdparty/jimfs/build.mill                      | 4 +++-
 example/thirdparty/mockito/build.mill                    | 3 ++-
 example/thirdparty/netty/build.mill                      | 3 ++-
 .../feature/codesig-javamodule/resources/build.mill      | 4 +++-
 integration/ide/bsp-server/resources/project/build.mill  | 7 ++++---
 integration/ide/gen-idea/resources/hello-idea/build.mill | 5 +++--
 .../zinc-incremental-compilation/resources/build.mill    | 3 ++-
 libs/javalib/src/mill/javalib/NativeImageModule.scala    | 5 +++--
 .../src/mill/javalib/checkstyle/CheckstyleModule.scala   | 9 +++++----
 libs/javalib/src/mill/javalib/pmd/PmdModule.scala        | 9 ++++-----
 19 files changed, 58 insertions(+), 31 deletions(-)

diff --git a/example/javalib/module/8-annotation-processors/build.mill b/example/javalib/module/8-annotation-processors/build.mill
index ccb1a6d4b478..b85518c4bffe 100644
--- a/example/javalib/module/8-annotation-processors/build.mill
+++ b/example/javalib/module/8-annotation-processors/build.mill
@@ -1,6 +1,8 @@
 package build
+
 import mill.*, javalib.*
 import java.io.File
+import mill.api.opt.*
 
 object foo extends JavaModule {
   def compileMvnDeps = Seq(mvn"org.projectlombok:lombok:1.18.34")
@@ -40,9 +42,9 @@ object bar extends JavaModule {
     defaultResolver().classpath(Seq(mvn"org.projectlombok:lombok:1.18.34"))
   }
 
-  def javacOptions = Seq(
+  def javacOptions = Opts(
     "-processorpath",
-    processors().map(_.path).mkString(File.pathSeparator)
+    Opt.mkPath(processors().map(_.path), sep = File.pathSeparator)
   )
 
   object test extends JavaTests, TestModule.Junit4
diff --git a/example/javalib/publishing/7-native-image/build.mill b/example/javalib/publishing/7-native-image/build.mill
index 592d42bece56..45d8ab8c42ec 100644
--- a/example/javalib/publishing/7-native-image/build.mill
+++ b/example/javalib/publishing/7-native-image/build.mill
@@ -1,12 +1,14 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, javalib.*
+import mill.api.opt.*
 import mill.api.ModuleRef
 
 object foo extends JavaModule, NativeImageModule {
   def jvmId = "graalvm-community:17.0.7"
 
-  def nativeImageOptions = Seq("--no-fallback")
+  def nativeImageOptions = Opts("--no-fallback")
 }
 
 //// SNIPPET:END
diff --git a/example/javalib/publishing/8-native-image-libs/build.mill b/example/javalib/publishing/8-native-image-libs/build.mill
index b0f0e96470cf..1463ae033c8c 100644
--- a/example/javalib/publishing/8-native-image-libs/build.mill
+++ b/example/javalib/publishing/8-native-image-libs/build.mill
@@ -1,6 +1,8 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, javalib.*
+import mill.api.opt.*
 import mill.api.ModuleRef
 
 object foo extends JavaModule, NativeImageModule {
@@ -10,7 +12,7 @@ object foo extends JavaModule, NativeImageModule {
     mvn"org.slf4j:slf4j-nop:2.0.7"
   )
 
-  def nativeImageOptions = Seq(
+  def nativeImageOptions = Opts(
     "--no-fallback",
     "-H:IncludeResourceBundles=net.sourceforge.argparse4j.internal.ArgumentParserImpl",
     "-Os"
diff --git a/example/kotlinlib/module/5-compilation-execution-flags/build.mill b/example/kotlinlib/module/5-compilation-execution-flags/build.mill
index 59a6341be8c3..c1b268d34c2d 100644
--- a/example/kotlinlib/module/5-compilation-execution-flags/build.mill
+++ b/example/kotlinlib/module/5-compilation-execution-flags/build.mill
@@ -11,7 +11,7 @@ object `package` extends KotlinModule {
   def forkArgs = Opts("-Xmx4g", "-Dmy.jvm.property=hello")
   def forkEnv = Map("MY_ENV_VAR" -> opt"WORLD")
 
-  def kotlincOptions = super.kotlincOptions() ++ Seq("-Werror")
+  def kotlincOptions = super.kotlincOptions() ++ Opt("-Werror")
 }
 
 // You can pass flags to the Kotlin compiler via `kotlincOptions`.
diff --git a/example/kotlinlib/publishing/8-native-image-libs/build.mill b/example/kotlinlib/publishing/8-native-image-libs/build.mill
index 81469e6e0175..3ef491f5d7a2 100644
--- a/example/kotlinlib/publishing/8-native-image-libs/build.mill
+++ b/example/kotlinlib/publishing/8-native-image-libs/build.mill
@@ -1,12 +1,14 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, kotlinlib.*
+import mill.api.opt.*
 import mill.api.ModuleRef
 
 object foo extends KotlinModule, NativeImageModule {
   def kotlinVersion = "1.9.24"
 
-  def nativeImageOptions = Seq(
+  def nativeImageOptions = Opts(
     "--no-fallback",
     "-Os",
     "--initialize-at-build-time=com.github.ajalt.mordant.internal.nativeimage.NativeImagePosixMppImpls"
diff --git a/example/scalalib/linting/3-acyclic/build.mill b/example/scalalib/linting/3-acyclic/build.mill
index 2b25b5a36fc3..0496a4a68cab 100644
--- a/example/scalalib/linting/3-acyclic/build.mill
+++ b/example/scalalib/linting/3-acyclic/build.mill
@@ -15,13 +15,16 @@
 // shown below:
 
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
+
 
 object `package` extends ScalaModule {
   def scalaVersion = "2.13.16"
   def compileMvnDeps = Seq(mvn"com.lihaoyi:::acyclic:0.3.18")
   def scalacPluginMvnDeps = Seq(mvn"com.lihaoyi:::acyclic:0.3.18")
-  def scalacOptions = Seq("-P:acyclic:force")
+  def scalacOptions = Opts("-P:acyclic:force")
 }
 
 /** See Also: src/Foo.scala */
diff --git a/example/scalalib/native/4-common-config/build.mill b/example/scalalib/native/4-common-config/build.mill
index 69dd6a3742d9..f34782f4e1cd 100644
--- a/example/scalalib/native/4-common-config/build.mill
+++ b/example/scalalib/native/4-common-config/build.mill
@@ -1,5 +1,7 @@
 package build
+
 import mill.*, scalalib.*, scalanativelib.*, scalanativelib.api.*
+import mill.api.opt.*
 
 object `package` extends ScalaNativeModule {
   def scalaVersion = "3.3.6"
@@ -18,7 +20,7 @@ object `package` extends ScalaNativeModule {
   def nativeIncrementalCompilation: T[Boolean] = true
 
   // Set nativeLinkingOptions path to a directory named `target`.
-  def nativeLinkingOptions = Seq("-L" + moduleDir.toString + "/target")
+  def nativeLinkingOptions = Opts(opt"-L${moduleDir}/target")
 
   // Set nativeWorkdir directory to `newDir`
   def nativeWorkdir = Task.dest / "newDir"
diff --git a/example/scalalib/publishing/7-native-image/build.mill b/example/scalalib/publishing/7-native-image/build.mill
index c6fdbae6d875..33c8c2609c68 100644
--- a/example/scalalib/publishing/7-native-image/build.mill
+++ b/example/scalalib/publishing/7-native-image/build.mill
@@ -1,6 +1,8 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
 import mill.api.ModuleRef
 
 object foo extends ScalaModule, NativeImageModule {
@@ -8,7 +10,7 @@ object foo extends ScalaModule, NativeImageModule {
 
   def jvmId = "graalvm-community:17.0.7"
 
-  def nativeImageOptions = Seq("--no-fallback")
+  def nativeImageOptions = Opts("--no-fallback")
 }
 
 //// SNIPPET:END
diff --git a/example/scalalib/publishing/8-native-image-libs/build.mill b/example/scalalib/publishing/8-native-image-libs/build.mill
index a41da94a385c..9e718ce87009 100644
--- a/example/scalalib/publishing/8-native-image-libs/build.mill
+++ b/example/scalalib/publishing/8-native-image-libs/build.mill
@@ -1,6 +1,8 @@
 //// SNIPPET:BUILD
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
 import mill.api.ModuleRef
 
 object foo extends ScalaModule, NativeImageModule {
@@ -8,7 +10,7 @@ object foo extends ScalaModule, NativeImageModule {
 
   def nativeImageOptions = Seq("--no-fallback", "-Os")
 
-  def mvnDeps = Seq(
+  def mvnDeps = Opts(
     mvn"com.lihaoyi::scalatags:0.13.1",
     mvn"com.lihaoyi::mainargs:0.7.7"
   )
diff --git a/example/thirdparty/jimfs/build.mill b/example/thirdparty/jimfs/build.mill
index 54d57487e3f2..6d60bac1a1e1 100644
--- a/example/thirdparty/jimfs/build.mill
+++ b/example/thirdparty/jimfs/build.mill
@@ -1,5 +1,7 @@
 package build
+
 import mill.*, javalib.*, publish.*
+import mill.api.opt.*
 
 def sharedCompileMvnDeps = Task {
   Seq(
@@ -26,7 +28,7 @@ object jimfs extends PublishModule, MavenModule {
     mvn"com.google.guava:guava:31.1-android"
   )
 
-  def javacOptions = Seq("-processor", "com.google.auto.service.processor.AutoServiceProcessor")
+  def javacOptions = Opts("-processor", "com.google.auto.service.processor.AutoServiceProcessor")
 
   object test extends MavenTests, TestModule.Junit4 {
     def junit4Version = "4.13.2"
diff --git a/example/thirdparty/mockito/build.mill b/example/thirdparty/mockito/build.mill
index 90484c256c73..bbf8c2adb8b5 100644
--- a/example/thirdparty/mockito/build.mill
+++ b/example/thirdparty/mockito/build.mill
@@ -1,4 +1,5 @@
 package build
+
 import mill.*, javalib.*
 import mill.api.ModuleRef
 import mill.api.opt.*
@@ -40,7 +41,7 @@ object libraries {
 
 trait MockitoModule extends MavenModule {
   def jvmId = "temurin:11.0.24"
-  def javacOptions = Seq("-encoding", "UTF-8")
+  def javacOptions = Opts("-encoding", "UTF-8")
   def testModuleDeps: Seq[JavaModule] = Nil
   def testMvnDeps: T[Seq[Dep]] = Seq()
   def testRuntimeMvnDeps: T[Seq[Dep]] = Seq()
diff --git a/example/thirdparty/netty/build.mill b/example/thirdparty/netty/build.mill
index 98a0d25ee981..a3502af2fd5f 100644
--- a/example/thirdparty/netty/build.mill
+++ b/example/thirdparty/netty/build.mill
@@ -6,6 +6,7 @@ package build
 
 import mill.*, javalib.*
 import mill.api.BuildCtx
+import mill.api.opt.*
 
 // TODO:
 //   testsuite-shading
@@ -15,7 +16,7 @@ import mill.api.BuildCtx
 def isOSX = System.getProperty("os.name").toLowerCase.contains("mac")
 
 trait NettyBaseModule extends MavenModule {
-  def javacOptions = Seq("-source", "1.8", "-target", "1.8")
+  def javacOptions = Opts("-source", "1.8", "-target", "1.8")
 }
 trait NettyBaseTestSuiteModule extends NettyBaseModule, TestModule.Junit5 {
   def testSandboxWorkingDir = false
diff --git a/integration/feature/codesig-javamodule/resources/build.mill b/integration/feature/codesig-javamodule/resources/build.mill
index b6250b170825..c62bfa48b186 100644
--- a/integration/feature/codesig-javamodule/resources/build.mill
+++ b/integration/feature/codesig-javamodule/resources/build.mill
@@ -1,8 +1,10 @@
 package build
+
 import mill._, javalib._
+import mill.api.opt.*
 
 object foo extends JavaModule {
-  def javacOptions = Seq("-source", "11", "-target", "11")
+  def javacOptions = Opts("-source", "11", "-target", "11")
   def sources = Task {
     println("Foo generating sources...")
     os.write(
diff --git a/integration/ide/bsp-server/resources/project/build.mill b/integration/ide/bsp-server/resources/project/build.mill
index 2bdf4141df51..a553ca82d0bd 100644
--- a/integration/ide/bsp-server/resources/project/build.mill
+++ b/integration/ide/bsp-server/resources/project/build.mill
@@ -1,6 +1,7 @@
 package build
 
-import mill._
+import mill.*
+import mill.api.opt.*
 
 // testing a simple Java module
 object `hello-java` extends scalalib.JavaModule {
@@ -94,7 +95,7 @@ object delayed extends scalalib.ScalaModule {
 
 object diag extends scalalib.ScalaModule {
   def scalaVersion = Option(System.getenv("TEST_SCALA_2_13_VERSION")).getOrElse(???)
-  def scalacOptions = Seq("-deprecation")
+  def scalacOptions = Opts("-deprecation")
 
   @deprecated("deprecated", "0.0.1")
   def thing = 2
@@ -103,7 +104,7 @@ object diag extends scalalib.ScalaModule {
 
   object many extends scalalib.ScalaModule {
     def scalaVersion = Option(System.getenv("TEST_SCALA_2_13_VERSION")).getOrElse(???)
-    def scalacOptions = Seq("-deprecation")
+    def scalacOptions = Opts("-deprecation")
   }
 }
 
diff --git a/integration/ide/gen-idea/resources/hello-idea/build.mill b/integration/ide/gen-idea/resources/hello-idea/build.mill
index 3446c1b129b4..eecb34362979 100644
--- a/integration/ide/gen-idea/resources/hello-idea/build.mill
+++ b/integration/ide/gen-idea/resources/hello-idea/build.mill
@@ -1,8 +1,9 @@
 package build
 
-import mill._
+import mill.*
 import mill.scalajslib.ScalaJSModule
 import mill.scalalib.{Dep, DepSyntax, JavaModule, ScalaModule, TestModule}
+import mill.api.opt.*
 
 trait HelloIdeaModule extends scalalib.ScalaModule {
   def scalaVersion = "2.12.5"
@@ -63,5 +64,5 @@ object moduleF extends JavaModule {
 object moduleG extends Cross[ModuleG]("2.12.5", "2.13.6")
 trait ModuleG extends ScalaModule with Cross.Module[String] {
   def scalaVersion = crossValue
-  def scalacOptions = Seq("-Xsource:3")
+  def scalacOptions = Opts("-Xsource:3")
 }
diff --git a/integration/invalidation/zinc-incremental-compilation/resources/build.mill b/integration/invalidation/zinc-incremental-compilation/resources/build.mill
index 740140ec52c2..265dd514d363 100644
--- a/integration/invalidation/zinc-incremental-compilation/resources/build.mill
+++ b/integration/invalidation/zinc-incremental-compilation/resources/build.mill
@@ -2,12 +2,13 @@ package build
 // Issue https://github.com/com-lihaoyi/mill/issues/1901
 import mill._
 import mill.scalalib._
+import mill.api.opt.*
 
 object app extends SbtModule {
 
   def scalaVersion = "2.13.16"
 
-  def scalacOptions = Seq("-Vclasspath")
+  def scalacOptions = Opts("-Vclasspath")
 
   def mvnDeps = Seq(
     mvn"io.getquill::quill-sql:3.18.0"
diff --git a/libs/javalib/src/mill/javalib/NativeImageModule.scala b/libs/javalib/src/mill/javalib/NativeImageModule.scala
index 8877aa16f4d2..499d687a2e8b 100644
--- a/libs/javalib/src/mill/javalib/NativeImageModule.scala
+++ b/libs/javalib/src/mill/javalib/NativeImageModule.scala
@@ -2,6 +2,7 @@ package mill.javalib
 
 import mill.*
 import mill.constants.Util
+import mill.api.opt.*
 
 import scala.util.Properties
 
@@ -32,7 +33,7 @@ trait NativeImageModule extends WithJvmWorkerModule {
     val executeableName = "native-executable"
     val command = Seq.newBuilder[String]
       .+=(nativeImageTool().path.toString)
-      .++=(nativeImageOptions())
+      .++=(nativeImageOptions().toStringSeq)
       .+=("-cp")
       .+=(nativeImageClasspath().iterator.map(_.path).mkString(java.io.File.pathSeparator))
       .+=(finalMainClass())
@@ -57,7 +58,7 @@ trait NativeImageModule extends WithJvmWorkerModule {
   /**
    * Additional options for the `native-image` Tool.
    */
-  def nativeImageOptions: T[Seq[String]] = Seq()
+  def nativeImageOptions: T[Opts] = Opts()
 
   /**
    * Path to the [[https://www.graalvm.org/latest/reference-manual/native-image/ `native-image` Tool]].
diff --git a/libs/javalib/src/mill/javalib/checkstyle/CheckstyleModule.scala b/libs/javalib/src/mill/javalib/checkstyle/CheckstyleModule.scala
index c7ee10c4b789..b86588989c7d 100644
--- a/libs/javalib/src/mill/javalib/checkstyle/CheckstyleModule.scala
+++ b/libs/javalib/src/mill/javalib/checkstyle/CheckstyleModule.scala
@@ -1,7 +1,8 @@
 package mill.javalib.checkstyle
 
 import mill._
-import mill.api.{PathRef}
+import mill.api.PathRef
+import mill.api.opt.*
 import mill.javalib.{DepSyntax, JavaModule}
 import mill.util.Jvm
 import mill.api.BuildCtx
@@ -27,7 +28,7 @@ trait CheckstyleModule extends JavaModule {
   protected def checkstyle0(stdout: Boolean, leftover: mainargs.Leftover[String]) = Task.Anon {
 
     val output = checkstyleOutput().path
-    val args = checkstyleOptions() ++
+    val args = checkstyleOptions().toStringSeq ++
       Seq("-c", checkstyleConfig().path.toString()) ++
       Seq("-f", checkstyleFormat()) ++
       (if (stdout) Seq.empty else Seq("-o", output.toString())) ++
@@ -106,8 +107,8 @@ trait CheckstyleModule extends JavaModule {
   /**
    * Additional arguments for Checkstyle.
    */
-  def checkstyleOptions: T[Seq[String]] = Task {
-    Seq.empty[String]
+  def checkstyleOptions: T[Opts] = Task {
+    Opts()
   }
 
   /**
diff --git a/libs/javalib/src/mill/javalib/pmd/PmdModule.scala b/libs/javalib/src/mill/javalib/pmd/PmdModule.scala
index 226fa0eb4169..ebf7c360364b 100644
--- a/libs/javalib/src/mill/javalib/pmd/PmdModule.scala
+++ b/libs/javalib/src/mill/javalib/pmd/PmdModule.scala
@@ -3,6 +3,7 @@ package mill.javalib.pmd
 import mill.*
 import mill.api.{Discover, ExternalModule, TaskCtx}
 import mill.api.daemon.experimental
+import mill.api.opt.*
 import mill.javalib.api.Versions
 import mill.javalib.{CoursierModule, Dep, DepSyntax, OfflineSupportModule}
 import mill.util.{Jvm, Version}
@@ -51,8 +52,8 @@ trait PmdModule extends CoursierModule, OfflineSupportModule {
       )
 
       val args =
-        if (isPmd6OrOlder(this.pmdVersion())) pmdOptions() ++ baseArgs
-        else pmdOptions() ++ (Seq("check") ++ baseArgs)
+        if (isPmd6OrOlder(this.pmdVersion())) pmdOptions().toStringSeq ++ baseArgs
+        else pmdOptions().toStringSeq ++ (Seq("check") ++ baseArgs)
       val mainCls =
         if (isPmd6OrOlder(this.pmdVersion())) "net.sourceforge.pmd.PMD"
         else "net.sourceforge.pmd.cli.PmdCli"
@@ -148,9 +149,7 @@ trait PmdModule extends CoursierModule, OfflineSupportModule {
   def pmdRulesets: T[Seq[PathRef]] = Task.Sources(moduleDir / "pmd-ruleset.xml")
 
   /** Additional arguments for PMD. */
-  def pmdOptions: T[Seq[String]] = Task {
-    Seq.empty[String]
-  }
+  def pmdOptions: T[Opts] = Task { Opts() }
 
   /** User language of the JVM running PMD. */
   def pmdLanguage: T[Option[String]] = Task.Input {

From 138cda3104109f05c371512502a538a29e4c79e3 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Fri, 7 Nov 2025 15:15:42 +0100
Subject: [PATCH 26/67] Fix opts in YAML

---
 example/cli/header/2-tasks-yaml/build.mill.yaml        | 2 +-
 example/javalib/config/4-native-image/build.mill.yaml  | 2 +-
 example/javalib/script/6-packaging/Bar.java            | 2 +-
 example/kotlinlib/script/6-packaging/Bar.kt            | 2 +-
 example/scalalib/config/4-native-image/build.mill.yaml | 2 +-
 example/scalalib/script/6-packaging/Bar.scala          | 2 +-
 6 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/example/cli/header/2-tasks-yaml/build.mill.yaml b/example/cli/header/2-tasks-yaml/build.mill.yaml
index 883e669c3d6f..6d567304d4c1 100644
--- a/example/cli/header/2-tasks-yaml/build.mill.yaml
+++ b/example/cli/header/2-tasks-yaml/build.mill.yaml
@@ -1,2 +1,2 @@
 mill-build:
-  scalacOptions: ["-verbose"]
+  scalacOptions: [["-verbose"]]
diff --git a/example/javalib/config/4-native-image/build.mill.yaml b/example/javalib/config/4-native-image/build.mill.yaml
index 686ac50ff159..a09851c2fe07 100644
--- a/example/javalib/config/4-native-image/build.mill.yaml
+++ b/example/javalib/config/4-native-image/build.mill.yaml
@@ -1,3 +1,3 @@
 extends: [mill.javalib.JavaModule, mill.javalib.NativeImageModule]
 jvmId: "graalvm-community:24"
-nativeImageOptions: ["--no-fallback"]
\ No newline at end of file
+nativeImageOptions: [["--no-fallback"]]
\ No newline at end of file
diff --git a/example/javalib/script/6-packaging/Bar.java b/example/javalib/script/6-packaging/Bar.java
index 0b71b3355f94..e6ff417050e8 100644
--- a/example/javalib/script/6-packaging/Bar.java
+++ b/example/javalib/script/6-packaging/Bar.java
@@ -1,5 +1,5 @@
 //| jvmId: "graalvm-community:17"
-//| nativeImageOptions: ["--no-fallback"]
+//| nativeImageOptions: [["--no-fallback"]]
 public class Bar {
   public static void main(String[] args) {
     System.out.println("Hello World");
diff --git a/example/kotlinlib/script/6-packaging/Bar.kt b/example/kotlinlib/script/6-packaging/Bar.kt
index 055d54c7cf65..a1ecccdc3894 100644
--- a/example/kotlinlib/script/6-packaging/Bar.kt
+++ b/example/kotlinlib/script/6-packaging/Bar.kt
@@ -1,6 +1,6 @@
 //| kotlinVersion: 2.0.20
 //| jvmId: "graalvm-community:17"
-//| nativeImageOptions: ["--no-fallback"]
+//| nativeImageOptions: [["--no-fallback"]]
 fun main(args: Array<String>) {
     println("Hello World")
 }
diff --git a/example/scalalib/config/4-native-image/build.mill.yaml b/example/scalalib/config/4-native-image/build.mill.yaml
index 47a8dc0e737c..af857aff1805 100644
--- a/example/scalalib/config/4-native-image/build.mill.yaml
+++ b/example/scalalib/config/4-native-image/build.mill.yaml
@@ -1,4 +1,4 @@
 extends: [mill.scalalib.ScalaModule, mill.scalalib.NativeImageModule]
 scalaVersion: 3.7.3
 jvmId: "graalvm-community:24"
-nativeImageOptions: ["--no-fallback"]
+nativeImageOptions: [["--no-fallback"]]
diff --git a/example/scalalib/script/6-packaging/Bar.scala b/example/scalalib/script/6-packaging/Bar.scala
index e56b76464801..8fe18949de53 100644
--- a/example/scalalib/script/6-packaging/Bar.scala
+++ b/example/scalalib/script/6-packaging/Bar.scala
@@ -1,6 +1,6 @@
 //| scalaVersion: 3.7.3
 //| jvmId: "graalvm-community:17"
-//| nativeImageOptions: ["--no-fallback"]
+//| nativeImageOptions: [["--no-fallback"]]
 @main def main() = {
   println("Hello World")
 }

From 1274f2429a1c43268678d2a299a0a9c7fb859369 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Fri, 7 Nov 2025 15:15:55 +0100
Subject: [PATCH 27/67] Fix opts usage

---
 example/scalalib/publishing/8-native-image-libs/build.mill | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/example/scalalib/publishing/8-native-image-libs/build.mill b/example/scalalib/publishing/8-native-image-libs/build.mill
index 9e718ce87009..345d347aa649 100644
--- a/example/scalalib/publishing/8-native-image-libs/build.mill
+++ b/example/scalalib/publishing/8-native-image-libs/build.mill
@@ -8,7 +8,7 @@ import mill.api.ModuleRef
 object foo extends ScalaModule, NativeImageModule {
   def scalaVersion = "3.7.1"
 
-  def nativeImageOptions = Seq("--no-fallback", "-Os")
+  def nativeImageOptions = Opts("--no-fallback", "-Os")
 
   def mvnDeps = Opts(
     mvn"com.lihaoyi::scalatags:0.13.1",

From 7a777a6405034c02048ef94d6cfe0105fa325ad3 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Fri, 7 Nov 2025 15:20:37 +0100
Subject: [PATCH 28/67] Fix import

---
 libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
index 35120b18ad9a..a4f1624d4e51 100644
--- a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
+++ b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
@@ -101,14 +101,14 @@ class BuildWriter(build: BuildSpec, renderCrossValueInTask: String = "crossValue
   private def renderBaseModuleImports(baseModule: ModuleSpec) = {
     val wildcards = mutable.SortedSet.empty[String]
     import baseModule.*
-    wildcards += "mill.api" += "mill.api.opts"
+    wildcards += "mill.api" += "mill.api.opt"
     wildcards ++= configs.flatMap(imports)
     renderLines(wildcards.map(s => s"import $s.*"))
   }
 
   private def renderModuleImports(module: ModuleSpec) = {
     val wildcards = mutable.SortedSet.empty[String]
-    wildcards += "mill.api" += "mill.api.opts"
+    wildcards += "mill.api" += "mill.api.opt"
     for spec <- module.sequence do {
       import spec.*
       if (supertypes.isEmpty || crossConfigs.nonEmpty) wildcards += "mill"

From e639d9088a68e59db7937974957a9dec570d50b0 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sat, 8 Nov 2025 17:25:41 +0100
Subject: [PATCH 29/67] Add support to read Opts from simplified JSON, like `[
 "opt1", "opt2" ]`

---
 core/api/src/mill/api/opt/Opts.scala          | 27 ++++++++++++++++---
 .../api/test/src/mill/api/opt/OptsTests.scala |  9 +++++++
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/core/api/src/mill/api/opt/Opts.scala b/core/api/src/mill/api/opt/Opts.scala
index e3d79e1d9399..5611457fceb3 100644
--- a/core/api/src/mill/api/opt/Opts.scala
+++ b/core/api/src/mill/api/opt/Opts.scala
@@ -46,9 +46,30 @@ object Opts {
     new Opts(groups.filter(!_.isEmpty))
   }
 
+//  given jsonReadWriter: upickle.ReadWriter[Opts] =
+//    upickle.readwriter[Seq[OptGroup]].bimap(
+//      _.value,
+//      Opts(_*)
+//    )
+
   given jsonReadWriter: upickle.ReadWriter[Opts] =
-    upickle.readwriter[Seq[OptGroup]].bimap(
-      _.value,
-      Opts(_*)
+    upickle.readwriter[ujson.Value].bimap(
+      { opts =>
+        // We always serialize as a seq of groups
+        upickle.transform(opts.value).to[ujson.Value]
+      },
+      {
+        case arr: ujson.Arr =>
+          Opts(
+            arr.value.map {
+              // The default case, a seq of groups
+              case e: ujson.Str => OptGroup(upickle.read[Opt](e))
+              // special case, a flat Opt, so we can also read simple ["opt1", "opt2"] arrays
+              // which is what we want to use in YAML build files
+              case g => upickle.read[OptGroup](g)
+            }.toSeq*
+          )
+      }
     )
+
 }
diff --git a/core/api/test/src/mill/api/opt/OptsTests.scala b/core/api/test/src/mill/api/opt/OptsTests.scala
index bb811966b22a..8a56eeef3bb0 100644
--- a/core/api/test/src/mill/api/opt/OptsTests.scala
+++ b/core/api/test/src/mill/api/opt/OptsTests.scala
@@ -121,6 +121,15 @@ class OptsTests extends TestSuite {
         val back = upickle.read[Opts](json)
         assert(opts1 == back)
       }
+      test("read-flat-opt-sequences") {
+        val flatOpts = Seq("opt1", "opt2")
+        val expected = Opts("opt1", "opt2")
+        assert(flatOpts == expected.toStringSeq)
+
+        val json = upickle.write(flatOpts)
+        val back = upickle.read[Opts](json)
+        assert(back == expected)
+      }
 //      test("with-mapping-home") {
 //        val json = upickle.write(opts1)
 //        assertGoldenLiteral(

From 3480bd8a744afb86147fc5b87bf941cd217944c9 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sat, 8 Nov 2025 17:25:59 +0100
Subject: [PATCH 30/67] fix example

---
 example/kotlinlib/module/7-resources/build.mill | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/example/kotlinlib/module/7-resources/build.mill b/example/kotlinlib/module/7-resources/build.mill
index cb2f7d5f5b55..c54877799690 100644
--- a/example/kotlinlib/module/7-resources/build.mill
+++ b/example/kotlinlib/module/7-resources/build.mill
@@ -11,7 +11,7 @@ object foo extends KotlinModule {
   object test extends KotlinTests, TestModule.Junit5 {
     def otherFiles = Task.Source("other-files")
 
-    def forkEnv = super.forkEnv() ++ Opt(
+    def forkEnv = super.forkEnv() ++ Map(
       "OTHER_FILES_DIR" -> Opt(otherFiles().path)
     )
 

From 410dee89b5eb7a9855470d1df83e4118025eaa01 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sat, 8 Nov 2025 17:26:16 +0100
Subject: [PATCH 31/67] Revert "Fix opts in YAML"

This reverts commit 138cda3104109f05c371512502a538a29e4c79e3.
---
 example/cli/header/2-tasks-yaml/build.mill.yaml        | 2 +-
 example/javalib/config/4-native-image/build.mill.yaml  | 2 +-
 example/javalib/script/6-packaging/Bar.java            | 2 +-
 example/kotlinlib/script/6-packaging/Bar.kt            | 2 +-
 example/scalalib/config/4-native-image/build.mill.yaml | 2 +-
 example/scalalib/script/6-packaging/Bar.scala          | 2 +-
 6 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/example/cli/header/2-tasks-yaml/build.mill.yaml b/example/cli/header/2-tasks-yaml/build.mill.yaml
index 6d567304d4c1..883e669c3d6f 100644
--- a/example/cli/header/2-tasks-yaml/build.mill.yaml
+++ b/example/cli/header/2-tasks-yaml/build.mill.yaml
@@ -1,2 +1,2 @@
 mill-build:
-  scalacOptions: [["-verbose"]]
+  scalacOptions: ["-verbose"]
diff --git a/example/javalib/config/4-native-image/build.mill.yaml b/example/javalib/config/4-native-image/build.mill.yaml
index a09851c2fe07..686ac50ff159 100644
--- a/example/javalib/config/4-native-image/build.mill.yaml
+++ b/example/javalib/config/4-native-image/build.mill.yaml
@@ -1,3 +1,3 @@
 extends: [mill.javalib.JavaModule, mill.javalib.NativeImageModule]
 jvmId: "graalvm-community:24"
-nativeImageOptions: [["--no-fallback"]]
\ No newline at end of file
+nativeImageOptions: ["--no-fallback"]
\ No newline at end of file
diff --git a/example/javalib/script/6-packaging/Bar.java b/example/javalib/script/6-packaging/Bar.java
index e6ff417050e8..0b71b3355f94 100644
--- a/example/javalib/script/6-packaging/Bar.java
+++ b/example/javalib/script/6-packaging/Bar.java
@@ -1,5 +1,5 @@
 //| jvmId: "graalvm-community:17"
-//| nativeImageOptions: [["--no-fallback"]]
+//| nativeImageOptions: ["--no-fallback"]
 public class Bar {
   public static void main(String[] args) {
     System.out.println("Hello World");
diff --git a/example/kotlinlib/script/6-packaging/Bar.kt b/example/kotlinlib/script/6-packaging/Bar.kt
index a1ecccdc3894..055d54c7cf65 100644
--- a/example/kotlinlib/script/6-packaging/Bar.kt
+++ b/example/kotlinlib/script/6-packaging/Bar.kt
@@ -1,6 +1,6 @@
 //| kotlinVersion: 2.0.20
 //| jvmId: "graalvm-community:17"
-//| nativeImageOptions: [["--no-fallback"]]
+//| nativeImageOptions: ["--no-fallback"]
 fun main(args: Array<String>) {
     println("Hello World")
 }
diff --git a/example/scalalib/config/4-native-image/build.mill.yaml b/example/scalalib/config/4-native-image/build.mill.yaml
index af857aff1805..47a8dc0e737c 100644
--- a/example/scalalib/config/4-native-image/build.mill.yaml
+++ b/example/scalalib/config/4-native-image/build.mill.yaml
@@ -1,4 +1,4 @@
 extends: [mill.scalalib.ScalaModule, mill.scalalib.NativeImageModule]
 scalaVersion: 3.7.3
 jvmId: "graalvm-community:24"
-nativeImageOptions: [["--no-fallback"]]
+nativeImageOptions: ["--no-fallback"]
diff --git a/example/scalalib/script/6-packaging/Bar.scala b/example/scalalib/script/6-packaging/Bar.scala
index 8fe18949de53..e56b76464801 100644
--- a/example/scalalib/script/6-packaging/Bar.scala
+++ b/example/scalalib/script/6-packaging/Bar.scala
@@ -1,6 +1,6 @@
 //| scalaVersion: 3.7.3
 //| jvmId: "graalvm-community:17"
-//| nativeImageOptions: [["--no-fallback"]]
+//| nativeImageOptions: ["--no-fallback"]
 @main def main() = {
   println("Hello World")
 }

From 4c9f723bdef59ec2e0903fcc3e72d0d6b53724dc Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sat, 8 Nov 2025 17:49:24 +0100
Subject: [PATCH 32/67] fix test

---
 .../kotlinlib/module/5-compilation-execution-flags/build.mill   | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/example/kotlinlib/module/5-compilation-execution-flags/build.mill b/example/kotlinlib/module/5-compilation-execution-flags/build.mill
index c1b268d34c2d..5acf64eca924 100644
--- a/example/kotlinlib/module/5-compilation-execution-flags/build.mill
+++ b/example/kotlinlib/module/5-compilation-execution-flags/build.mill
@@ -11,7 +11,7 @@ object `package` extends KotlinModule {
   def forkArgs = Opts("-Xmx4g", "-Dmy.jvm.property=hello")
   def forkEnv = Map("MY_ENV_VAR" -> opt"WORLD")
 
-  def kotlincOptions = super.kotlincOptions() ++ Opt("-Werror")
+  def kotlincOptions = super.kotlincOptions() ++ Opts("-Werror")
 }
 
 // You can pass flags to the Kotlin compiler via `kotlincOptions`.

From 1926d924884e7b57cbaa502d454391b84439e707 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sat, 8 Nov 2025 18:04:36 +0100
Subject: [PATCH 33/67] Use Opts for
 `ScalaNativeModule.native{Compile,Linking}Options`

---
 example/scalalib/native/4-common-config/build.mill  |  2 +-
 .../src/mill/scalanativelib/ScalaNativeModule.scala | 13 +++++++------
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/example/scalalib/native/4-common-config/build.mill b/example/scalalib/native/4-common-config/build.mill
index f34782f4e1cd..2eeac56e7f55 100644
--- a/example/scalalib/native/4-common-config/build.mill
+++ b/example/scalalib/native/4-common-config/build.mill
@@ -20,7 +20,7 @@ object `package` extends ScalaNativeModule {
   def nativeIncrementalCompilation: T[Boolean] = true
 
   // Set nativeLinkingOptions path to a directory named `target`.
-  def nativeLinkingOptions = Opts(opt"-L${moduleDir}/target")
+  def nativeLinkingOptions = Opts(opt"-L${moduleDir / "target"}")
 
   // Set nativeWorkdir directory to `newDir`
   def nativeWorkdir = Task.dest / "newDir"
diff --git a/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala b/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
index 5255b12c556b..a0e3b4f3c744 100644
--- a/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
+++ b/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
@@ -7,6 +7,7 @@ import mill.api.{CrossVersion, Result, TaskCtx}
 import mill.api.daemon.internal.bsp.ScalaBuildTarget
 import mill.javalib.api.JvmWorkerUtil
 import mill.api.daemon.internal.{ScalaNativeModuleApi, ScalaPlatform, internal}
+import mill.api.opt.*
 import mill.javalib.RunModule
 import mill.javalib.testrunner.{TestResult, TestRunner, TestRunnerUtils}
 import mill.scalalib.{Dep, DepSyntax, Lib, SbtModule, ScalaModule, TestModule}
@@ -177,13 +178,13 @@ trait ScalaNativeModule extends ScalaModule with ScalaNativeModuleApi { outer =>
   def nativeTarget: T[Option[String]] = Task { None }
 
   // Options that are passed to clang during compilation
-  def nativeCompileOptions = Task {
-    withScalaNativeBridge.apply().apply(_.discoverCompileOptions())
+  def nativeCompileOptions: T[Opts] = Task {
+    Opts(withScalaNativeBridge.apply().apply(_.discoverCompileOptions()))
   }
 
   // Options that are passed to clang during linking
-  def nativeLinkingOptions = Task {
-    withScalaNativeBridge.apply().apply(_.discoverLinkingOptions())
+  def nativeLinkingOptions: T[Opts] = Task {
+    Opts(withScalaNativeBridge.apply().apply(_.discoverLinkingOptions()))
   }
 
   // Whether to link `@stub` methods, or ignore them
@@ -245,8 +246,8 @@ trait ScalaNativeModule extends ScalaModule with ScalaNativeModuleApi { outer =>
       nativeClang().path.toIO,
       nativeClangPP().path.toIO,
       nativeTarget(),
-      nativeCompileOptions(),
-      nativeLinkingOptions(),
+      nativeCompileOptions().toStringSeq,
+      nativeLinkingOptions().toStringSeq,
       nativeGC(),
       nativeLinkStubs(),
       nativeLTO().value,

From 2c4d96e981f77738378446b54f97eae93493cf78 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sat, 8 Nov 2025 18:04:45 +0100
Subject: [PATCH 34/67] fix tests

---
 example/scalalib/publishing/8-native-image-libs/build.mill | 2 +-
 example/scalalib/spark/1-hello-spark/build.mill            | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/example/scalalib/publishing/8-native-image-libs/build.mill b/example/scalalib/publishing/8-native-image-libs/build.mill
index 345d347aa649..654343707ada 100644
--- a/example/scalalib/publishing/8-native-image-libs/build.mill
+++ b/example/scalalib/publishing/8-native-image-libs/build.mill
@@ -10,7 +10,7 @@ object foo extends ScalaModule, NativeImageModule {
 
   def nativeImageOptions = Opts("--no-fallback", "-Os")
 
-  def mvnDeps = Opts(
+  def mvnDeps = Seq(
     mvn"com.lihaoyi::scalatags:0.13.1",
     mvn"com.lihaoyi::mainargs:0.7.7"
   )
diff --git a/example/scalalib/spark/1-hello-spark/build.mill b/example/scalalib/spark/1-hello-spark/build.mill
index 854fed482c3e..8e3253f55f33 100644
--- a/example/scalalib/spark/1-hello-spark/build.mill
+++ b/example/scalalib/spark/1-hello-spark/build.mill
@@ -10,7 +10,7 @@ object foo extends ScalaModule {
     mvn"org.apache.spark::spark-sql:3.5.4"
   )
 
-  def forkArgs = Seq("--add-opens", "java.base/sun.nio.ch=ALL-UNNAMED")
+  def forkArgs = Opts("--add-opens", "java.base/sun.nio.ch=ALL-UNNAMED")
 
   object test extends ScalaTests {
     def mvnDeps = Seq(mvn"com.lihaoyi::utest:0.9.1")

From 5441806c867f10830462e20d34f91137913bde9d Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sat, 8 Nov 2025 18:40:33 +0100
Subject: [PATCH 35/67] fix tests

---
 .../plugins/2-plugin-integration-tests/build.mill          | 6 ++++--
 example/javalib/web/5-todo-micronaut/build.mill            | 2 +-
 example/large/multifile/11-helper-files/build.mill         | 6 ++++--
 example/large/multifile/11-helper-files/foo/package.mill   | 7 +++++--
 example/scalalib/spark/3-semi-realistic/build.mill         | 2 +-
 integration/ide/bsp-server/resources/project/build.mill    | 2 +-
 6 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/example/extending/plugins/2-plugin-integration-tests/build.mill b/example/extending/plugins/2-plugin-integration-tests/build.mill
index d058ad20b937..c61fce19a127 100644
--- a/example/extending/plugins/2-plugin-integration-tests/build.mill
+++ b/example/extending/plugins/2-plugin-integration-tests/build.mill
@@ -5,7 +5,9 @@
 // command. Integration tests require further build configuration, which is shown below:
 
 package build
+
 import mill.*, scalalib.*, publish.*
+import mill.api.opt.*
 import mill.util.BuildInfo.{millVersion, millBinPlatform}
 
 object myplugin extends ScalaModule, PublishModule {
@@ -22,8 +24,8 @@ object myplugin extends ScalaModule, PublishModule {
     def mvnDeps = Seq(mvn"com.lihaoyi::mill-testkit:$millVersion")
     def forkEnv = Task {
       Map(
-        "MILL_EXECUTABLE_PATH" -> millExecutable.assembly().path.toString,
-        "MILL_USER_TEST_REPO" -> publishLocalTestRepo().path.toString
+        "MILL_EXECUTABLE_PATH" -> Opt(millExecutable.assembly().path),
+        "MILL_USER_TEST_REPO" -> Opt(publishLocalTestRepo().path)
       )
     }
 
diff --git a/example/javalib/web/5-todo-micronaut/build.mill b/example/javalib/web/5-todo-micronaut/build.mill
index ecf307a75b8f..30d4767911b5 100644
--- a/example/javalib/web/5-todo-micronaut/build.mill
+++ b/example/javalib/web/5-todo-micronaut/build.mill
@@ -55,7 +55,7 @@ trait MicronautModule extends MavenModule {
   def javacOptions = Opts(
     OptGroup(
       "-processorpath",
-      Opt.mkPath(processors().map(_.path), ":")
+      Opt.mkPath(processors().map(_.path), sep = ":")
     ),
     "-parameters",
     "-Amicronaut.processing.incremental=true",
diff --git a/example/large/multifile/11-helper-files/build.mill b/example/large/multifile/11-helper-files/build.mill
index 4962a774c0b9..3932a0afe0a6 100644
--- a/example/large/multifile/11-helper-files/build.mill
+++ b/example/large/multifile/11-helper-files/build.mill
@@ -3,12 +3,14 @@
 // as your `build.mill` or a `package.mill`.
 
 package build
+
 import mill.*, scalalib.*
+import mill.api.opt.*
 
 object `package` extends MyModule {
   def forkEnv = Map(
-    "MY_SCALA_VERSION" -> build.scalaVersion(),
-    "MY_PROJECT_VERSION" -> foo.myProjectVersion
+    "MY_SCALA_VERSION" -> Opt(build.scalaVersion()),
+    "MY_PROJECT_VERSION" -> Opt(foo.myProjectVersion)
   )
 }
 
diff --git a/example/large/multifile/11-helper-files/foo/package.mill b/example/large/multifile/11-helper-files/foo/package.mill
index 6a701b2ce09e..cfc83be4b475 100644
--- a/example/large/multifile/11-helper-files/foo/package.mill
+++ b/example/large/multifile/11-helper-files/foo/package.mill
@@ -1,8 +1,11 @@
 package build.foo
+
 import mill.*, scalalib.*
+import mill.api.opt.*
+
 object `package` extends build.MyModule {
   def forkEnv = Map(
-    "MY_SCALA_VERSION" -> build.myScalaVersion,
-    "MY_PROJECT_VERSION" -> myProjectVersion
+    "MY_SCALA_VERSION" -> Opt(build.myScalaVersion),
+    "MY_PROJECT_VERSION" -> Opt(myProjectVersion)
   )
 }
diff --git a/example/scalalib/spark/3-semi-realistic/build.mill b/example/scalalib/spark/3-semi-realistic/build.mill
index a2cf1d6c54af..2220c4953313 100644
--- a/example/scalalib/spark/3-semi-realistic/build.mill
+++ b/example/scalalib/spark/3-semi-realistic/build.mill
@@ -18,7 +18,7 @@ object `package` extends ScalaModule {
     def mvnDeps = Seq(mvn"com.lihaoyi::utest:0.9.1")
     def testFramework = "utest.runner.Framework"
 
-    def forkArgs = Seq("--add-opens", "java.base/sun.nio.ch=ALL-UNNAMED")
+    def forkArgs = Opts("--add-opens", "java.base/sun.nio.ch=ALL-UNNAMED")
   }
 
 }
diff --git a/integration/ide/bsp-server/resources/project/build.mill b/integration/ide/bsp-server/resources/project/build.mill
index a553ca82d0bd..5de96d88844d 100644
--- a/integration/ide/bsp-server/resources/project/build.mill
+++ b/integration/ide/bsp-server/resources/project/build.mill
@@ -59,7 +59,7 @@ object app extends scalalib.JavaModule {
     mvn"com.mysql:mysql-connector-j:9.1.0"
   )
 
-  def forkEnv = Map("MY_ENV_VAR" -> "my-value")
+  def forkEnv = Map("MY_ENV_VAR" -> opt"my-value")
 
   object test extends JavaTests {
     def testFramework = "com.novocode.junit.JUnitFramework"

From 008972db35adb5aba14d2e0e3a25132d7d93c191 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 10:51:53 +0100
Subject: [PATCH 36/67] Move setDaemon calls to use-site

---
 libs/daemon/client/src/mill/client/FileToStreamTailer.java | 1 -
 runner/daemon/src/mill/daemon/TailManager.scala            | 3 +++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/libs/daemon/client/src/mill/client/FileToStreamTailer.java b/libs/daemon/client/src/mill/client/FileToStreamTailer.java
index 856172052c15..cf879fba1deb 100644
--- a/libs/daemon/client/src/mill/client/FileToStreamTailer.java
+++ b/libs/daemon/client/src/mill/client/FileToStreamTailer.java
@@ -22,7 +22,6 @@ public class FileToStreamTailer extends Thread implements AutoCloseable {
   public FileToStreamTailer(File file, PrintStream stream, int intervalMsec) {
     super("FileToStreamTailerThread");
     this.intervalMsec = intervalMsec;
-    setDaemon(true);
     this.file = file;
     this.stream = stream;
   }
diff --git a/runner/daemon/src/mill/daemon/TailManager.scala b/runner/daemon/src/mill/daemon/TailManager.scala
index aba119d35a53..f329b80f1ee3 100644
--- a/runner/daemon/src/mill/daemon/TailManager.scala
+++ b/runner/daemon/src/mill/daemon/TailManager.scala
@@ -29,6 +29,9 @@ class TailManager(daemonDir: os.Path) extends AutoCloseable {
     tailerRefreshIntervalMillis
   )
 
+  stdoutTailer.setDaemon(true)
+  stderrTailer.setDaemon(true)
+
   stdoutTailer.start()
   stderrTailer.start()
 

From 2e91883823d4388baeb7a528bc95e9328762dcfc Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 10:57:19 +0100
Subject: [PATCH 37/67] Fix potential logic error (used variable)

---
 .../mill/androidlib/databinding/AndroidDataBindingImpl.scala    | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libs/androidlib/databinding/impl/src/mill/androidlib/databinding/AndroidDataBindingImpl.scala b/libs/androidlib/databinding/impl/src/mill/androidlib/databinding/AndroidDataBindingImpl.scala
index 5389e89b5d1a..ab2830329767 100644
--- a/libs/androidlib/databinding/impl/src/mill/androidlib/databinding/AndroidDataBindingImpl.scala
+++ b/libs/androidlib/databinding/impl/src/mill/androidlib/databinding/AndroidDataBindingImpl.scala
@@ -83,7 +83,7 @@ class AndroidDataBindingImpl extends AndroidDataBindingWorker {
 
     private def toFile(canonicalName: String): File = {
       val asPath = canonicalName.replace('.', '/')
-      os.Path(s"base/${asPath}.java").toIO
+      os.Path(s"${base}/${asPath}.java").toIO
     }
 
     override def deleteFile(canonicalName: String): Unit = {

From dc065d57f3ceb131d8c987be8baf3f409341649e Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 11:05:03 +0100
Subject: [PATCH 38/67] Read and show a ignore reason from the
 `ignoreErrorsOnCI` file

---
 testkit/src/mill/testkit/UtestExampleTestSuite.scala | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/testkit/src/mill/testkit/UtestExampleTestSuite.scala b/testkit/src/mill/testkit/UtestExampleTestSuite.scala
index 8ed32891d271..5819d743001a 100644
--- a/testkit/src/mill/testkit/UtestExampleTestSuite.scala
+++ b/testkit/src/mill/testkit/UtestExampleTestSuite.scala
@@ -28,14 +28,16 @@ object UtestExampleTestSuite extends TestSuite {
           )
         }
 
+      val ignoreFile = workspaceSourcePath / "ignoreErrorsOnCI"
       val ignoreErrors = System.getenv("CI") != null &&
-        os.exists(workspaceSourcePath / "ignoreErrorsOnCI")
+        os.exists(ignoreFile)
       if (ignoreErrors)
         try run()
         catch {
           case _: TimeoutException =>
             System.err.println(
-              s"Found ignoreErrorsOnCI under $workspaceSourcePath, ignoring timeout exception"
+              s"Found ignoreErrorsOnCI under $workspaceSourcePath, ignoring timeout exception" +
+                Option(os.read(ignoreFile)).filter(_.isBlank).mkString("Reason: ", "", "")
             )
         }
       else

From ac90771c948ca5ab7c4bc21650f6807b3813ffe1 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 11:06:21 +0100
Subject: [PATCH 39/67] Ignore bin-incompat mill-scalafix example for now

---
 example/scalalib/config/3-linting/ignoreErrorsOnCI | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 example/scalalib/config/3-linting/ignoreErrorsOnCI

diff --git a/example/scalalib/config/3-linting/ignoreErrorsOnCI b/example/scalalib/config/3-linting/ignoreErrorsOnCI
new file mode 100644
index 000000000000..7b87248a8116
--- /dev/null
+++ b/example/scalalib/config/3-linting/ignoreErrorsOnCI
@@ -0,0 +1 @@
+mill-scalafix does not have a binary compatible release yet
\ No newline at end of file

From 288aa4e4b9172f3823915b2f126e4caa0b66e20c Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 13:33:43 +0100
Subject: [PATCH 40/67] Update expected test outcomes

---
 .../buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala  | 2 +-
 libs/init/gradle/test/resources/expected/gradle-6-0/build.mill  | 2 +-
 .../gradle/test/resources/expected/gradle-7-0/app/package.mill  | 2 +-
 libs/init/gradle/test/resources/expected/gradle-7-0/build.mill  | 2 +-
 .../gradle/test/resources/expected/gradle-7-0/list/package.mill | 2 +-
 .../expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala  | 2 +-
 .../test/resources/expected/gradle-7-0/utilities/package.mill   | 2 +-
 .../gradle/test/resources/expected/gradle-8-0/app/package.mill  | 2 +-
 libs/init/gradle/test/resources/expected/gradle-8-0/build.mill  | 2 +-
 .../gradle/test/resources/expected/gradle-8-0/list/package.mill | 2 +-
 .../expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala  | 2 +-
 .../test/resources/expected/gradle-8-0/utilities/package.mill   | 2 +-
 .../test/resources/expected/gradle-9-0-0/app/package.mill       | 2 +-
 .../init/gradle/test/resources/expected/gradle-9-0-0/build.mill | 2 +-
 .../test/resources/expected/gradle-9-0-0/list/package.mill      | 2 +-
 .../gradle-9-0-0/mill-build/src/ProjectBaseModule.scala         | 2 +-
 .../gradle-9-0-0/mill-build/src/ProjectPublishModule.scala      | 2 +-
 .../test/resources/expected/gradle-9-0-0/utilities/package.mill | 2 +-
 .../test/resources/expected/with-args/gradle-8-0/build.mill     | 2 +-
 .../init/maven/test/resources/expected/maven-samples/build.mill | 2 +-
 .../maven-samples/mill-build/src/MavenSamplesBaseModule.scala   | 2 +-
 .../resources/expected/maven-samples/multi-module/package.mill  | 2 +-
 .../expected/maven-samples/multi-module/server/package.mill     | 2 +-
 .../expected/maven-samples/multi-module/webapp/package.mill     | 2 +-
 .../resources/expected/maven-samples/single-module/package.mill | 2 +-
 libs/init/maven/test/resources/expected/quickstart/build.mill   | 2 +-
 libs/init/maven/test/resources/expected/spring-start/build.mill | 2 +-
 .../test/resources/expected/with-args/maven-samples/build.mill  | 2 +-
 .../test/resources/expected/with-args/quickstart/build.mill     | 2 +-
 libs/init/sbt/test/resources/expected/cross-version/build.mill  | 2 +-
 .../resources/expected/sbt-multi-project-example/build.mill     | 2 +-
 .../expected/sbt-multi-project-example/common/package.mill      | 2 +-
 .../mill-build/src/SbtMultiProjectExampleBaseModule.scala       | 2 +-
 .../expected/sbt-multi-project-example/multi1/package.mill      | 2 +-
 .../expected/sbt-multi-project-example/multi2/package.mill      | 2 +-
 .../sbt-multi-project-example/nested/nested/package.mill        | 2 +-
 .../expected/sbt-multi-project-example/nested/package.mill      | 2 +-
 .../sbt/test/resources/expected/scala-seed-project/build.mill   | 2 +-
 38 files changed, 38 insertions(+), 38 deletions(-)

diff --git a/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala b/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala
index 580fffd79a97..814eb1c778ee 100644
--- a/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala
+++ b/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala
@@ -16,7 +16,7 @@ class BuildGenChecker(sourceRoot: os.Path, scalafmtConfigFile: os.Path) {
       generate: => Unit,
       sourceRel: os.SubPath,
       expectedRel: os.SubPath,
-      updateSnapshots: Boolean = false // pass true to update test data on disk
+      updateSnapshots: Boolean = sys.env.getOrElse("UTEST_UPDATE_GOLDEN_TESTS", "0").equals("1") // pass true to update test data on disk
   )(using
       tp: TestPath
   ): Boolean = {
diff --git a/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill b/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill
index 8316a7b8af8c..f882ae36d47a 100644
--- a/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-6-0/build.mill
@@ -2,7 +2,7 @@
 package build
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/app/package.mill b/libs/init/gradle/test/resources/expected/gradle-7-0/app/package.mill
index 4c98e7e89b8f..8ccbf444bf97 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/app/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/app/package.mill
@@ -1,7 +1,7 @@
 package build.app
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/build.mill b/libs/init/gradle/test/resources/expected/gradle-7-0/build.mill
index 7dbc474fa239..cb36ea2ea721 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/build.mill
@@ -3,6 +3,6 @@ package build
 
 import mill.*
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 
 object `package` extends Module {}
diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/list/package.mill b/libs/init/gradle/test/resources/expected/gradle-7-0/list/package.mill
index 8f4d7e36d5b1..450d3dfe752d 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/list/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/list/package.mill
@@ -1,7 +1,7 @@
 package build.list
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala b/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala
index cf5cf15e8f6c..9006e19bb870 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/mill-build/src/ProjectBaseModule.scala
@@ -1,7 +1,7 @@
 package millbuild
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 
 trait ProjectBaseModule extends MavenModule {
diff --git a/libs/init/gradle/test/resources/expected/gradle-7-0/utilities/package.mill b/libs/init/gradle/test/resources/expected/gradle-7-0/utilities/package.mill
index 649cd0be4c57..de2132d9798c 100644
--- a/libs/init/gradle/test/resources/expected/gradle-7-0/utilities/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-7-0/utilities/package.mill
@@ -1,7 +1,7 @@
 package build.utilities
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/app/package.mill b/libs/init/gradle/test/resources/expected/gradle-8-0/app/package.mill
index 7a3c062e24b5..b21421a5222f 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/app/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/app/package.mill
@@ -1,7 +1,7 @@
 package build.app
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/build.mill b/libs/init/gradle/test/resources/expected/gradle-8-0/build.mill
index 7dbc474fa239..cb36ea2ea721 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/build.mill
@@ -3,6 +3,6 @@ package build
 
 import mill.*
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 
 object `package` extends Module {}
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill b/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill
index b2ebb0a17310..4639e3c38896 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/list/package.mill
@@ -1,7 +1,7 @@
 package build.list
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.errorprone.*
 import millbuild.*
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala b/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala
index cf5cf15e8f6c..9006e19bb870 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/mill-build/src/ProjectBaseModule.scala
@@ -1,7 +1,7 @@
 package millbuild
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 
 trait ProjectBaseModule extends MavenModule {
diff --git a/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill b/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill
index 444b7517dfbd..fcd7f7aa675b 100644
--- a/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-8-0/utilities/package.mill
@@ -1,7 +1,7 @@
 package build.utilities
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.errorprone.*
 import millbuild.*
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/app/package.mill b/libs/init/gradle/test/resources/expected/gradle-9-0-0/app/package.mill
index 7d79151c9bdc..148fb81209d6 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/app/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/app/package.mill
@@ -1,7 +1,7 @@
 package build.app
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import millbuild.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/build.mill b/libs/init/gradle/test/resources/expected/gradle-9-0-0/build.mill
index 13a2db102d16..74ee3455e8b0 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/build.mill
@@ -4,6 +4,6 @@ package build
 
 import mill.*
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 
 object `package` extends Module {}
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/list/package.mill b/libs/init/gradle/test/resources/expected/gradle-9-0-0/list/package.mill
index f255608b29e8..893529fc4077 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/list/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/list/package.mill
@@ -1,7 +1,7 @@
 package build.list
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala b/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala
index 61164d62d78b..c6c7787b2f85 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectBaseModule.scala
@@ -1,7 +1,7 @@
 package millbuild
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 
 trait ProjectBaseModule extends MavenModule {
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectPublishModule.scala b/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectPublishModule.scala
index 7bfc0715ca4a..5a40117a4893 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectPublishModule.scala
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/mill-build/src/ProjectPublishModule.scala
@@ -1,7 +1,7 @@
 package millbuild
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 
diff --git a/libs/init/gradle/test/resources/expected/gradle-9-0-0/utilities/package.mill b/libs/init/gradle/test/resources/expected/gradle-9-0-0/utilities/package.mill
index 058c8926a0f9..e3c88690edaa 100644
--- a/libs/init/gradle/test/resources/expected/gradle-9-0-0/utilities/package.mill
+++ b/libs/init/gradle/test/resources/expected/gradle-9-0-0/utilities/package.mill
@@ -1,7 +1,7 @@
 package build.utilities
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill b/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill
index 4ffd8d096d8a..67cbb7bd8ef2 100644
--- a/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill
+++ b/libs/init/gradle/test/resources/expected/with-args/gradle-8-0/build.mill
@@ -3,7 +3,7 @@ package build
 
 import mill.*
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.errorprone.*
 
diff --git a/libs/init/maven/test/resources/expected/maven-samples/build.mill b/libs/init/maven/test/resources/expected/maven-samples/build.mill
index a937819422ac..397d2b4624d1 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/build.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/build.mill
@@ -2,7 +2,7 @@
 package build
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/maven-samples/mill-build/src/MavenSamplesBaseModule.scala b/libs/init/maven/test/resources/expected/maven-samples/mill-build/src/MavenSamplesBaseModule.scala
index e4cf622b96ab..e6179eec98f2 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/mill-build/src/MavenSamplesBaseModule.scala
+++ b/libs/init/maven/test/resources/expected/maven-samples/mill-build/src/MavenSamplesBaseModule.scala
@@ -1,7 +1,7 @@
 package millbuild
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 
diff --git a/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill b/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill
index 41eb89d68f24..3dd952c0c7e0 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/multi-module/package.mill
@@ -1,7 +1,7 @@
 package build.`multi-module`
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill b/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill
index 957a2a20f442..b379d8b3ea96 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/multi-module/server/package.mill
@@ -1,7 +1,7 @@
 package build.`multi-module`.server
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill b/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill
index 38151735a17d..ed1321f53229 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/multi-module/webapp/package.mill
@@ -1,7 +1,7 @@
 package build.`multi-module`.webapp
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill b/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill
index ab5ddd26937d..0fb8eaeebfb3 100644
--- a/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill
+++ b/libs/init/maven/test/resources/expected/maven-samples/single-module/package.mill
@@ -1,7 +1,7 @@
 package build.`single-module`
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/quickstart/build.mill b/libs/init/maven/test/resources/expected/quickstart/build.mill
index 62ce7136f3a9..472b4c5b0ba1 100644
--- a/libs/init/maven/test/resources/expected/quickstart/build.mill
+++ b/libs/init/maven/test/resources/expected/quickstart/build.mill
@@ -2,7 +2,7 @@
 package build
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.errorprone.*
 import mill.javalib.publish.*
diff --git a/libs/init/maven/test/resources/expected/spring-start/build.mill b/libs/init/maven/test/resources/expected/spring-start/build.mill
index 421b11487658..75aa1eb5b290 100644
--- a/libs/init/maven/test/resources/expected/spring-start/build.mill
+++ b/libs/init/maven/test/resources/expected/spring-start/build.mill
@@ -2,7 +2,7 @@
 package build
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import millbuild.*
diff --git a/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill b/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill
index a25a11501ea7..a20df9557534 100644
--- a/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill
+++ b/libs/init/maven/test/resources/expected/with-args/maven-samples/build.mill
@@ -2,7 +2,7 @@
 package build
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 
diff --git a/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill b/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill
index cc05bdfae586..3347cdf925a3 100644
--- a/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill
+++ b/libs/init/maven/test/resources/expected/with-args/quickstart/build.mill
@@ -2,7 +2,7 @@
 package build
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.errorprone.*
 import mill.javalib.publish.*
diff --git a/libs/init/sbt/test/resources/expected/cross-version/build.mill b/libs/init/sbt/test/resources/expected/cross-version/build.mill
index 32dd2a646f82..e0857503f96c 100644
--- a/libs/init/sbt/test/resources/expected/cross-version/build.mill
+++ b/libs/init/sbt/test/resources/expected/cross-version/build.mill
@@ -4,7 +4,7 @@ package build
 
 import mill.*
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/build.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/build.mill
index 7dbc474fa239..cb36ea2ea721 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/build.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/build.mill
@@ -3,6 +3,6 @@ package build
 
 import mill.*
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 
 object `package` extends Module {}
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill
index 70b067d0d497..1d8071e6931f 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/common/package.mill
@@ -1,7 +1,7 @@
 package build.common
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala
index a37140cd775b..7ef6be3b7d17 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/mill-build/src/SbtMultiProjectExampleBaseModule.scala
@@ -1,7 +1,7 @@
 package millbuild
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill
index 9ff351267bcf..a32677d07517 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi1/package.mill
@@ -1,7 +1,7 @@
 package build.multi1
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi2/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi2/package.mill
index 646a226255b9..b3f2653397ba 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi2/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/multi2/package.mill
@@ -1,7 +1,7 @@
 package build.multi2
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill
index c7380175478a..7fab989b1d99 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/nested/package.mill
@@ -1,7 +1,7 @@
 package build.nested.nested
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/package.mill b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/package.mill
index 3aa5969e613d..bccecf3d1d4a 100644
--- a/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/package.mill
+++ b/libs/init/sbt/test/resources/expected/sbt-multi-project-example/nested/package.mill
@@ -2,6 +2,6 @@ package build.nested
 
 import mill.*
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 
 object `package` extends Module {}
diff --git a/libs/init/sbt/test/resources/expected/scala-seed-project/build.mill b/libs/init/sbt/test/resources/expected/scala-seed-project/build.mill
index 5fcef3a10116..f7a63203aa09 100644
--- a/libs/init/sbt/test/resources/expected/scala-seed-project/build.mill
+++ b/libs/init/sbt/test/resources/expected/scala-seed-project/build.mill
@@ -2,7 +2,7 @@
 package build
 
 import mill.api.*
-import mill.api.opts.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*

From 59b5e1c5cb7b9c0b75bf54eed1ab0814f6438354 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 15:12:29 +0100
Subject: [PATCH 41/67] Test fixes

---
 example/thirdparty/arrow/build.mill   | 17 +++++++++--------
 example/thirdparty/mockito/build.mill |  2 +-
 example/thirdparty/netty/build.mill   |  4 ++--
 3 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/example/thirdparty/arrow/build.mill b/example/thirdparty/arrow/build.mill
index a18923d6375b..25e73e1978af 100644
--- a/example/thirdparty/arrow/build.mill
+++ b/example/thirdparty/arrow/build.mill
@@ -3,6 +3,7 @@ package build
 import mill.*, kotlinlib.*, kotlinlib.js.*
 import mill.scalalib.CoursierModule
 import mill.api.Result
+import mill.api.opt.*
 import mill.javalib.testrunner.TestResult
 import mill.javalib.api.CompilationResult
 import mill.kotlinlib.kover.KoverModule
@@ -113,7 +114,7 @@ object `package` extends Module {
 
     def kotlinExplicitApi: T[Boolean] = true
 
-    protected def commonSourcesArg(sources: Seq[PathRef], commonSourcesDirName: String): String = {
+    protected def commonSourcesArg(sources: Seq[PathRef], commonSourcesDirName: String): Opt = {
       val files =
         sources
           .flatMap(pr => os.walk(pr.path))
@@ -123,7 +124,7 @@ object `package` extends Module {
       // This is allowed with `-Xcommon-sources` flag, but probably will be prohibited in the future.
       // Ideally, it should be in the [[KotlinModule]] directly, but this implies [[KotlinModule]] should know where
       // common sources are, making some assumptions about layout.
-      s"-Xcommon-sources=${files.mkString(",")}"
+      Opt.mkPath(files, sep = ",", prefix = "-Xcommon-sources=")
     }
 
     trait ArrowPlatformModule extends KotlinMavenModule, PlatformKotlinModule { outer =>
@@ -148,7 +149,7 @@ object `package` extends Module {
 
       override def kotlincOptions =
         super.kotlincOptions() ++
-          Seq("-Xexpect-actual-classes", commonSourcesArg(sources(), "commonMain"))
+          Opts("-Xexpect-actual-classes", commonSourcesArg(sources(), "commonMain"))
 
       trait ArrowPlatformTests extends KotlinMavenTests {
 
@@ -252,7 +253,7 @@ object `package` extends Module {
               libraries.coroutinesTestJvm,
               libraries.kotestAssertionsCoreJvm
             )
-            def kotlincOptions = super.kotlincOptions() ++ Seq("-Xcontext-receivers")
+            def kotlincOptions = super.kotlincOptions() ++ Opts("-Xcontext-receivers")
           }
         }
         object js extends ArrowPlatformJsModule {
@@ -354,7 +355,7 @@ object `package` extends Module {
         object jvm extends ArrowPlatformJvmModule {
           def moduleDeps = super.moduleDeps ++ Seq(`arrow-atomic`.jvm, `arrow-annotations`.jvm)
           def mvnDeps = super.mvnDeps() ++ Seq(libraries.kotlinxSerializationCoreJvm)
-          def kotlincOptions = super.kotlincOptions() ++ Seq("-Xcontext-receivers")
+          def kotlincOptions = super.kotlincOptions() ++ Opts("-Xcontext-receivers")
           object test extends ArrowPlatformJvmTests {
             def moduleDeps = super.moduleDeps ++ Seq(fx.`arrow-fx-coroutines`.jvm)
 
@@ -521,7 +522,7 @@ object `package` extends Module {
             def testTimeout = 60_000L
           }
         }
-        val additionalKotlincOptions = Seq("-Xconsistent-data-class-copy-visibility")
+        val additionalKotlincOptions = Opts("-Xconsistent-data-class-copy-visibility")
 
       }
     }
@@ -569,7 +570,7 @@ object `package` extends Module {
           def moduleDeps = super.moduleDeps ++ Seq(core.`arrow-core`.jvm)
           object test extends ArrowPlatformJvmTests {
             def kotlincOptions =
-              super.kotlincOptions() ++ Seq(commonSourcesArg(jvm.test.sources(), "commonTest"))
+              super.kotlincOptions() ++ Opts(commonSourcesArg(jvm.test.sources(), "commonTest"))
             def mvnDeps = super.mvnDeps() ++ Seq(
               libraries.kotlinTestJunit5,
               libraries.coroutinesTestJvm,
@@ -582,7 +583,7 @@ object `package` extends Module {
           def moduleDeps = super.moduleDeps ++ Seq(core.`arrow-core`.js)
           object test extends ArrowPlatformJsTests {
             def kotlincOptions =
-              super.kotlincOptions() ++ Seq(commonSourcesArg(js.test.sources(), "commonTest"))
+              super.kotlincOptions() ++ Opts(commonSourcesArg(js.test.sources(), "commonTest"))
             def mvnDeps = super.mvnDeps() ++ Seq(
               libraries.coroutinesTestJs,
               libraries.kotestAssertionsCoreJs,
diff --git a/example/thirdparty/mockito/build.mill b/example/thirdparty/mockito/build.mill
index bbf8c2adb8b5..8f654127ab14 100644
--- a/example/thirdparty/mockito/build.mill
+++ b/example/thirdparty/mockito/build.mill
@@ -46,7 +46,7 @@ trait MockitoModule extends MavenModule {
   def testMvnDeps: T[Seq[Dep]] = Seq()
   def testRuntimeMvnDeps: T[Seq[Dep]] = Seq()
   def testFramework = "com.novocode.junit.JUnitFramework"
-  def testForkArgs: T[Opts] = Seq()
+  def testForkArgs: T[Opts] = Opts()
 
   def testFilteredSources: T[Seq[os.Path]] = Task { Seq() }
 
diff --git a/example/thirdparty/netty/build.mill b/example/thirdparty/netty/build.mill
index a3502af2fd5f..f4ad93883229 100644
--- a/example/thirdparty/netty/build.mill
+++ b/example/thirdparty/netty/build.mill
@@ -54,7 +54,7 @@ trait NettyBaseTestSuiteModule extends NettyBaseModule, TestModule.Junit5 {
     mvn"org.bouncycastle:bctls-jdk15on:1.69"
   )
 
-  def forkArgs = Seq(
+  def forkArgs = Opts(
     "-DnativeImage.handlerMetadataGroupId=io.netty",
     "-Dio.netty.bootstrap.extensions=serviceload",
     "-XX:+AllowRedefinitionToAddDeleteMethods",
@@ -82,7 +82,7 @@ trait NettyModule extends NettyBaseModule {
     def moduleDeps = super.moduleDeps ++ testModuleDeps
     def mvnDeps = super.mvnDeps() ++ testMvnDeps()
     def forkWorkingDir = NettyModule.this.moduleDir
-    def forkArgs = super.forkArgs() ++ Seq(
+    def forkArgs = super.forkArgs() ++ Opts(
       "-Dnativeimage.handlerMetadataArtifactId=netty-" + NettyModule.this.moduleSegments.last.value
     )
 

From 1d4c16086ff35e10f69ecb05e8ae47cd46570dc1 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 15:53:39 +0100
Subject: [PATCH 42/67] Added `mkPath` to test case

---
 core/api/test/src/mill/api/opt/OptsTests.scala | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/core/api/test/src/mill/api/opt/OptsTests.scala b/core/api/test/src/mill/api/opt/OptsTests.scala
index 8a56eeef3bb0..4b8639fc144b 100644
--- a/core/api/test/src/mill/api/opt/OptsTests.scala
+++ b/core/api/test/src/mill/api/opt/OptsTests.scala
@@ -35,7 +35,8 @@ class OptsTests extends TestSuite {
     // some files as ArgGroup
     OptGroup(sources2*),
     // Mixed ArgGroup
-    OptGroup("--extra", opt"-Xplugin=${plugin1}") ++ OptGroup(sources1*)
+    OptGroup("--extra", opt"-Xplugin=${plugin1}") ++ OptGroup(sources1*),
+    Opt.mkPath(sources1, sep = ":", prefix = "-special-files=")
   )
 
   val expectedOpts1 = Opts(
@@ -55,6 +56,9 @@ class OptsTests extends TestSuite {
       Opt("-Xplugin=", plugin1),
       Opt(srcDir1),
       Opt(srcDir2)
+    ),
+    OptGroup(
+      Opt.mkPath(sources1, sep = ":", prefix = "-special-files=")
     )
   )
 
@@ -114,7 +118,7 @@ class OptsTests extends TestSuite {
 
         assertGoldenLiteral(
           json,
-          "[[\"-deprecation\"],[\"-verbose\"],[\"--release\",\"17\"],[[\"-Xplugin=\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}]],[[\"-Xplugin:\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}]],[[{\"path\":\"/tmp/opts-test/work/src1\"}]],[[{\"path\":\"/tmp/opts-test/work/src2\"}]],[[{\"path\":\"/tmp/opts-test/work/src3\"}],[{\"path\":\"/tmp/opts-test/work/src4\"}]],[\"--extra\",[\"-Xplugin=\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}],[{\"path\":\"/tmp/opts-test/work/src1\"}],[{\"path\":\"/tmp/opts-test/work/src2\"}]]]"
+          "[[\"-deprecation\"],[\"-verbose\"],[\"--release\",\"17\"],[[\"-Xplugin=\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}]],[[\"-Xplugin:\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}]],[[{\"path\":\"/tmp/opts-test/work/src1\"}]],[[{\"path\":\"/tmp/opts-test/work/src2\"}]],[[{\"path\":\"/tmp/opts-test/work/src3\"}],[{\"path\":\"/tmp/opts-test/work/src4\"}]],[\"--extra\",[\"-Xplugin=\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}],[{\"path\":\"/tmp/opts-test/work/src1\"}],[{\"path\":\"/tmp/opts-test/work/src2\"}]],[[\"-special-files=\",{\"path\":\"/tmp/opts-test/work/src1\"},\":\",{\"path\":\"/tmp/opts-test/work/src2\"}]]]"
         )
 
         assert(json.split("\\Q$HOME\\E").size == 1 + 0)

From 63f1abf9392ee15b301b8df1b25df02993a6a1ba Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 16:05:03 +0100
Subject: [PATCH 43/67] fix tests

---
 example/thirdparty/android-compose-samples/build.mill     | 4 ++--
 .../ide/bsp-server/resources/snapshots/diagnostics.json   | 8 ++++----
 .../invalidation/multi-level-editing/resources/build.mill | 3 ++-
 3 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/example/thirdparty/android-compose-samples/build.mill b/example/thirdparty/android-compose-samples/build.mill
index 84d8d6c5e6f8..38645bda3e3f 100644
--- a/example/thirdparty/android-compose-samples/build.mill
+++ b/example/thirdparty/android-compose-samples/build.mill
@@ -35,7 +35,7 @@ object JetLagged extends mill.api.Module {
 
     override def androidApplicationId = "com.example.jetlagged"
 
-    override def kotlincOptions = super.kotlincOptions() ++ Seq(
+    override def kotlincOptions = super.kotlincOptions() ++ Opts(
       "-jvm-target",
       "17"
     )
@@ -142,7 +142,7 @@ object JetNews extends mill.api.Module {
 
     override def androidApplicationId = "com.example.jetnews"
 
-    override def kotlincOptions = super.kotlincOptions() ++ Seq(
+    override def kotlincOptions = super.kotlincOptions() ++ Opts(
       "-jvm-target",
       "17"
     )
diff --git a/integration/ide/bsp-server/resources/snapshots/diagnostics.json b/integration/ide/bsp-server/resources/snapshots/diagnostics.json
index bd6e771f2830..626ff49c91b2 100644
--- a/integration/ide/bsp-server/resources/snapshots/diagnostics.json
+++ b/integration/ide/bsp-server/resources/snapshots/diagnostics.json
@@ -22,11 +22,11 @@
       {
         "range": {
           "start": {
-            "line": 101,
+            "line": 102,
             "character": 17
           },
           "end": {
-            "line": 101,
+            "line": 102,
             "character": 22
           }
         },
@@ -52,11 +52,11 @@
       {
         "range": {
           "start": {
-            "line": 101,
+            "line": 102,
             "character": 17
           },
           "end": {
-            "line": 101,
+            "line": 102,
             "character": 22
           }
         },
diff --git a/integration/invalidation/multi-level-editing/resources/build.mill b/integration/invalidation/multi-level-editing/resources/build.mill
index 95ee301ebe79..80add7536aa9 100644
--- a/integration/invalidation/multi-level-editing/resources/build.mill
+++ b/integration/invalidation/multi-level-editing/resources/build.mill
@@ -1,10 +1,11 @@
 package build
 
 import mill._, scalalib._
+import mill.api.opt.*
 import scalatags.Text.all._
 
 object foo extends JavaModule {
   def forkEnv = Map(
-    "snippet" -> frag(h1("hello"), p("world"), p(constant.Constant.scalatagsVersion)).render
+    "snippet" -> Opt(frag(h1("hello"), p("world"), p(constant.Constant.scalatagsVersion)).render)
   )
 }

From bf8723b19f0dc938969a4f29ccc2132ed071a14d Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 18:51:10 +0100
Subject: [PATCH 44/67] .

---
 core/api/test/src/mill/api/opt/OptsTests.scala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/core/api/test/src/mill/api/opt/OptsTests.scala b/core/api/test/src/mill/api/opt/OptsTests.scala
index 4b8639fc144b..dd08c7949b6d 100644
--- a/core/api/test/src/mill/api/opt/OptsTests.scala
+++ b/core/api/test/src/mill/api/opt/OptsTests.scala
@@ -79,7 +79,8 @@ class OptsTests extends TestSuite {
       "--extra",
       s"-Xplugin=${plugin1.toString()}"
     ) ++
-    sources1.map(_.toString())
+    sources1.map(_.toString()) ++
+    Seq(sources1.mkString("-special-files=", ":", ""))
 
   override def tests: Tests = Tests {
     test("OptGroup.isEmpty") {

From 2ec818981fd5399ee2acbaca05faf67bb66ba2c1 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 19:58:36 +0100
Subject: [PATCH 45/67] Futher minimize json serialization

---
 core/api/src/mill/api/opt/Opts.scala           | 11 +++++++++--
 core/api/test/src/mill/api/opt/OptsTests.scala |  2 +-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/core/api/src/mill/api/opt/Opts.scala b/core/api/src/mill/api/opt/Opts.scala
index 5611457fceb3..357745dac25c 100644
--- a/core/api/src/mill/api/opt/Opts.scala
+++ b/core/api/src/mill/api/opt/Opts.scala
@@ -53,10 +53,17 @@ object Opts {
 //    )
 
   given jsonReadWriter: upickle.ReadWriter[Opts] =
-    upickle.readwriter[ujson.Value].bimap(
+    upickle.readwriter[ujson.Arr].bimap(
       { opts =>
         // We always serialize as a seq of groups
-        upickle.transform(opts.value).to[ujson.Value]
+        opts.value.map { group =>
+          if(group.size == 1 && !group.head.containsPaths) {
+            ujson.Str(group.head.toString())
+          }
+          else
+            upickle.transform(group).to[ujson.Value]
+        }
+//        upickle.transform(opts.value).to[ujson.Value]
       },
       {
         case arr: ujson.Arr =>
diff --git a/core/api/test/src/mill/api/opt/OptsTests.scala b/core/api/test/src/mill/api/opt/OptsTests.scala
index dd08c7949b6d..bd9cb196bab0 100644
--- a/core/api/test/src/mill/api/opt/OptsTests.scala
+++ b/core/api/test/src/mill/api/opt/OptsTests.scala
@@ -119,7 +119,7 @@ class OptsTests extends TestSuite {
 
         assertGoldenLiteral(
           json,
-          "[[\"-deprecation\"],[\"-verbose\"],[\"--release\",\"17\"],[[\"-Xplugin=\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}]],[[\"-Xplugin:\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}]],[[{\"path\":\"/tmp/opts-test/work/src1\"}]],[[{\"path\":\"/tmp/opts-test/work/src2\"}]],[[{\"path\":\"/tmp/opts-test/work/src3\"}],[{\"path\":\"/tmp/opts-test/work/src4\"}]],[\"--extra\",[\"-Xplugin=\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}],[{\"path\":\"/tmp/opts-test/work/src1\"}],[{\"path\":\"/tmp/opts-test/work/src2\"}]],[[\"-special-files=\",{\"path\":\"/tmp/opts-test/work/src1\"},\":\",{\"path\":\"/tmp/opts-test/work/src2\"}]]]"
+          "[\"-deprecation\",\"-verbose\",[\"--release\",\"17\"],[[\"-Xplugin=\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}]],[[\"-Xplugin:\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}]],[[{\"path\":\"/tmp/opts-test/work/src1\"}]],[[{\"path\":\"/tmp/opts-test/work/src2\"}]],[[{\"path\":\"/tmp/opts-test/work/src3\"}],[{\"path\":\"/tmp/opts-test/work/src4\"}]],[\"--extra\",[\"-Xplugin=\",{\"path\":\"/tmp/opts-test/.cache/plugin1\"}],[{\"path\":\"/tmp/opts-test/work/src1\"}],[{\"path\":\"/tmp/opts-test/work/src2\"}]],[[\"-special-files=\",{\"path\":\"/tmp/opts-test/work/src1\"},\":\",{\"path\":\"/tmp/opts-test/work/src2\"}]]]"
         )
 
         assert(json.split("\\Q$HOME\\E").size == 1 + 0)

From 5085667be131183d3b1818cdd0e018b5618ccf3a Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Sun, 9 Nov 2025 21:08:30 +0100
Subject: [PATCH 46/67] .

---
 example/thirdparty/commons-io/build.mill | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/example/thirdparty/commons-io/build.mill b/example/thirdparty/commons-io/build.mill
index 7d2705f73fa8..4f6812fda255 100644
--- a/example/thirdparty/commons-io/build.mill
+++ b/example/thirdparty/commons-io/build.mill
@@ -7,7 +7,7 @@ import contrib.jmh.JmhModule
 
 object `package` extends PublishModule, MavenModule {
   def jvmId = "temurin:11.0.24"
-  def javacOptions = Seq("-encoding", "UTF-8")
+  def javacOptions = Opts("-encoding", "UTF-8")
   def publishVersion = "2.17.0-SNAPSHOT"
 
   def pomSettings = PomSettings(

From 4ee23218e9edffc311ce4a75dba29e2cf3eacb85 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Mon, 24 Nov 2025 17:22:27 +0100
Subject: [PATCH 47/67] Fix merge errors

---
 libs/javalib/src/mill/javalib/JavaModule.scala  | 17 ++++++++++-------
 .../src/mill/javalib/bsp/BspRunModule.scala     |  4 ++--
 .../javalib/spring/boot/SpringBootModule.scala  |  7 ++++---
 .../checkstyle/CheckstyleModuleTest.scala       |  3 ++-
 .../src/mill/scalalib/ScalaModule.scala         |  2 +-
 5 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/libs/javalib/src/mill/javalib/JavaModule.scala b/libs/javalib/src/mill/javalib/JavaModule.scala
index ce017e2a71fa..291ce774bfc7 100644
--- a/libs/javalib/src/mill/javalib/JavaModule.scala
+++ b/libs/javalib/src/mill/javalib/JavaModule.scala
@@ -298,10 +298,10 @@ trait JavaModule
     if (annotationProcessorsMvnDeps().nonEmpty)
       Opts(
         "-processorpath",
-        Opt.mkPath(annotationProcessorsResolvedMvnDeps(), sep = File.pathSeparator)
+        Opt.mkPath(annotationProcessorsResolvedMvnDeps().map(_.path), sep = File.pathSeparator)
       )
     else
-      Seq.empty
+      Opts()
 
   }
 
@@ -905,11 +905,14 @@ trait JavaModule
       os.makeDir.all(compileGenSources)
     }
 
-    val jOpts = JavaCompilerOptions.split(Seq(
-      "-s",
-      compileGenSources.toString
-    ) ++ javacOptions().toStringSeq ++ mandatoryJavacOptions().toStringSeq ++ annotationProcessorsJavacOptions())
-
+    val jOpts = JavaCompilerOptions.split(
+      Opts(
+        OptGroup("-s", compileGenSources.toString),
+        javacOptions(),
+        mandatoryJavacOptions(),
+        annotationProcessorsJavacOptions()
+      ).toStringSeq
+    )
     val worker = jvmWorker().internalWorker()
 
     worker.apply(
diff --git a/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala b/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala
index ceddbbcddfc3..726d31b00947 100644
--- a/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala
+++ b/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala
@@ -21,9 +21,9 @@ private[mill] trait BspRunModule(runModule: RunModule) extends mill.api.Module {
 
     override private[mill] def bspJvmRunEnvironment: Task.Simple[(
         runClasspath: Seq[Path],
-        forkArgs: Seq[String],
+        forkArgs: Opts,
         forkWorkingDir: Path,
-        forkEnv: Map[String, String],
+        forkEnv: Map[String, Opt],
         mainClass: Option[String],
         localMainClasses: Seq[String]
     )] =
diff --git a/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala b/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala
index 4d30ef6cbe79..c8adfa858993 100644
--- a/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala
+++ b/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala
@@ -2,6 +2,7 @@ package mill.javalib.spring.boot
 
 import mill.{T, Task}
 import mill.api.{ModuleRef, PathRef}
+import mill.api.opt.*
 import mill.javalib.{Dep, DepSyntax, JavaModule, NativeImageModule}
 
 import java.util.Properties
@@ -150,7 +151,7 @@ trait SpringBootModule extends JavaModule {
     /**
      * Enables AOT for running the application under this module
      */
-    override def forkArgs = super.forkArgs() ++ Seq("-Dspring.aot.enabled=true")
+    override def forkArgs = super.forkArgs() ++ Opts("-Dspring.aot.enabled=true")
 
     override def generatedSources: Task.Simple[Seq[PathRef]] = Task {
       val aotGeneratedSources = Seq(PathRef(outer.springBootProcessAOT().path / "sources"))
@@ -174,7 +175,7 @@ trait SpringBootModule extends JavaModule {
    * parent module as a native GraalVM application, provided the [[outer.springBootProcessAOT]] works.
    */
   trait NativeSpringBootBuildModule extends SpringBootOptimisedBuildModule, NativeImageModule {
-    override def nativeImageOptions: Task.Simple[Seq[String]] = Task {
+    override def nativeImageOptions: Task.Simple[Opts] = Task {
 
       val nativeImageArgs: Seq[String] =
         val nativeImageProps = outer.springBootAOTNativeProperties().path
@@ -188,7 +189,7 @@ trait SpringBootModule extends JavaModule {
         } else
           Seq.empty
 
-      nativeImageArgs
+      Opts(nativeImageArgs)
     }
   }
 
diff --git a/libs/javalib/test/src/mill/javalib/checkstyle/CheckstyleModuleTest.scala b/libs/javalib/test/src/mill/javalib/checkstyle/CheckstyleModuleTest.scala
index 6aae9114b9c8..9c5969f7e391 100644
--- a/libs/javalib/test/src/mill/javalib/checkstyle/CheckstyleModuleTest.scala
+++ b/libs/javalib/test/src/mill/javalib/checkstyle/CheckstyleModuleTest.scala
@@ -3,6 +3,7 @@ package mill.javalib.checkstyle
 import mill._
 import mainargs.Leftover
 import mill.api.Discover
+import mill.api.opt.*
 import mill.javalib.JavaModule
 import mill.testkit.{TestRootModule, UnitTester}
 import utest._
@@ -135,7 +136,7 @@ object CheckstyleModuleTest extends TestSuite {
 
     object module extends TestRootModule with JavaModule with CheckstyleModule {
       override def checkstyleFormat: T[String] = format
-      override def checkstyleOptions: T[Seq[String]] = options
+      override def checkstyleOptions: T[Opts] = Opts(options)
       override def checkstyleVersion: T[String] = version
       lazy val millDiscover = Discover[this.type]
     }
diff --git a/libs/scalalib/src/mill/scalalib/ScalaModule.scala b/libs/scalalib/src/mill/scalalib/ScalaModule.scala
index 1b7fe85cd932..cd68fed866ef 100644
--- a/libs/scalalib/src/mill/scalalib/ScalaModule.scala
+++ b/libs/scalalib/src/mill/scalalib/ScalaModule.scala
@@ -343,7 +343,7 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
             scalaOrganization(),
             scalaDocClasspath(),
             scalacPluginClasspath(),
-            options ++ compileCp ++ scalaDocOptions().toStringSeq ++ files.map(_.toString())
+            options ++ compileCp ++ scalaDocOptions().toStringSeq ++ files.map(_.toString()),
             workDir = Task.dest
           ),
           javaHome = javaHome().map(_.path)

From 505c29f7a6346aa6f5269547c5caf6f1a78ec0c0 Mon Sep 17 00:00:00 2001
From: "autofix-ci[bot]" <114827586+autofix-ci[bot]@users.noreply.github.com>
Date: Mon, 24 Nov 2025 16:32:05 +0000
Subject: [PATCH 48/67] [autofix.ci] apply automated fixes

---
 core/api/src/mill/api/opt/Opts.scala                         | 5 ++---
 example/scalalib/linting/3-acyclic/build.mill                | 1 -
 .../test/src/mill/main/buildgen/BuildGenChecker.scala        | 5 ++++-
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/core/api/src/mill/api/opt/Opts.scala b/core/api/src/mill/api/opt/Opts.scala
index 357745dac25c..f5ff05dafc71 100644
--- a/core/api/src/mill/api/opt/Opts.scala
+++ b/core/api/src/mill/api/opt/Opts.scala
@@ -57,10 +57,9 @@ object Opts {
       { opts =>
         // We always serialize as a seq of groups
         opts.value.map { group =>
-          if(group.size == 1 && !group.head.containsPaths) {
+          if (group.size == 1 && !group.head.containsPaths) {
             ujson.Str(group.head.toString())
-          }
-          else
+          } else
             upickle.transform(group).to[ujson.Value]
         }
 //        upickle.transform(opts.value).to[ujson.Value]
diff --git a/example/scalalib/linting/3-acyclic/build.mill b/example/scalalib/linting/3-acyclic/build.mill
index d337442bb9b2..52c1113179cc 100644
--- a/example/scalalib/linting/3-acyclic/build.mill
+++ b/example/scalalib/linting/3-acyclic/build.mill
@@ -19,7 +19,6 @@ package build
 import mill.*, scalalib.*
 import mill.api.opt.*
 
-
 object `package` extends ScalaModule {
   def scalaVersion = "2.13.16"
   def compileMvnDeps = Seq(mvn"com.lihaoyi:::acyclic:0.3.18")
diff --git a/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala b/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala
index 814eb1c778ee..b3fe2a844d2e 100644
--- a/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala
+++ b/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala
@@ -16,7 +16,10 @@ class BuildGenChecker(sourceRoot: os.Path, scalafmtConfigFile: os.Path) {
       generate: => Unit,
       sourceRel: os.SubPath,
       expectedRel: os.SubPath,
-      updateSnapshots: Boolean = sys.env.getOrElse("UTEST_UPDATE_GOLDEN_TESTS", "0").equals("1") // pass true to update test data on disk
+      updateSnapshots: Boolean = sys.env.getOrElse(
+        "UTEST_UPDATE_GOLDEN_TESTS",
+        "0"
+      ).equals("1") // pass true to update test data on disk
   )(using
       tp: TestPath
   ): Boolean = {

From 602cbf86c5ebadb89f974a85aed48ea4d1740cc2 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 25 Nov 2025 11:11:48 +0100
Subject: [PATCH 49/67] Update test expectations

---
 .../crossproject-cross-version/build.mill     |  2 +
 .../dummy/package.mill                        |  2 +
 .../full/package.mill                         |  2 +
 .../jvm-util/package.mill                     |  2 +
 .../CrossprojectCrossVersionBaseModule.scala  | 10 ++-
 .../pure/package.mill                         |  2 +
 .../crossproject-cross-version/build.mill     | 85 ++++++++++---------
 .../sbt-multi-project-example/build.mill      | 10 ++-
 8 files changed, 66 insertions(+), 49 deletions(-)

diff --git a/libs/init/sbt/test/resources/expected/crossproject-cross-version/build.mill b/libs/init/sbt/test/resources/expected/crossproject-cross-version/build.mill
index 8958ea4ccb06..cb36ea2ea721 100644
--- a/libs/init/sbt/test/resources/expected/crossproject-cross-version/build.mill
+++ b/libs/init/sbt/test/resources/expected/crossproject-cross-version/build.mill
@@ -2,5 +2,7 @@
 package build
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 
 object `package` extends Module {}
diff --git a/libs/init/sbt/test/resources/expected/crossproject-cross-version/dummy/package.mill b/libs/init/sbt/test/resources/expected/crossproject-cross-version/dummy/package.mill
index fb171f025b3f..0874687d9e50 100644
--- a/libs/init/sbt/test/resources/expected/crossproject-cross-version/dummy/package.mill
+++ b/libs/init/sbt/test/resources/expected/crossproject-cross-version/dummy/package.mill
@@ -1,6 +1,8 @@
 package build.dummy
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalajslib.*
diff --git a/libs/init/sbt/test/resources/expected/crossproject-cross-version/full/package.mill b/libs/init/sbt/test/resources/expected/crossproject-cross-version/full/package.mill
index 54f283b6c7df..cf4bc261c8f9 100644
--- a/libs/init/sbt/test/resources/expected/crossproject-cross-version/full/package.mill
+++ b/libs/init/sbt/test/resources/expected/crossproject-cross-version/full/package.mill
@@ -1,6 +1,8 @@
 package build.full
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalajslib.*
diff --git a/libs/init/sbt/test/resources/expected/crossproject-cross-version/jvm-util/package.mill b/libs/init/sbt/test/resources/expected/crossproject-cross-version/jvm-util/package.mill
index 8cf027b7e48d..d9d7f53f396a 100644
--- a/libs/init/sbt/test/resources/expected/crossproject-cross-version/jvm-util/package.mill
+++ b/libs/init/sbt/test/resources/expected/crossproject-cross-version/jvm-util/package.mill
@@ -1,6 +1,8 @@
 package build.`jvm-util`
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/crossproject-cross-version/mill-build/src/CrossprojectCrossVersionBaseModule.scala b/libs/init/sbt/test/resources/expected/crossproject-cross-version/mill-build/src/CrossprojectCrossVersionBaseModule.scala
index 58a0bef3f3b4..37ee87d9d0c6 100644
--- a/libs/init/sbt/test/resources/expected/crossproject-cross-version/mill-build/src/CrossprojectCrossVersionBaseModule.scala
+++ b/libs/init/sbt/test/resources/expected/crossproject-cross-version/mill-build/src/CrossprojectCrossVersionBaseModule.scala
@@ -1,5 +1,7 @@
 package millbuild
 
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
@@ -9,15 +11,15 @@ trait CrossprojectCrossVersionBaseModule
 
   def mvnDeps = super.mvnDeps() ++ Seq(Deps.upickle)
 
-  def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+  def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
     (scalaVersion() match {
-      case "2.12.20" => Seq(
+      case "2.12.20" => Opts(
           "-Xlint:_,-unused",
           "-Ywarn-numeric-widen",
           "-Ywarn-unused:_,-nowarn,-privates"
         )
-      case "2.13.14" => Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-      case "3.7.1"   => Seq("-Wunused")
+      case "2.13.14" => Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+      case "3.7.1"   => Opts("-Wunused")
       case _         => Nil
     })
 
diff --git a/libs/init/sbt/test/resources/expected/crossproject-cross-version/pure/package.mill b/libs/init/sbt/test/resources/expected/crossproject-cross-version/pure/package.mill
index 1446ca396c25..a0e5ad32a2d2 100644
--- a/libs/init/sbt/test/resources/expected/crossproject-cross-version/pure/package.mill
+++ b/libs/init/sbt/test/resources/expected/crossproject-cross-version/pure/package.mill
@@ -1,6 +1,8 @@
 package build.pure
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalajslib.*
diff --git a/libs/init/sbt/test/resources/expected/with-args/crossproject-cross-version/build.mill b/libs/init/sbt/test/resources/expected/with-args/crossproject-cross-version/build.mill
index 1a225aabe646..8e109d7178ee 100644
--- a/libs/init/sbt/test/resources/expected/with-args/crossproject-cross-version/build.mill
+++ b/libs/init/sbt/test/resources/expected/with-args/crossproject-cross-version/build.mill
@@ -2,6 +2,8 @@
 package build
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalajslib.*
@@ -26,16 +28,16 @@ object `package` extends Module {
 
       def artifactName = "dummy"
 
-      def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+      def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
         (scalaVersion() match {
-          case "2.12.20" => Seq(
+          case "2.12.20" => Opts(
               "-Xlint:_,-unused",
               "-Ywarn-numeric-widen",
               "-Ywarn-unused:_,-nowarn,-privates"
             )
           case "2.13.14" =>
-            Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-          case "3.7.1" => Seq("-Wunused")
+            Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+          case "3.7.1" => Opts("-Wunused")
           case _       => Nil
         })
 
@@ -62,16 +64,16 @@ object `package` extends Module {
 
       def artifactName = "dummy"
 
-      def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+      def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
         (scalaVersion() match {
-          case "2.12.20" => Seq(
+          case "2.12.20" => Opts(
               "-Xlint:_,-unused",
               "-Ywarn-numeric-widen",
               "-Ywarn-unused:_,-nowarn,-privates"
             )
           case "2.13.14" =>
-            Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-          case "3.7.1" => Seq("-Wunused")
+            Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+          case "3.7.1" => Opts("-Wunused")
           case _       => Nil
         })
 
@@ -101,16 +103,16 @@ object `package` extends Module {
 
       def artifactName = "dummy"
 
-      def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+      def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
         (scalaVersion() match {
-          case "2.12.20" => Seq(
+          case "2.12.20" => Opts(
               "-Xlint:_,-unused",
               "-Ywarn-numeric-widen",
               "-Ywarn-unused:_,-nowarn,-privates"
             )
           case "2.13.14" =>
-            Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-          case "3.7.1" => Seq("-Wunused")
+            Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+          case "3.7.1" => Opts("-Wunused")
           case _       => Nil
         })
 
@@ -144,16 +146,16 @@ object `package` extends Module {
 
       def artifactName = "full"
 
-      def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+      def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
         (scalaVersion() match {
-          case "2.12.20" => Seq(
+          case "2.12.20" => Opts(
               "-Xlint:_,-unused",
               "-Ywarn-numeric-widen",
               "-Ywarn-unused:_,-nowarn,-privates"
             )
           case "2.13.14" =>
-            Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-          case "3.7.1" => Seq("-Wunused")
+            Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+          case "3.7.1" => Opts("-Wunused")
           case _       => Nil
         })
 
@@ -182,16 +184,16 @@ object `package` extends Module {
 
       def artifactName = "full"
 
-      def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+      def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
         (scalaVersion() match {
-          case "2.12.20" => Seq(
+          case "2.12.20" => Opts(
               "-Xlint:_,-unused",
               "-Ywarn-numeric-widen",
               "-Ywarn-unused:_,-nowarn,-privates"
             )
           case "2.13.14" =>
-            Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-          case "3.7.1" => Seq("-Wunused")
+            Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+          case "3.7.1" => Opts("-Wunused")
           case _       => Nil
         })
 
@@ -221,16 +223,16 @@ object `package` extends Module {
 
       def artifactName = "full"
 
-      def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+      def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
         (scalaVersion() match {
-          case "2.12.20" => Seq(
+          case "2.12.20" => Opts(
               "-Xlint:_,-unused",
               "-Ywarn-numeric-widen",
               "-Ywarn-unused:_,-nowarn,-privates"
             )
           case "2.13.14" =>
-            Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-          case "3.7.1" => Seq("-Wunused")
+            Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+          case "3.7.1" => Opts("-Wunused")
           case _       => Nil
         })
 
@@ -256,16 +258,17 @@ object `package` extends Module {
 
     def artifactName = "jvmutil"
 
-    def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+    def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
       (scalaVersion() match {
-        case "2.12.20" => Seq(
+        case "2.12.20" => Opts(
             "-Xlint:_,-unused",
             "-Ywarn-numeric-widen",
             "-Ywarn-unused:_,-nowarn,-privates"
           )
-        case "2.13.14" => Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-        case "3.7.1"   => Seq("-Wunused")
-        case _         => Nil
+        case "2.13.14" =>
+          Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+        case "3.7.1" => Opts("-Wunused")
+        case _       => Nil
       })
 
     def pomSettings = PomSettings(
@@ -296,16 +299,16 @@ object `package` extends Module {
 
       def artifactName = "pure"
 
-      def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+      def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
         (scalaVersion() match {
-          case "2.12.20" => Seq(
+          case "2.12.20" => Opts(
               "-Xlint:_,-unused",
               "-Ywarn-numeric-widen",
               "-Ywarn-unused:_,-nowarn,-privates"
             )
           case "2.13.14" =>
-            Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-          case "3.7.1" => Seq("-Wunused")
+            Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+          case "3.7.1" => Opts("-Wunused")
           case _       => Nil
         })
 
@@ -332,16 +335,16 @@ object `package` extends Module {
 
       def artifactName = "pure"
 
-      def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+      def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
         (scalaVersion() match {
-          case "2.12.20" => Seq(
+          case "2.12.20" => Opts(
               "-Xlint:_,-unused",
               "-Ywarn-numeric-widen",
               "-Ywarn-unused:_,-nowarn,-privates"
             )
           case "2.13.14" =>
-            Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-          case "3.7.1" => Seq("-Wunused")
+            Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+          case "3.7.1" => Opts("-Wunused")
           case _       => Nil
         })
 
@@ -371,16 +374,16 @@ object `package` extends Module {
 
       def artifactName = "pure"
 
-      def scalacOptions = super.scalacOptions() ++ Seq("-deprecation") ++
+      def scalacOptions = super.scalacOptions() ++ Opts("-deprecation") ++
         (scalaVersion() match {
-          case "2.12.20" => Seq(
+          case "2.12.20" => Opts(
               "-Xlint:_,-unused",
               "-Ywarn-numeric-widen",
               "-Ywarn-unused:_,-nowarn,-privates"
             )
           case "2.13.14" =>
-            Seq("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
-          case "3.7.1" => Seq("-Wunused")
+            Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
+          case "3.7.1" => Opts("-Wunused")
           case _       => Nil
         })
 
diff --git a/libs/init/sbt/test/resources/expected/with-args/sbt-multi-project-example/build.mill b/libs/init/sbt/test/resources/expected/with-args/sbt-multi-project-example/build.mill
index 905a39c01346..ac61e34904c1 100644
--- a/libs/init/sbt/test/resources/expected/with-args/sbt-multi-project-example/build.mill
+++ b/libs/init/sbt/test/resources/expected/with-args/sbt-multi-project-example/build.mill
@@ -2,6 +2,8 @@
 package build
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
@@ -23,7 +25,7 @@ object `package` extends Module {
 
     def scalaVersion = "2.12.3"
 
-    def scalacOptions = super.scalacOptions() ++ Seq(
+    def scalacOptions = super.scalacOptions() ++ Opts(
       "-unchecked",
       "-feature",
       "-language:existentials",
@@ -102,7 +104,7 @@ object `package` extends Module {
 
     def scalaVersion = "2.12.3"
 
-    def scalacOptions = super.scalacOptions() ++ Seq(
+    def scalacOptions = super.scalacOptions() ++ Opts(
       "-unchecked",
       "-feature",
       "-language:existentials",
@@ -181,7 +183,7 @@ object `package` extends Module {
 
     def scalaVersion = "2.12.3"
 
-    def scalacOptions = super.scalacOptions() ++ Seq(
+    def scalacOptions = super.scalacOptions() ++ Opts(
       "-unchecked",
       "-feature",
       "-language:existentials",
@@ -251,7 +253,7 @@ object `package` extends Module {
 
       def scalaVersion = "2.12.3"
 
-      def scalacOptions = super.scalacOptions() ++ Seq(
+      def scalacOptions = super.scalacOptions() ++ Opts(
         "-unchecked",
         "-feature",
         "-language:existentials",

From e720525aed7869aa96681b3d88b1b135a00ed81f Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 25 Nov 2025 11:33:53 +0100
Subject: [PATCH 50/67] Update test

---
 .../sbt/test/resources/crossproject/project/build.properties    | 2 +-
 libs/init/sbt/test/resources/expected/crossproject/build.mill   | 2 ++
 .../sbt/test/resources/expected/crossproject/dummy/package.mill | 2 ++
 .../sbt/test/resources/expected/crossproject/full/package.mill  | 2 ++
 .../crossproject/mill-build/src/CrossprojectBaseModule.scala    | 2 ++
 .../sbt/test/resources/expected/crossproject/pure/package.mill  | 2 ++
 6 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/libs/init/sbt/test/resources/crossproject/project/build.properties b/libs/init/sbt/test/resources/crossproject/project/build.properties
index fdb242913064..e33f4dab9e9f 100644
--- a/libs/init/sbt/test/resources/crossproject/project/build.properties
+++ b/libs/init/sbt/test/resources/crossproject/project/build.properties
@@ -1 +1 @@
-sbt.version=1.8.3
\ No newline at end of file
+sbt.version=1.10.10
\ No newline at end of file
diff --git a/libs/init/sbt/test/resources/expected/crossproject/build.mill b/libs/init/sbt/test/resources/expected/crossproject/build.mill
index 8958ea4ccb06..cb36ea2ea721 100644
--- a/libs/init/sbt/test/resources/expected/crossproject/build.mill
+++ b/libs/init/sbt/test/resources/expected/crossproject/build.mill
@@ -2,5 +2,7 @@
 package build
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 
 object `package` extends Module {}
diff --git a/libs/init/sbt/test/resources/expected/crossproject/dummy/package.mill b/libs/init/sbt/test/resources/expected/crossproject/dummy/package.mill
index 80d36507e8bf..54d80571e833 100644
--- a/libs/init/sbt/test/resources/expected/crossproject/dummy/package.mill
+++ b/libs/init/sbt/test/resources/expected/crossproject/dummy/package.mill
@@ -1,6 +1,8 @@
 package build.dummy
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalajslib.*
diff --git a/libs/init/sbt/test/resources/expected/crossproject/full/package.mill b/libs/init/sbt/test/resources/expected/crossproject/full/package.mill
index 48bb1ba0c703..8f28765f4f5d 100644
--- a/libs/init/sbt/test/resources/expected/crossproject/full/package.mill
+++ b/libs/init/sbt/test/resources/expected/crossproject/full/package.mill
@@ -1,6 +1,8 @@
 package build.full
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalajslib.*
diff --git a/libs/init/sbt/test/resources/expected/crossproject/mill-build/src/CrossprojectBaseModule.scala b/libs/init/sbt/test/resources/expected/crossproject/mill-build/src/CrossprojectBaseModule.scala
index 75ed6f4db394..3cec27918ab8 100644
--- a/libs/init/sbt/test/resources/expected/crossproject/mill-build/src/CrossprojectBaseModule.scala
+++ b/libs/init/sbt/test/resources/expected/crossproject/mill-build/src/CrossprojectBaseModule.scala
@@ -1,5 +1,7 @@
 package millbuild
 
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalalib.*
diff --git a/libs/init/sbt/test/resources/expected/crossproject/pure/package.mill b/libs/init/sbt/test/resources/expected/crossproject/pure/package.mill
index da498f3e6d4f..12006dc0b3b7 100644
--- a/libs/init/sbt/test/resources/expected/crossproject/pure/package.mill
+++ b/libs/init/sbt/test/resources/expected/crossproject/pure/package.mill
@@ -1,6 +1,8 @@
 package build.pure
 
 import mill.*
+import mill.api.*
+import mill.api.opt.*
 import mill.javalib.*
 import mill.javalib.publish.*
 import mill.scalajslib.*

From 5f8ffa5e74d69e1f616f4e21130de927b58bc2de Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 25 Nov 2025 11:39:51 +0100
Subject: [PATCH 51/67] .

---
 .../test/src/mill/main/buildgen/BuildGenChecker.scala     | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala b/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala
index b3fe2a844d2e..dcf1ea7db6c7 100644
--- a/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala
+++ b/libs/init/buildgen/test/src/mill/main/buildgen/BuildGenChecker.scala
@@ -16,10 +16,10 @@ class BuildGenChecker(sourceRoot: os.Path, scalafmtConfigFile: os.Path) {
       generate: => Unit,
       sourceRel: os.SubPath,
       expectedRel: os.SubPath,
-      updateSnapshots: Boolean = sys.env.getOrElse(
-        "UTEST_UPDATE_GOLDEN_TESTS",
-        "0"
-      ).equals("1") // pass true to update test data on disk
+      // pass true to update test data on disk
+      updateSnapshots: Boolean = sys.env
+        .getOrElse("UTEST_UPDATE_GOLDEN_TESTS", "0")
+        .equals("1")
   )(using
       tp: TestPath
   ): Boolean = {

From 9a72ca5be935f255b27707afe213eb563e6ba026 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 25 Nov 2025 11:46:52 +0100
Subject: [PATCH 52/67] fix/disable test for unsupported mill-scalafix

---
 example/scalalib/config/4-linting/build.mill | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/example/scalalib/config/4-linting/build.mill b/example/scalalib/config/4-linting/build.mill
index 76c06f9dc444..191285abbf23 100644
--- a/example/scalalib/config/4-linting/build.mill
+++ b/example/scalalib/config/4-linting/build.mill
@@ -48,14 +48,16 @@ object Foo {
 /** See Also: .scalafix.conf */
 /** Usage
 
-> ./mill __.fix
-
-> cat src/Foo.scala
-package foo
-object Foo {
-  def main(args: Array[String]) = {
-    println("Hello world")
-  }
-}
+> ./mill __.fix # mill-scalafix not compatible with new Opts class
+error: fix java.lang.ClassCastException: class mill.api.opt.Opts cannot be cast to class scala.collection.immutable.Seq ...
 
 */
+////
+////> cat src/Foo.scala
+////package foo
+////object Foo {
+////  def main(args: Array[String]) = {
+////    println("Hello world")
+////  }
+////}
+

From d74278c763d94de786ef7a434c6f882ec0c7178d Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 25 Nov 2025 12:49:27 +0100
Subject: [PATCH 53/67] Add missing import

---
 example/thirdparty/android-compose-samples/build.mill | 1 +
 1 file changed, 1 insertion(+)

diff --git a/example/thirdparty/android-compose-samples/build.mill b/example/thirdparty/android-compose-samples/build.mill
index 38645bda3e3f..c235cf8e159b 100644
--- a/example/thirdparty/android-compose-samples/build.mill
+++ b/example/thirdparty/android-compose-samples/build.mill
@@ -1,4 +1,5 @@
 import mill.*, androidlib.*, kotlinlib.*
+import mill.api.opt.*
 
 object Versions {
   val kotlinVersion = "2.1.20"

From 6018b98e77f0e1a19c47a2d8859c7ae0a87e56aa Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 25 Nov 2025 13:09:47 +0100
Subject: [PATCH 54/67] Fix BuildWriter for Opts

---
 libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
index a4f1624d4e51..7f280b2eb2ce 100644
--- a/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
+++ b/libs/init/buildgen/src/mill/main/buildgen/BuildWriter.scala
@@ -630,7 +630,7 @@ class BuildWriter(build: BuildSpec, renderCrossValueInTask: String = "crossValue
           }
           s""" ++ ($renderCrossValueInTask match {
              |  ${renderLines(crossCases)}
-             |  case _ => Nil
+             |  case _ => Opts()
              |})""".stripMargin
         }
       }

From 7a172e356349bbc7a025966d95d4b0617b1a8ec4 Mon Sep 17 00:00:00 2001
From: "autofix-ci[bot]" <114827586+autofix-ci[bot]@users.noreply.github.com>
Date: Tue, 25 Nov 2025 12:26:03 +0000
Subject: [PATCH 55/67] [autofix.ci] apply automated fixes

---
 example/scalalib/config/4-linting/build.mill | 1 -
 1 file changed, 1 deletion(-)

diff --git a/example/scalalib/config/4-linting/build.mill b/example/scalalib/config/4-linting/build.mill
index 191285abbf23..cd33a4410fa2 100644
--- a/example/scalalib/config/4-linting/build.mill
+++ b/example/scalalib/config/4-linting/build.mill
@@ -60,4 +60,3 @@ error: fix java.lang.ClassCastException: class mill.api.opt.Opts cannot be cast
 ////    println("Hello world")
 ////  }
 ////}
-

From d31db2bffc98ce12bcae6a32099e02d1f32560e5 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 25 Nov 2025 13:28:08 +0100
Subject: [PATCH 56/67] Update test literals

---
 .../ide/bsp-server/resources/snapshots/diagnostics.json   | 8 ++++----
 integration/ide/bsp-server/resources/snapshots/logging    | 2 +-
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/integration/ide/bsp-server/resources/snapshots/diagnostics.json b/integration/ide/bsp-server/resources/snapshots/diagnostics.json
index 61958c399914..6e23832a0ab0 100644
--- a/integration/ide/bsp-server/resources/snapshots/diagnostics.json
+++ b/integration/ide/bsp-server/resources/snapshots/diagnostics.json
@@ -22,11 +22,11 @@
       {
         "range": {
           "start": {
-            "line": 106,
+            "line": 107,
             "character": 17
           },
           "end": {
-            "line": 106,
+            "line": 107,
             "character": 22
           }
         },
@@ -52,11 +52,11 @@
       {
         "range": {
           "start": {
-            "line": 106,
+            "line": 107,
             "character": 17
           },
           "end": {
-            "line": 106,
+            "line": 107,
             "character": 22
           }
         },
diff --git a/integration/ide/bsp-server/resources/snapshots/logging b/integration/ide/bsp-server/resources/snapshots/logging
index cf76c5fc8a5e..7314e1c6999e 100644
--- a/integration/ide/bsp-server/resources/snapshots/logging
+++ b/integration/ide/bsp-server/resources/snapshots/logging
@@ -6,7 +6,7 @@ bsp] BSP server started
 bsp-init-mill-build/build.mill-60] compiling * Scala sources to * ...
 bsp-init-mill-build/build.mill-60] done compiling
 bsp-init-build.mill-60] compiling * Scala sources to * ...
-bsp-init-build.mill-60] [warn] build.mill:107:18
+bsp-init-build.mill-60] [warn] build.mill:108:18
 bsp-init-build.mill-60]   def theThing = thing
 bsp-init-build.mill-60]                  ^^^^^
 bsp-init-build.mill-60] method thing in object diag is deprecated since 0.0.1: deprecated

From 7e874920ee8b9063992991f080e5ae76840125bc Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 25 Nov 2025 13:32:17 +0100
Subject: [PATCH 57/67] Bootstrap Patch

---
 ci/mill-bootstrap.patch                | 9 +++++++++
 mill-build/src/millbuild/package.scala | 1 +
 2 files changed, 10 insertions(+)
 create mode 100644 mill-build/src/millbuild/package.scala

diff --git a/ci/mill-bootstrap.patch b/ci/mill-bootstrap.patch
index e69de29bb2d1..fe0181203a8a 100644
--- a/ci/mill-bootstrap.patch
+++ b/ci/mill-bootstrap.patch
@@ -0,0 +1,9 @@
+diff --git a/mill-build/src/millbuild/package.scala b/mill-build/src/millbuild/package.scala
+index b64a97c8e44..9335010790c 100644
+--- a/mill-build/src/millbuild/package.scala
++++ b/mill-build/src/millbuild/package.scala
+@@ -1 +1,3 @@
+-package object millbuild {}
++package object millbuild {
++  implicit def toOpts(seq: Seq[String]): mill.api.opt.Opts = mill.api.opt.Opts(seq)
++}
diff --git a/mill-build/src/millbuild/package.scala b/mill-build/src/millbuild/package.scala
new file mode 100644
index 000000000000..b64a97c8e441
--- /dev/null
+++ b/mill-build/src/millbuild/package.scala
@@ -0,0 +1 @@
+package object millbuild {}

From b005074ad6f2513a6c2cac43a9ed4f1e59c3a691 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 25 Nov 2025 22:49:58 +0100
Subject: [PATCH 58/67] Update mill-bootstrap.patch

---
 ci/mill-bootstrap.patch | 402 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 393 insertions(+), 9 deletions(-)

diff --git a/ci/mill-bootstrap.patch b/ci/mill-bootstrap.patch
index fe0181203a8a..e616918e6256 100644
--- a/ci/mill-bootstrap.patch
+++ b/ci/mill-bootstrap.patch
@@ -1,9 +1,393 @@
-diff --git a/mill-build/src/millbuild/package.scala b/mill-build/src/millbuild/package.scala
-index b64a97c8e44..9335010790c 100644
---- a/mill-build/src/millbuild/package.scala
-+++ b/mill-build/src/millbuild/package.scala
-@@ -1 +1,3 @@
--package object millbuild {}
-+package object millbuild {
-+  implicit def toOpts(seq: Seq[String]): mill.api.opt.Opts = mill.api.opt.Opts(seq)
-+}
+diff --git a/dist/package.mill b/dist/package.mill
+index d9d8e2e05f1..af820b9c1d1 100644
+--- a/dist/package.mill
++++ b/dist/package.mill
+@@ -13,6 +13,7 @@ import scala.util.Using
+ import scala.util.Properties
+ import java.nio.file.Files
+ import java.nio.file.attribute.PosixFilePermission
++import mill.api.opt.*
+ 
+ trait DistModule extends Module {
+   // All modules that we want to aggregate as part of this `dev` assembly.
+@@ -173,7 +174,7 @@ object `package` extends MillJavaModule with DistModule {
+ 
+   def localBinName = "mill-assembly.jar"
+ 
+-  private def millClientJvmArgs = Seq(
++  private def millClientJvmArgs = Opts(
+     // Avoid reserving a lot of memory for the client, as the client only forward information
+     "-Xmx128m"
+   )
+@@ -206,7 +207,7 @@ object `package` extends MillJavaModule with DistModule {
+       mainClass = mainClass().get,
+       shellClassPath = classpath,
+       cmdClassPath = Seq(classpathJar.toString()),
+-      jvmArgs = jvmArgs
++      jvmArgs = jvmArgs.toStringSeq
+     )
+ 
+     os.write(outputPath, script)
+@@ -220,7 +221,7 @@ object `package` extends MillJavaModule with DistModule {
+     mill.scalalib.Assembly.Rule.ExcludePattern("mill/local-test-overrides/.*")
+   )
+ 
+-  def forkArgs = millClientJvmArgs ++ Seq(
++  def forkArgs = millClientJvmArgs ++ Opts(
+     // Workaround for Zinc/JNA bug
+     // https://github.com/sbt/sbt/blame/6718803ee6023ab041b045a6988fafcfae9d15b5/main/src/main/scala/sbt/Main.scala#L130
+     "-Djna.nosys=true"
+@@ -243,7 +244,7 @@ object `package` extends MillJavaModule with DistModule {
+         try {
+           os.call(
+             cmd = (launcher().path.toString, rest),
+-            env = forkEnv() ++ build.dist.localTestOverridesEnv(),
++            env = (forkEnv() ++ build.dist.localTestOverridesEnv()).view.mapValues(_.toString).toMap,
+             cwd = wd,
+             stdin = os.Inherit,
+             stdout = os.Inherit,
+@@ -357,7 +358,7 @@ object `package` extends MillJavaModule with DistModule {
+ 
+     def executableRaw = nativeImage()
+ 
+-    def nativeImageOptions = Seq(
++    def nativeImageOptions = Opts(
+       "--no-fallback",
+       "--enable-url-protocols=https",
+       "-Os",
+diff --git a/dist/scripts/package.mill b/dist/scripts/package.mill
+index 89262ef2b35..4afac12dd18 100644
+--- a/dist/scripts/package.mill
++++ b/dist/scripts/package.mill
+@@ -7,6 +7,7 @@ import mill.*
+ import mill.scalalib.*
+ import mill.api.Result
+ import millbuild.*
++import mill.api.opt.*
+ 
+ object `package` extends mill.Module { scripts =>
+ 
+@@ -77,9 +78,9 @@ object `package` extends mill.Module { scripts =>
+     def mvnDeps = Seq(millbuild.Deps.osLib, millbuild.Deps.pprint)
+     def utestVersion = millbuild.Deps.TestDeps.utest.version
+     def forkEnv = Map(
+-      "MILL_TEST_SH_SCRIPT" -> millSh.compile0().path.toString,
+-      "MILL_TEST_BAT_SCRIPT" -> millBat.compile0().path.toString,
+-      "MILL_NATIVE_SUFFIX" -> build.dist.native.nativeSuffix()
++      "MILL_TEST_SH_SCRIPT" -> Opt(millSh.compile0().path),
++      "MILL_TEST_BAT_SCRIPT" -> Opt(millBat.compile0().path),
++      "MILL_NATIVE_SUFFIX" -> Opt(build.dist.native.nativeSuffix())
+     )
+   }
+   def scriptsModules: Seq[BootstrapScriptModule] = Seq(millSh, millBat)
+diff --git a/example/package.mill b/example/package.mill
+index e4e29b18b5e..2f5e02d302e 100644
+--- a/example/package.mill
++++ b/example/package.mill
+@@ -14,6 +14,7 @@ import mill.T
+ import mill.api.Cross
+ import mill.api.BuildCtx
+ import mill.testkit.Chunk
++import mill.api.opt.*
+ 
+ object `package` extends Module {
+   def exampleModules: Seq[ExampleCrossModule] = moduleInternal
+@@ -238,8 +239,8 @@ object `package` extends Module {
+     def runClasspath = build.libs.util.test.runClasspath()
+     def localRunClasspath = build.testkit.localRunClasspath()
+ 
+-    def forkEnv = Map("LANG" -> "C")
+-    def forkArgs: T[Seq[String]] = Task(List.empty[String])
++    def forkEnv = Map("LANG" -> opt"C")
++    def forkArgs: T[Opts] = Task(Opts())
+ 
+     /**
+      * Parses a `build.mill` for specific comments and return the split-by-type content
+diff --git a/integration/package.mill b/integration/package.mill
+index cd35e6d0763..870b0b99f39 100644
+--- a/integration/package.mill
++++ b/integration/package.mill
+@@ -16,6 +16,7 @@ import mill.javalib.testrunner.TestResult
+ import millbuild.*
+ import upickle.implicits.namedTuples.default.given
+ import mill.api.BuildCtx
++import mill.api.opt.*
+ 
+ object `package` extends mill.Module {
+   // We compile the test code once and then offer multiple modes to
+@@ -48,8 +49,8 @@ object `package` extends mill.Module {
+     def resources: T[Seq[PathRef]]
+     def runClasspath: T[Seq[PathRef]]
+     def localRunClasspath: T[Seq[PathRef]]
+-    def forkEnv: T[Map[String, String]]
+-    def forkArgs: T[Seq[String]]
++    def forkEnv: T[Map[String, Opt]]
++    def forkArgs: T[Opts]
+     def testExclusive = false
+ 
+     trait ModeModule extends MillBaseTestsModule {
+@@ -59,7 +60,7 @@ object `package` extends mill.Module {
+         localOutFolder().withValue(()) { sharedOutFolder =>
+           val testModuleUtil = new mill.javalib.TestModuleUtil(
+             testUseArgsFile(),
+-            forkArgs(),
++            forkArgs().toStringSeq,
+             Seq.empty[String],
+             jvmWorker().scalalibClasspath(),
+             resources(),
+@@ -69,7 +70,8 @@ object `package` extends mill.Module {
+             args,
+             testForkGrouping(),
+             jvmWorker().testrunnerEntrypointClasspath(),
+-            allForkEnv() ++ Option.when(useSharedOut) { "MILL_TEST_SHARED_OUTPUT_DIR" -> "1" },
++            allForkEnv().view.mapValues(_.toString).toMap ++
++              Option.when(useSharedOut) { "MILL_TEST_SHARED_OUTPUT_DIR" -> "1" },
+             testSandboxWorkingDir = !useSharedOut,
+             if (useSharedOut) sharedOutFolder else forkWorkingDir(),
+             testReportXml(),
+@@ -85,15 +87,15 @@ object `package` extends mill.Module {
+       def mode: String = moduleSegments.parts.last
+       def scalaVersion = Deps.scalaVersion
+ 
+-      def forkEnv = Task {
++      def forkEnv: T[Map[String, Opt]] = Task {
+         super.forkEnv() ++
+           IntegrationTestModule.this.forkEnv() ++
+           Map(
+-            "MILL_INTEGRATION_DAEMON_MODE" -> (mode == "daemon").toString,
+-            "MILL_INTEGRATION_IS_PACKAGED_LAUNCHER" -> millIntegrationIsPackagedLauncher().toString,
+-            "MILL_LAUNCHER" -> build.dist.bootstrapLauncher().path.toString,
+-            "MILL_LAUNCHER_BAT" -> build.dist.bootstrapLauncherBat().path.toString,
+-            "MILL_INTEGRATION_LAUNCHER" -> millIntegrationLauncher().path.toString
++            "MILL_INTEGRATION_DAEMON_MODE" -> Opt((mode == "daemon").toString),
++            "MILL_INTEGRATION_IS_PACKAGED_LAUNCHER" -> Opt(millIntegrationIsPackagedLauncher().toString),
++            "MILL_LAUNCHER" -> Opt(build.dist.bootstrapLauncher().path),
++            "MILL_LAUNCHER_BAT" -> Opt(build.dist.bootstrapLauncherBat().path),
++            "MILL_INTEGRATION_LAUNCHER" -> Opt(millIntegrationLauncher().path)
+           ) ++
+           (if (millIntegrationIsPackagedLauncher()) Map() else build.dist.localTestOverridesEnv())
+       }
+@@ -155,12 +157,12 @@ object `package` extends mill.Module {
+       def enableBsp = false
+     }
+     override def moduleDeps = super[IntegrationTestModule].moduleDeps
+-    def forkArgs: T[Seq[String]] = Task(List.empty[String])
++    def forkArgs: T[Opts] = Task(Opts())
+     def forkEnv = super.forkEnv() ++ Seq(
+       // "UTEST_UPDATE_GOLDEN_TESTS" -> "1",
+-      "MILL_PROJECT_ROOT" -> BuildCtx.workspaceRoot.toString,
+-      "TEST_SCALA_2_13_VERSION" -> Deps.testScala213Version,
+-      "TEST_KOTLIN_VERSION" -> Deps.kotlinCompiler.version
++      "MILL_PROJECT_ROOT" -> Opt(BuildCtx.workspaceRoot),
++      "TEST_SCALA_2_13_VERSION" -> Opt(Deps.testScala213Version),
++      "TEST_KOTLIN_VERSION" -> Opt(Deps.kotlinCompiler.version)
+     )
+   }
+   trait IdeIntegrationCrossModule extends IntegrationCrossModule {
+@@ -168,7 +170,7 @@ object `package` extends mill.Module {
+     def mvnDeps = super.mvnDeps() ++ Seq(
+       Deps.bsp4j
+     )
+-    def forkArgs = super.forkArgs() ++ Seq(
++    def forkArgs = super.forkArgs() ++ Opts(
+       s"-Dmill.integration.coursier-version=${Deps.coursier.dep.versionConstraint.asString}"
+     )
+   }
+diff --git a/libs/javalib/package.mill b/libs/javalib/package.mill
+index 7ed81bc6843..c580c7fc5f1 100644
+--- a/libs/javalib/package.mill
++++ b/libs/javalib/package.mill
+@@ -15,6 +15,7 @@ import mill.contrib.buildinfo.BuildInfo
+ import mill.T
+ import mill.api.Cross
+ import millbuild.*
++import mill.api.opt.*
+ 
+ object `package` extends MillStableScalaModule {
+ 
+@@ -44,7 +45,7 @@ object `package` extends MillStableScalaModule {
+     )
+   def testForkEnv = {
+     val locale = if (Properties.isMac) "en_US.UTF-8" else "C.utf8"
+-    super.testForkEnv() ++ Map("LC_ALL" -> locale)
++    super.testForkEnv() ++ Map("LC_ALL" -> Opt(locale))
+   }
+ 
+   override def mimaBinaryIssueFilters: T[Seq[ProblemFilter]] =
+diff --git a/libs/scalalib/package.mill b/libs/scalalib/package.mill
+index acb0ed5daa8..8317f1d6971 100644
+--- a/libs/scalalib/package.mill
++++ b/libs/scalalib/package.mill
+@@ -13,6 +13,7 @@ import mill.contrib.buildinfo.BuildInfo
+ import mill.T
+ import mill.api.Cross
+ import millbuild.*
++import mill.api.opt.*
+ 
+ object `package` extends MillStableScalaModule {
+   def moduleDeps = Seq(build.libs.javalib)
+@@ -30,6 +31,6 @@ object `package` extends MillStableScalaModule {
+ 
+   def testForkEnv = {
+     val locale = if (Properties.isMac) "en_US.UTF-8" else "C.utf8"
+-    super.testForkEnv() ++ Map("LC_ALL" -> locale)
++    super.testForkEnv() ++ Map("LC_ALL" -> Opt(locale))
+   }
+ }
+diff --git a/mill-build/src/millbuild/MillBaseTestsModule.scala b/mill-build/src/millbuild/MillBaseTestsModule.scala
+index 80fdd82d16c..0b10e69157c 100644
+--- a/mill-build/src/millbuild/MillBaseTestsModule.scala
++++ b/mill-build/src/millbuild/MillBaseTestsModule.scala
+@@ -2,10 +2,11 @@ package millbuild
+ 
+ import mill.*
+ import mill.scalalib.*
++import mill.api.opt.*
+ 
+ trait MillBaseTestsModule extends TestModule {
+   def forkArgs = Task {
+-    Seq(
++    Opts(
+       s"-DMILL_SCALA_3_NEXT_VERSION=${Deps.scalaVersion}",
+       s"-DMILL_SCALA_2_13_VERSION=${Deps.scala2Version}",
+       s"-DMILL_SCALA_2_12_VERSION=${Deps.workerScalaVersion212}",
+diff --git a/mill-build/src/millbuild/MillJavaModule.scala b/mill-build/src/millbuild/MillJavaModule.scala
+index f82f0d10ec5..089349df5c5 100644
+--- a/mill-build/src/millbuild/MillJavaModule.scala
++++ b/mill-build/src/millbuild/MillJavaModule.scala
+@@ -8,6 +8,7 @@ import mill.T
+ import mill.Task
+ import mill.scalalib.Dep
+ import mill.scalalib.JavaModule
++import mill.api.opt.*
+ 
+ import java.io.File
+ 
+@@ -59,13 +60,13 @@ trait MillJavaModule extends JavaModule {
+   }
+ 
+   def testMvnDeps: T[Seq[Dep]] = Seq(Deps.TestDeps.utest)
+-  def testForkEnv: T[Map[String, String]] = forkEnv() ++ localTestOverridesEnv()
++  def testForkEnv: T[Map[String, Opt]] = forkEnv() ++ localTestOverridesEnv()
+   def testModuleDeps: Seq[JavaModule] =
+     if (this == build.core.api) Seq(build.core.api)
+     else Seq(this, build.core.api.test)
+ 
+-  def localTestOverridesEnv = Task {
+-    val localRepos = localTestRepositories().map(_.path.toString).mkString(File.pathSeparator)
++  def localTestOverridesEnv: T[Seq[(String, Opt)]] = Task {
++    val localRepos = Opt.mkPath(localTestRepositories().map(_.path), sep = File.pathSeparator)
+     Seq("MILL_LOCAL_TEST_REPO" -> localRepos)
+   }
+ 
+@@ -107,14 +108,14 @@ trait MillJavaModule extends JavaModule {
+   }
+ 
+   override def javacOptions = Task {
+-    super.javacOptions() ++ ciJavacOptions() ++ Seq(
++    super.javacOptions() ++ ciJavacOptions() ++ Opts(
+       "-Xlint",
+       "-Xlint:-serial", // we don't care about java serialization
+       "-Xlint:-try" // TODO: a bunch of code needs reviewing with this lint)
+     )
+   }
+ 
+-  def javadocOptions = super.javadocOptions() ++ Seq(
++  def javadocOptions = super.javadocOptions() ++ Opts(
+     // Disable warnings for missing documentation comments or tags (for example,
+     // a missing comment or class, or a missing @return tag or similar tag on a method).
+     // We have many methods without JavaDoc comments, so those warnings are useless
+diff --git a/mill-build/src/millbuild/MillPublishJavaModule.scala b/mill-build/src/millbuild/MillPublishJavaModule.scala
+index 342747153c6..6730228fe51 100644
+--- a/mill-build/src/millbuild/MillPublishJavaModule.scala
++++ b/mill-build/src/millbuild/MillPublishJavaModule.scala
+@@ -4,6 +4,7 @@ import build_.package_ as build
+ import mill.Task
+ import mill.scalalib.PublishModule
+ import mill.scalalib.publish.{Developer, License, PomSettings, VersionControl}
++import mill.api.opt.*
+ 
+ trait MillPublishJavaModule extends MillJavaModule with PublishModule {
+ 
+@@ -14,7 +15,7 @@ trait MillPublishJavaModule extends MillJavaModule with PublishModule {
+   )
+   def pomSettings = MillPublishJavaModule.commonPomSettings(artifactName())
+   def javacOptions =
+-    super.javacOptions() ++ Seq("--release", "11", "-encoding", "UTF-8", "-deprecation")
++    super.javacOptions() ++ Opts("--release", "11", "-encoding", "UTF-8", "-deprecation")
+ }
+ 
+ object MillPublishJavaModule {
+diff --git a/runner/bsp/package.mill b/runner/bsp/package.mill
+index 5c511891e2d..0cdae63e94e 100644
+--- a/runner/bsp/package.mill
++++ b/runner/bsp/package.mill
+@@ -4,6 +4,7 @@ import mill._
+ import mill.contrib.buildinfo.BuildInfo
+ import mill.T
+ import millbuild.*
++import mill.api.opt.*
+ 
+ object `package` extends MillPublishScalaModule with BuildInfo {
+   def compileModuleDeps = Seq(build.libs.javalib)
+@@ -24,7 +25,7 @@ object `package` extends MillPublishScalaModule with BuildInfo {
+ 
+   override lazy val test: MillScalaTests = new Test {}
+   trait Test extends MillScalaTests {
+-    def forkEnv: T[Map[String, String]] = Task {
++    def forkEnv: T[Map[String, Opt]] = Task {
+       // We try to fetch this dependency with coursier in the tests
+       worker.publishLocalCached()
+       super.forkEnv()
+diff --git a/runner/codesig/package.mill b/runner/codesig/package.mill
+index c10f27f0ec5..579df2817e2 100644
+--- a/runner/codesig/package.mill
++++ b/runner/codesig/package.mill
+@@ -3,6 +3,7 @@ import mill._
+ import mill.scalalib._
+ import millbuild.*
+ import mill.api.BuildCtx
++import mill.api.opt.*
+ 
+ /**
+  * The CodeSig module implements a conservative call-graph analyzer at the JVM
+@@ -27,14 +28,14 @@ object `package` extends MillPublishScalaModule {
+ 
+     def testLogFolder = Task { Task.dest }
+ 
+-    def caseEnvs[V](f1: CaseModule => Task[V])(s: String, f2: V => String) = {
++    def caseEnvs[V](f1: CaseModule => Task[V])(s: String, f2: V => Opt) = {
+       Task.traverse(caseKeys) { i => f1(cases(i)).map(v => s"MILL_TEST_${s}_$i" -> f2(v)) }
+     }
+     def forkEnv = Task {
+-      Map("MILL_TEST_LOGS" -> testLogFolder().toString) ++
+-        caseEnvs(_.compile)("CLASSES", _.classes.path.toString)() ++
+-        caseEnvs(_.compileClasspath)("CLASSPATH", _.map(_.path).mkString(","))() ++
+-        caseEnvs(_.sources)("SOURCES", _.head.path.toString)()
++      Map("MILL_TEST_LOGS" -> Opt(testLogFolder())) ++
++        caseEnvs(_.compile)("CLASSES", cr => Opt(cr.classes.path))() ++
++        caseEnvs(_.compileClasspath)("CLASSPATH", prs => Opt.mkPath(prs.map(_.path), sep = ","))() ++
++        caseEnvs(_.sources)("SOURCES", s => Opt(s.head.path))()
+     }
+ 
+     object cases extends Cross[CaseModule](caseKeys)
+diff --git a/testkit/package.mill b/testkit/package.mill
+index a8693a3834c..3b713079acf 100644
+--- a/testkit/package.mill
++++ b/testkit/package.mill
+@@ -3,6 +3,7 @@ package build.testkit
+ import mill._
+ import millbuild.*
+ import mill.api.BuildCtx
++import mill.api.opt.*
+ 
+ /**
+  * Mill unit/integration/example-testing helpers. Both used internally
+@@ -20,6 +21,6 @@ object `package` extends MillPublishScalaModule {
+ 
+   def forkEnv =
+     super.forkEnv() ++
+-      Map("MILL_EXECUTABLE_PATH" -> build.dist.launcher().path.toString()) ++
++      Map("MILL_EXECUTABLE_PATH" -> Opt(build.dist.launcher().path)) ++
+       build.dist.testForkEnv()
+ }

From a405b6cd616d63ba4f098ca5929c7ac4a58e08c4 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 26 Nov 2025 10:58:40 +0100
Subject: [PATCH 59/67] Update patchfile

---
 ci/mill-bootstrap.patch | 209 +++++++++++++++++++++++++++++++++++-----
 1 file changed, 183 insertions(+), 26 deletions(-)

diff --git a/ci/mill-bootstrap.patch b/ci/mill-bootstrap.patch
index e616918e6256..b61bdddf65ce 100644
--- a/ci/mill-bootstrap.patch
+++ b/ci/mill-bootstrap.patch
@@ -1,5 +1,35 @@
+diff --git a/contrib/package.mill b/contrib/package.mill
+index e27aacc8c66..5f46f114336 100644
+--- a/contrib/package.mill
++++ b/contrib/package.mill
+@@ -13,6 +13,7 @@ import mill.contrib.buildinfo.BuildInfo
+ import mill.T
+ import mill.api.Cross
+ import millbuild.*
++import mill.api.opt.*
+ 
+ /**
+  * [[build.contrib]] contains user-contributed Mill plugins that satisfy
+@@ -66,7 +67,7 @@ object `package` extends mill.Module {
+ 
+     def testArgs = Task {
+       super.testArgs() ++
+-        Seq(
++        Opts(
+           s"-DTEST_PLAY_VERSION_2_6=${Deps.Play_2_6.playVersion}",
+           s"-DTEST_PLAY_VERSION_2_7=${Deps.Play_2_7.playVersion}",
+           s"-DTEST_PLAY_VERSION_2_8=${Deps.Play_2_8.playVersion}",
+@@ -114,7 +115,7 @@ object `package` extends mill.Module {
+ 
+     def testArgs = Task {
+       super.testArgs() ++
+-        Seq(
++        Opts(
+           s"-DMILL_SCOVERAGE2_VERSION=${Deps.scalacScoverage2Plugin.version}"
+         )
+     }
 diff --git a/dist/package.mill b/dist/package.mill
-index d9d8e2e05f1..af820b9c1d1 100644
+index d9d8e2e05f1..e8b4b6fa802 100644
 --- a/dist/package.mill
 +++ b/dist/package.mill
 @@ -13,6 +13,7 @@ import scala.util.Using
@@ -10,30 +40,21 @@ index d9d8e2e05f1..af820b9c1d1 100644
  
  trait DistModule extends Module {
    // All modules that we want to aggregate as part of this `dev` assembly.
-@@ -173,7 +174,7 @@ object `package` extends MillJavaModule with DistModule {
- 
-   def localBinName = "mill-assembly.jar"
- 
--  private def millClientJvmArgs = Seq(
-+  private def millClientJvmArgs = Opts(
-     // Avoid reserving a lot of memory for the client, as the client only forward information
-     "-Xmx128m"
-   )
-@@ -206,7 +207,7 @@ object `package` extends MillJavaModule with DistModule {
-       mainClass = mainClass().get,
-       shellClassPath = classpath,
-       cmdClassPath = Seq(classpathJar.toString()),
--      jvmArgs = jvmArgs
-+      jvmArgs = jvmArgs.toStringSeq
-     )
- 
-     os.write(outputPath, script)
+@@ -182,7 +183,7 @@ object `package` extends MillJavaModule with DistModule {
+     val isWin = scala.util.Properties.isWin
+     val outputPath = Task.dest / (if (isWin) "run.bat" else "run")
+ 
+-    val launcherForkArgs = testArgs()
++    val launcherForkArgs = testArgs().toStringSeq
+     val (millArgs, otherArgs) =
+       launcherForkArgs.partition(arg =>
+         arg.startsWith("-DMILL") && !arg.startsWith("-DMILL_VERSION")
 @@ -220,7 +221,7 @@ object `package` extends MillJavaModule with DistModule {
      mill.scalalib.Assembly.Rule.ExcludePattern("mill/local-test-overrides/.*")
    )
  
 -  def forkArgs = millClientJvmArgs ++ Seq(
-+  def forkArgs = millClientJvmArgs ++ Opts(
++  def forkArgs = Opts(millClientJvmArgs) ++ Opts(
      // Workaround for Zinc/JNA bug
      // https://github.com/sbt/sbt/blame/6718803ee6023ab041b045a6988fafcfae9d15b5/main/src/main/scala/sbt/Main.scala#L130
      "-Djna.nosys=true"
@@ -81,7 +102,7 @@ index 89262ef2b35..4afac12dd18 100644
    }
    def scriptsModules: Seq[BootstrapScriptModule] = Seq(millSh, millBat)
 diff --git a/example/package.mill b/example/package.mill
-index e4e29b18b5e..2f5e02d302e 100644
+index a8c10b9c31a..64eaae4b8a3 100644
 --- a/example/package.mill
 +++ b/example/package.mill
 @@ -14,6 +14,7 @@ import mill.T
@@ -193,8 +214,31 @@ index cd35e6d0763..870b0b99f39 100644
        s"-Dmill.integration.coursier-version=${Deps.coursier.dep.versionConstraint.asString}"
      )
    }
+diff --git a/libs/androidlib/package.mill b/libs/androidlib/package.mill
+index fc386b7efd2..e8b3ecf27c0 100644
+--- a/libs/androidlib/package.mill
++++ b/libs/androidlib/package.mill
+@@ -5,6 +5,7 @@ import mill.*
+ import mill.contrib.buildinfo.BuildInfo
+ import mill.javalib.*
+ import millbuild.*
++import mill.api.opt.*
+ 
+ // TODO when android hardcoded versions are fixed, generate a build info
+ // TODO change MillPublishScalaModule to MillStableScalaModule after mill version when androidlib is stable and released.
+@@ -56,8 +57,8 @@ object `package` extends MillPublishScalaModule with BuildInfo {
+ 
+   trait MillAndroidModule extends MillPublishScalaModule {
+     override def javacOptions = super.javacOptions() ++ {
+-      val release = Seq("-source", "1.8", "-target", "1.8")
+-      release ++ Seq("-encoding", "UTF-8", "-deprecation")
++      val release = Opts("-source", "1.8", "-target", "1.8")
++      release ++ Opts("-encoding", "UTF-8", "-deprecation")
+     }
+ 
+     override def repositoriesTask = Task.Anon {
 diff --git a/libs/javalib/package.mill b/libs/javalib/package.mill
-index 7ed81bc6843..c580c7fc5f1 100644
+index 45c676ffc4b..a751c5f0995 100644
 --- a/libs/javalib/package.mill
 +++ b/libs/javalib/package.mill
 @@ -15,6 +15,7 @@ import mill.contrib.buildinfo.BuildInfo
@@ -252,7 +296,7 @@ index 80fdd82d16c..0b10e69157c 100644
        s"-DMILL_SCALA_2_13_VERSION=${Deps.scala2Version}",
        s"-DMILL_SCALA_2_12_VERSION=${Deps.workerScalaVersion212}",
 diff --git a/mill-build/src/millbuild/MillJavaModule.scala b/mill-build/src/millbuild/MillJavaModule.scala
-index f82f0d10ec5..089349df5c5 100644
+index f82f0d10ec5..deb7e049a01 100644
 --- a/mill-build/src/millbuild/MillJavaModule.scala
 +++ b/mill-build/src/millbuild/MillJavaModule.scala
 @@ -8,6 +8,7 @@ import mill.T
@@ -263,6 +307,24 @@ index f82f0d10ec5..089349df5c5 100644
  
  import java.io.File
  
+@@ -15,13 +16,13 @@ import scala.util.Properties
+ 
+ trait MillJavaModule extends JavaModule {
+ 
+-  def testArgs: T[Seq[String]] = Task {
++  def testArgs: T[Opts] = Task {
+     // Workaround for Zinc/JNA bug
+     // https://github.com/sbt/sbt/blame/6718803ee6023ab041b045a6988fafcfae9d15b5/main/src/main/scala/sbt/Main.scala#L130
+-    val jnaArgs = Seq("-Djna.nosys=true")
++    val jnaArgs = Opts("-Djna.nosys=true")
+     val userLang =
+-      if (Properties.isMac || Properties.isWin) Seq("-Duser.language=en")
+-      else Nil
++      if (Properties.isMac || Properties.isWin) Opts("-Duser.language=en")
++      else Opts()
+     jnaArgs ++ userLang
+   }
+ 
 @@ -59,13 +60,13 @@ trait MillJavaModule extends JavaModule {
    }
  
@@ -280,7 +342,19 @@ index f82f0d10ec5..089349df5c5 100644
      Seq("MILL_LOCAL_TEST_REPO" -> localRepos)
    }
  
-@@ -107,14 +108,14 @@ trait MillJavaModule extends JavaModule {
+@@ -98,23 +99,23 @@ trait MillJavaModule extends JavaModule {
+     Task.env.get("CI").contains("1")
+   }
+ 
+-  def ciJavacOptions: Task[Seq[String]] = Task {
+-    if (isCI()) Seq(
++  def ciJavacOptions: Task[Opts] = Task {
++    if (isCI()) Opts(
+       // When in CI make the warnings fatal
+       "-Werror"
+     )
+-    else Nil
++    else Opts()
    }
  
    override def javacOptions = Task {
@@ -318,8 +392,84 @@ index 342747153c6..6730228fe51 100644
  }
  
  object MillPublishJavaModule {
+diff --git a/mill-build/src/millbuild/MillScalaModule.scala b/mill-build/src/millbuild/MillScalaModule.scala
+index fcd2fdcda0b..6ace3435eb4 100644
+--- a/mill-build/src/millbuild/MillScalaModule.scala
++++ b/mill-build/src/millbuild/MillScalaModule.scala
+@@ -5,6 +5,7 @@ import mill.scalalib.*
+ import mill.javalib.api.JvmWorkerUtil
+ import mill.api.BuildCtx
+ import com.goyeau.mill.scalafix.ScalafixModule
++import mill.api.opt.*
+ 
+ /**
+  * Some custom scala settings and test convenience
+@@ -24,19 +25,19 @@ trait MillScalaModule extends ScalaModule with MillJavaModule with ScalafixModul
+ 
+   def isScala3: T[Boolean] = Task { JvmWorkerUtil.isScala3(scalaVersion()) }
+ 
+-  def ciScalacOptions: T[Seq[String]] = Task {
++  def ciScalacOptions: T[Opts] = Task {
+     if (isCI()) {
+       // Turn warnings into errors on CI
+-      if (isScala3()) Seq("-Werror") else Seq("-Xfatal-warnings")
+-    } else Nil
++      if (isScala3()) Opts("-Werror") else Opts("-Xfatal-warnings")
++    } else Opts()
+   }
+ 
+   def scalacOptions =
+-    super.scalacOptions() ++ Seq(
++    super.scalacOptions() ++ Opts(
+       "-deprecation",
+       "-feature"
+     ) ++ ciScalacOptions() ++ (
+-      if (isScala3()) Seq(
++      if (isScala3()) Opts(
+         "-explain-cyclic",
+         "-Wunused:all",
+         "-Wconf:msg=An existential type that came from a Scala-2 classfile:silent",
+@@ -53,7 +54,7 @@ trait MillScalaModule extends ScalaModule with MillJavaModule with ScalafixModul
+         // "-Wimplausible-patterns",
+         // "-rewrite", "-source", "3.7-migration"
+       )
+-      else Seq(
++      else Opts(
+         "-P:acyclic:force",
+         "-Xlint:unused",
+         "-Xlint:adapted-args",
+@@ -90,7 +91,7 @@ trait MillScalaModule extends ScalaModule with MillJavaModule with ScalafixModul
+   trait MillScalaTests extends ScalaTests with MillJavaModule with MillBaseTestsModule
+       with ScalafixModule {
+     def scalafixConfig = Task { Some(BuildCtx.workspaceRoot / ".scalafix.conf") }
+-    def forkArgs = super.forkArgs() ++ outer.testArgs()
++    def forkArgs = super.forkArgs() ++ Opts(outer.testArgs())
+     def moduleDeps = outer.testModuleDeps
+     def mvnDeps = super.mvnDeps() ++ outer.testMvnDeps()
+     def forkEnv = super.forkEnv() ++ outer.testForkEnv()
+diff --git a/runner/autooverride/package.mill b/runner/autooverride/package.mill
+index 959d0d64e60..8967fbb3561 100644
+--- a/runner/autooverride/package.mill
++++ b/runner/autooverride/package.mill
+@@ -3,6 +3,7 @@ package build.runner.autooverride
+ import mill.*
+ import mill.scalalib.*
+ import millbuild.*
++import mill.api.opt.*
+ 
+ /**
+  * AutoOverride compiler plugin for Scala 3 that automatically implements
+@@ -40,7 +41,7 @@ object `package` extends Module {
+       millbuild.Deps.TestDeps.utest
+     )
+ 
+-    def scalacOptions = super.scalacOptions() ++ Seq(
++    def scalacOptions = super.scalacOptions() ++ Opts(
+       s"-Xplugin:${plugin.jar().path}",
+       s"-Xplugin-require:auto-override"
+     )
 diff --git a/runner/bsp/package.mill b/runner/bsp/package.mill
-index 5c511891e2d..0cdae63e94e 100644
+index 5c511891e2d..0d7cd65b780 100644
 --- a/runner/bsp/package.mill
 +++ b/runner/bsp/package.mill
 @@ -4,6 +4,7 @@ import mill._
@@ -330,7 +480,7 @@ index 5c511891e2d..0cdae63e94e 100644
  
  object `package` extends MillPublishScalaModule with BuildInfo {
    def compileModuleDeps = Seq(build.libs.javalib)
-@@ -24,7 +25,7 @@ object `package` extends MillPublishScalaModule with BuildInfo {
+@@ -24,13 +25,13 @@ object `package` extends MillPublishScalaModule with BuildInfo {
  
    override lazy val test: MillScalaTests = new Test {}
    trait Test extends MillScalaTests {
@@ -339,6 +489,13 @@ index 5c511891e2d..0cdae63e94e 100644
        // We try to fetch this dependency with coursier in the tests
        worker.publishLocalCached()
        super.forkEnv()
+     }
+ 
+-    def forkArgs = super.forkArgs() ++ Seq(s"-DBSP4J_VERSION=${Deps.bsp4j.version}")
++    def forkArgs = super.forkArgs() ++ Opts(s"-DBSP4J_VERSION=${Deps.bsp4j.version}")
+   }
+ 
+   object worker extends MillPublishScalaModule {
 diff --git a/runner/codesig/package.mill b/runner/codesig/package.mill
 index c10f27f0ec5..579df2817e2 100644
 --- a/runner/codesig/package.mill

From 399a3a9d4140fddafe599c79d9269be1799e5928 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 26 Nov 2025 13:04:48 +0100
Subject: [PATCH 60/67] Fix more tests

---
 example/thirdparty/commons-io/build.mill      |  1 +
 .../CrossprojectCrossVersionBaseModule.scala  |  2 +-
 .../crossproject-cross-version/build.mill     | 20 +++++++++----------
 3 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/example/thirdparty/commons-io/build.mill b/example/thirdparty/commons-io/build.mill
index 4f6812fda255..3d76730c0b2d 100644
--- a/example/thirdparty/commons-io/build.mill
+++ b/example/thirdparty/commons-io/build.mill
@@ -3,6 +3,7 @@
 package build
 
 import mill.*, javalib.*, publish.*
+import mill.api.opt.*
 import contrib.jmh.JmhModule
 
 object `package` extends PublishModule, MavenModule {
diff --git a/libs/init/sbt/test/resources/expected/crossproject-cross-version/mill-build/src/CrossprojectCrossVersionBaseModule.scala b/libs/init/sbt/test/resources/expected/crossproject-cross-version/mill-build/src/CrossprojectCrossVersionBaseModule.scala
index 37ee87d9d0c6..3746db9f9faf 100644
--- a/libs/init/sbt/test/resources/expected/crossproject-cross-version/mill-build/src/CrossprojectCrossVersionBaseModule.scala
+++ b/libs/init/sbt/test/resources/expected/crossproject-cross-version/mill-build/src/CrossprojectCrossVersionBaseModule.scala
@@ -20,7 +20,7 @@ trait CrossprojectCrossVersionBaseModule
         )
       case "2.13.14" => Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
       case "3.7.1"   => Opts("-Wunused")
-      case _         => Nil
+      case _         => Opts()
     })
 
   def publishVersion = "0.1.0-SNAPSHOT"
diff --git a/libs/init/sbt/test/resources/expected/with-args/crossproject-cross-version/build.mill b/libs/init/sbt/test/resources/expected/with-args/crossproject-cross-version/build.mill
index 8e109d7178ee..171dfd663500 100644
--- a/libs/init/sbt/test/resources/expected/with-args/crossproject-cross-version/build.mill
+++ b/libs/init/sbt/test/resources/expected/with-args/crossproject-cross-version/build.mill
@@ -38,7 +38,7 @@ object `package` extends Module {
           case "2.13.14" =>
             Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
           case "3.7.1" => Opts("-Wunused")
-          case _       => Nil
+          case _       => Opts()
         })
 
       def pomSettings = PomSettings(
@@ -74,7 +74,7 @@ object `package` extends Module {
           case "2.13.14" =>
             Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
           case "3.7.1" => Opts("-Wunused")
-          case _       => Nil
+          case _       => Opts()
         })
 
       def pomSettings = PomSettings(
@@ -113,7 +113,7 @@ object `package` extends Module {
           case "2.13.14" =>
             Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
           case "3.7.1" => Opts("-Wunused")
-          case _       => Nil
+          case _       => Opts()
         })
 
       def pomSettings = PomSettings(
@@ -156,7 +156,7 @@ object `package` extends Module {
           case "2.13.14" =>
             Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
           case "3.7.1" => Opts("-Wunused")
-          case _       => Nil
+          case _       => Opts()
         })
 
       def pomSettings = PomSettings(
@@ -194,7 +194,7 @@ object `package` extends Module {
           case "2.13.14" =>
             Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
           case "3.7.1" => Opts("-Wunused")
-          case _       => Nil
+          case _       => Opts()
         })
 
       def pomSettings = PomSettings(
@@ -233,7 +233,7 @@ object `package` extends Module {
           case "2.13.14" =>
             Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
           case "3.7.1" => Opts("-Wunused")
-          case _       => Nil
+          case _       => Opts()
         })
 
       def pomSettings = PomSettings(
@@ -268,7 +268,7 @@ object `package` extends Module {
         case "2.13.14" =>
           Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
         case "3.7.1" => Opts("-Wunused")
-        case _       => Nil
+        case _       => Opts()
       })
 
     def pomSettings = PomSettings(
@@ -309,7 +309,7 @@ object `package` extends Module {
           case "2.13.14" =>
             Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
           case "3.7.1" => Opts("-Wunused")
-          case _       => Nil
+          case _       => Opts()
         })
 
       def pomSettings = PomSettings(
@@ -345,7 +345,7 @@ object `package` extends Module {
           case "2.13.14" =>
             Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
           case "3.7.1" => Opts("-Wunused")
-          case _       => Nil
+          case _       => Opts()
         })
 
       def pomSettings = PomSettings(
@@ -384,7 +384,7 @@ object `package` extends Module {
           case "2.13.14" =>
             Opts("-Xlint:_,-unused", "-Wnumeric-widen", "-Wunused")
           case "3.7.1" => Opts("-Wunused")
-          case _       => Nil
+          case _       => Opts()
         })
 
       def pomSettings = PomSettings(

From b4598fdfe784ed870613cae54a4b07425266f042 Mon Sep 17 00:00:00 2001
From: "autofix-ci[bot]" <114827586+autofix-ci[bot]@users.noreply.github.com>
Date: Tue, 9 Dec 2025 16:23:54 +0000
Subject: [PATCH 61/67] [autofix.ci] apply automated fixes

---
 libs/javalib/src/mill/javalib/NativeImageModule.scala           | 1 -
 libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala           | 2 +-
 .../javalib/src/mill/javalib/spring/boot/SpringBootModule.scala | 1 +
 runner/meta/src/mill/meta/MillBuildRootModule.scala             | 2 +-
 4 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/libs/javalib/src/mill/javalib/NativeImageModule.scala b/libs/javalib/src/mill/javalib/NativeImageModule.scala
index f5b294cebb8f..ed65b4530918 100644
--- a/libs/javalib/src/mill/javalib/NativeImageModule.scala
+++ b/libs/javalib/src/mill/javalib/NativeImageModule.scala
@@ -1,7 +1,6 @@
 package mill.javalib
 
 import mill.*
-import mill.api.BuildCtx
 import mill.api.opt.*
 import mill.constants.{DaemonFiles, Util}
 import coursier.core.VariantSelector.ConfigurationBased
diff --git a/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala b/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala
index ed96f47f3477..9943bed77b54 100644
--- a/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala
+++ b/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala
@@ -3,7 +3,7 @@ package mill.javalib.bsp
 import java.nio.file.Path
 import mill.api.daemon.internal.bsp.BspJavaModuleApi
 import mill.Task
-import mill.api.daemon.internal.{EvaluatorApi, OptsApi, TaskApi, internal}
+import mill.api.daemon.internal.{EvaluatorApi, OptsApi, internal}
 import mill.api.ModuleCtx
 import mill.api.opt.*
 import mill.javalib.{JavaModule, SemanticDbJavaModule}
diff --git a/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala b/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala
index 8fdbd4cb64bb..3548f27729a7 100644
--- a/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala
+++ b/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala
@@ -180,6 +180,7 @@ trait SpringBootModule extends JavaModule {
    * parent module as a native GraalVM application, provided the [[outer.springBootProcessAOT]] works.
    */
   trait NativeSpringBootBuildModule extends SpringBootOptimisedBuildModule, NativeImageModule {
+
     /**
      * Uses the configuration path from both [[outer.springBootProcessAOT]] and
      * [[nativeMvnDepsMetadata]]
diff --git a/runner/meta/src/mill/meta/MillBuildRootModule.scala b/runner/meta/src/mill/meta/MillBuildRootModule.scala
index 72c0e08852f2..dc9b4cc0c27c 100644
--- a/runner/meta/src/mill/meta/MillBuildRootModule.scala
+++ b/runner/meta/src/mill/meta/MillBuildRootModule.scala
@@ -1,7 +1,7 @@
 package mill.meta
 
 import java.nio.file.Path
-import mill.api.{BuildCtx, Discover, PathRef, Result, Task}
+import mill.api.{BuildCtx, Result}
 import mill.*
 import mill.api.opt.*
 import mill.api.daemon.internal.internal

From 24af5e837ffca00b4ff6c3affa07a23ae7148e1e Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 9 Dec 2025 21:05:20 +0100
Subject: [PATCH 62/67] fix

---
 .../javalib/src/mill/javalib/spring/boot/SpringBootModule.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala b/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala
index 3548f27729a7..eb272d259c29 100644
--- a/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala
+++ b/libs/javalib/src/mill/javalib/spring/boot/SpringBootModule.scala
@@ -188,7 +188,7 @@ trait SpringBootModule extends JavaModule {
     override def nativeImageOptions: Task.Simple[Opts] = Task {
       val configurationsPath = outer.springBootProcessAOT().path / "resources/META-INF"
       super.nativeImageOptions() ++ Opts(
-        Opt("--configurations-path", configurationsPath)
+        OptGroup("--configurations-path", configurationsPath)
       )
     }
   }

From 64840f8192dfdb5e34ee62eb3f899e06017f9c2c Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Tue, 9 Dec 2025 21:23:54 +0100
Subject: [PATCH 63/67] fix test

---
 example/scalalib/config/4-linting/build.mill | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/example/scalalib/config/4-linting/build.mill b/example/scalalib/config/4-linting/build.mill
index da807d567d0d..d5931fac4807 100644
--- a/example/scalalib/config/4-linting/build.mill
+++ b/example/scalalib/config/4-linting/build.mill
@@ -49,7 +49,8 @@ object Foo {
 /** Usage
 
 > ./mill __.fix # mill-scalafix not compatible with new Opts class
-error: fix java.lang.ClassCastException: class mill.api.opt.Opts cannot be cast to class scala.collection.immutable.Seq ...
+error: fix
+error: java.lang.ClassCastException: class mill.api.opt.Opts cannot be cast to class scala.collection.immutable.Seq ...
 
 */
 ////

From 89e07d22703ad63f4021caa840a7738929a6e3c2 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 10 Dec 2025 09:56:39 +0100
Subject: [PATCH 64/67] fix test

---
 libs/javalib/src/mill/javalib/NativeImageModule.scala | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/libs/javalib/src/mill/javalib/NativeImageModule.scala b/libs/javalib/src/mill/javalib/NativeImageModule.scala
index ed65b4530918..94ade842da86 100644
--- a/libs/javalib/src/mill/javalib/NativeImageModule.scala
+++ b/libs/javalib/src/mill/javalib/NativeImageModule.scala
@@ -106,9 +106,10 @@ trait NativeImageModule extends WithJvmWorkerModule, OfflineSupportModule {
    * Additional options for the `native-image` Tool.
    */
   def nativeImageOptions: T[Opts] = Task {
-    Opts(nativeMvnDepsMetadata().map(md =>
-      Opt("--configurations-path", md.path / "META-INF")
-    ))
+    nativeMvnDepsMetadata() match {
+      case Some(md) => Opts(OptGroup("--configurations-path", md.path / "META-INF"))
+      case _ => Opts()
+    }
   }
 
   /**

From d495d6f56bb1c804375c9e0f9b8e1353cee10b52 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 10 Dec 2025 10:03:46 +0100
Subject: [PATCH 65/67] fix

---
 example/kotlinlib/web/9-spring-boot-aot/build.mill | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/example/kotlinlib/web/9-spring-boot-aot/build.mill b/example/kotlinlib/web/9-spring-boot-aot/build.mill
index ed5045b8a32d..c2be25c93781 100644
--- a/example/kotlinlib/web/9-spring-boot-aot/build.mill
+++ b/example/kotlinlib/web/9-spring-boot-aot/build.mill
@@ -1,6 +1,7 @@
 package build
 
 import mill.*
+import mill.api.opt.*
 import mill.kotlinlib.*
 import mill.javalib.spring.boot.SpringBootModule
 import mill.javalib.TestModule.{Junit4, Junit5}
@@ -13,7 +14,7 @@ object `package` extends SpringBootModule, KotlinModule { outer =>
   override def kotlinVersion = "2.2.21"
   override def artifactName: T[String] = "spring-boot-native-demo"
 
-  override def kotlincOptions: T[Seq[String]] = super.kotlincOptions() ++ Seq(
+  override def kotlincOptions: T[Opts] = super.kotlincOptions() ++ Opts(
     "-jvm-target",
     "17",
     "-Xjsr305=strict",

From 2e31c444b700902071f5452bc12b16e92db90b08 Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 10 Dec 2025 10:36:59 +0100
Subject: [PATCH 66/67] fix test

---
 .../ide/bsp-server/resources/snapshots/logging  | 17 +----------------
 1 file changed, 1 insertion(+), 16 deletions(-)

diff --git a/integration/ide/bsp-server/resources/snapshots/logging b/integration/ide/bsp-server/resources/snapshots/logging
index 32d101d8be60..f167fd75f7ec 100644
--- a/integration/ide/bsp-server/resources/snapshots/logging
+++ b/integration/ide/bsp-server/resources/snapshots/logging
@@ -3,24 +3,10 @@ bsp] BSP server started
 1-buildInitialize] Entered buildInitialize
 1-buildInitialize] Got client semanticdbVersion: * Enabling SemanticDB support.
 1-buildInitialize] buildInitialize took * msec
-<<<<<<< HEAD
-bsp-init-mill-build/build.mill-60] compiling * Scala sources to * ...
-bsp-init-mill-build/build.mill-60] done compiling
-bsp-init-build.mill-60] compiling * Scala sources to * ...
-bsp-init-build.mill-60] [warn] build.mill:108:18
-bsp-init-build.mill-60]   def theThing = thing
-bsp-init-build.mill-60]                  ^^^^^
-bsp-init-build.mill-60] method thing in object diag is deprecated since 0.0.1: deprecated
-bsp-init-build.mill-60] [warn] one warning found
-bsp-init-build.mill-60] done compiling
-bsp-init-113] compiling * Scala source to * ...
-bsp-init-113] done compiling
-bsp-init] BSP disabled for target file:///workspace/dependsOnSkipped because of its dependencies file:///workspace/skipped
-=======
 bsp-init-mill-build/build.mill-61] compiling * Scala sources to * ...
 bsp-init-mill-build/build.mill-61] done compiling
 bsp-init-build.mill-61] compiling * Scala sources to * ...
-bsp-init-build.mill-61] [warn] build.mill:107:18
+bsp-init-build.mill-61] [warn] build.mill:108:18
 bsp-init-build.mill-61]   def theThing = thing
 bsp-init-build.mill-61]                  ^^^^^
 bsp-init-build.mill-61] method thing in object diag is deprecated since 0.0.1: deprecated
@@ -30,7 +16,6 @@ bsp-init-build.mill-61] done compiling
 bsp-init-114] compiling * Scala source to * ...
 bsp-init-114] done compiling
 bsp-init] [warn] BSP disabled for target file:///workspace/dependsOnSkipped because of its dependencies file:///workspace/skipped
->>>>>>> origin/main
 bsp-init] BSP disabled for target file:///workspace/skipped via BspModuleApi#enableBsp
 bsp-init] SNAPSHOT
 2-workspaceBuildTargets] Entered workspaceBuildTargets

From 48db696e4458f30e7e6d3937d55eca080c1c3cbb Mon Sep 17 00:00:00 2001
From: Tobias Roeser <le.petit.fou@web.de>
Date: Wed, 10 Dec 2025 13:21:54 +0100
Subject: [PATCH 67/67] Add OptMap type alias for Map[String, Opt] and some
 convenience

---
 ci/mill-bootstrap.patch                       |  4 +--
 core/api/src/mill/api/opt/OptMap.scala        | 29 +++++++++++++++++++
 .../androidlib/hilt/AndroidHiltSupport.scala  | 12 ++++----
 .../javalib/src/mill/javalib/JavaModule.scala |  2 +-
 libs/javalib/src/mill/javalib/RunModule.scala | 10 +++----
 .../javalib/src/mill/javalib/TestModule.scala |  6 ++--
 .../src/mill/javalib/bsp/BspJavaModule.scala  |  2 +-
 .../src/mill/javalib/bsp/BspRunModule.scala   |  4 +--
 .../src/mill/kotlinlib/ksp/KspModule.scala    |  4 +--
 .../src/mill/scalalib/ScalaModule.scala       |  4 +--
 .../scalanativelib/ScalaNativeModule.scala    |  6 ++--
 .../mill/bsp/worker/MillJvmBuildServer.scala  |  5 ++--
 12 files changed, 59 insertions(+), 29 deletions(-)
 create mode 100644 core/api/src/mill/api/opt/OptMap.scala

diff --git a/ci/mill-bootstrap.patch b/ci/mill-bootstrap.patch
index b61bdddf65ce..04de08f24853 100644
--- a/ci/mill-bootstrap.patch
+++ b/ci/mill-bootstrap.patch
@@ -63,7 +63,7 @@ index d9d8e2e05f1..e8b4b6fa802 100644
            os.call(
              cmd = (launcher().path.toString, rest),
 -            env = forkEnv() ++ build.dist.localTestOverridesEnv(),
-+            env = (forkEnv() ++ build.dist.localTestOverridesEnv()).view.mapValues(_.toString).toMap,
++            env = (forkEnv() ++ build.dist.localTestOverridesEnv()).toStringMap,
              cwd = wd,
              stdin = os.Inherit,
              stdout = os.Inherit,
@@ -161,7 +161,7 @@ index cd35e6d0763..870b0b99f39 100644
              testForkGrouping(),
              jvmWorker().testrunnerEntrypointClasspath(),
 -            allForkEnv() ++ Option.when(useSharedOut) { "MILL_TEST_SHARED_OUTPUT_DIR" -> "1" },
-+            allForkEnv().view.mapValues(_.toString).toMap ++
++            allForkEnv().toStringMap ++
 +              Option.when(useSharedOut) { "MILL_TEST_SHARED_OUTPUT_DIR" -> "1" },
              testSandboxWorkingDir = !useSharedOut,
              if (useSharedOut) sharedOutFolder else forkWorkingDir(),
diff --git a/core/api/src/mill/api/opt/OptMap.scala b/core/api/src/mill/api/opt/OptMap.scala
new file mode 100644
index 000000000000..980f7851f5d7
--- /dev/null
+++ b/core/api/src/mill/api/opt/OptMap.scala
@@ -0,0 +1,29 @@
+package mill.api.opt
+
+import mill.api.daemon.internal.OptApi
+
+type OptMap = Map[String, Opt]
+
+extension (map: Map[String, OptApi]) {
+  def toStringMap: Map[String, String] = map.view.mapValues(_.toString()).toMap
+}
+
+object OptMap {
+
+  def apply(elems: (String, String | os.Path | Opt)*): OptMap = Map.from(
+    elems
+      .map { (k, v) =>
+        (
+          k,
+          v match {
+            case o: Opt => o
+            case o: (String | os.Path) => Opt(o)
+          }
+        )
+      }
+  )
+
+//  given jsonReadWriter: upickle.ReadWriter[OptMap] =
+//    upickle.readwriter[Map[String, Opt]].bimap(_.map, OptMap(_))
+
+}
diff --git a/libs/androidlib/src/mill/androidlib/hilt/AndroidHiltSupport.scala b/libs/androidlib/src/mill/androidlib/hilt/AndroidHiltSupport.scala
index 7186bf86c154..ec78c743b8d8 100644
--- a/libs/androidlib/src/mill/androidlib/hilt/AndroidHiltSupport.scala
+++ b/libs/androidlib/src/mill/androidlib/hilt/AndroidHiltSupport.scala
@@ -25,12 +25,12 @@ import mill.{T, Task}
 @mill.api.experimental
 trait AndroidHiltSupport extends KspModule, AndroidKotlinModule {
 
-  override def kspProcessorOptions: T[Map[String, Opt]] = Task {
-    super.kspProcessorOptions() ++ Map(
-      "dagger.fastInit" -> opt"enabled",
-      "dagger.hilt.android.internal.disableAndroidSuperclassValidation" -> opt"true",
-      "dagger.hilt.android.internal.projectType" -> opt"APP",
-      "dagger.hilt.internal.useAggregatingRootProcessor" -> opt"true"
+  override def kspProcessorOptions: T[OptMap] = Task {
+    super.kspProcessorOptions() ++ OptMap(
+      "dagger.fastInit" -> "enabled",
+      "dagger.hilt.android.internal.disableAndroidSuperclassValidation" -> "true",
+      "dagger.hilt.android.internal.projectType" -> "APP",
+      "dagger.hilt.internal.useAggregatingRootProcessor" -> "true"
     )
   }
 
diff --git a/libs/javalib/src/mill/javalib/JavaModule.scala b/libs/javalib/src/mill/javalib/JavaModule.scala
index 3399ada40216..31d5b906b89b 100644
--- a/libs/javalib/src/mill/javalib/JavaModule.scala
+++ b/libs/javalib/src/mill/javalib/JavaModule.scala
@@ -1277,7 +1277,7 @@ trait JavaModule
       val cmd = Seq(Jvm.jdkTool("jshell", javaHome().map(_.path))) ++ jshellArgs
       os.call(
         cmd = cmd,
-        env = allForkEnv().view.mapValues(_.toString).toMap,
+        env = allForkEnv().toStringMap,
         cwd = forkWorkingDir(),
         stdin = os.Inherit,
         stdout = os.Inherit
diff --git a/libs/javalib/src/mill/javalib/RunModule.scala b/libs/javalib/src/mill/javalib/RunModule.scala
index 805387b84e53..e9af66299397 100644
--- a/libs/javalib/src/mill/javalib/RunModule.scala
+++ b/libs/javalib/src/mill/javalib/RunModule.scala
@@ -37,16 +37,16 @@ trait RunModule extends WithJvmWorkerModule with RunModuleApi {
   /**
    * Any environment variables you want to pass to the forked JVM.
    */
-  def forkEnv: T[Map[String, Opt]] = Task { Map.empty[String, Opt] }
+  def forkEnv: T[OptMap] = Task { OptMap() }
 
   /**
    * Environment variables to pass to the forked JVM.
    *
    * Includes [[forkEnv]] and the variables defined by Mill itself.
    */
-  def allForkEnv: T[Map[String, Opt]] = Task {
-    forkEnv() ++ Map(
-      EnvVars.MILL_WORKSPACE_ROOT -> Opt(BuildCtx.workspaceRoot)
+  def allForkEnv: T[OptMap] = Task {
+    forkEnv() ++ OptMap(
+      EnvVars.MILL_WORKSPACE_ROOT -> BuildCtx.workspaceRoot
     )
   }
 
@@ -170,7 +170,7 @@ trait RunModule extends WithJvmWorkerModule with RunModuleApi {
       finalMainClassOpt(),
       runClasspath().map(_.path),
       forkArgs().toStringSeq,
-      allForkEnv().view.mapValues(_.toString()).toMap,
+      allForkEnv().toStringMap,
       runUseArgsFile(),
       javaHome().map(_.path),
       propagateEnv()
diff --git a/libs/javalib/src/mill/javalib/TestModule.scala b/libs/javalib/src/mill/javalib/TestModule.scala
index 5ecd1d709607..b2d49f53f9ca 100644
--- a/libs/javalib/src/mill/javalib/TestModule.scala
+++ b/libs/javalib/src/mill/javalib/TestModule.scala
@@ -226,8 +226,8 @@ trait TestModule
    */
   def testSandboxWorkingDir: T[Boolean] = true
 
-  override def allForkEnv: T[Map[String, Opt]] = Task {
-    super.allForkEnv() ++ Map(
+  override def allForkEnv: T[OptMap] = Task {
+    super.allForkEnv() ++ OptMap(
       EnvVars.MILL_TEST_RESOURCE_DIR -> Opt.mkPath(resources().map(_.path), sep = ";")
     )
   }
@@ -252,7 +252,7 @@ trait TestModule
         args(),
         testForkGrouping(),
         jvmWorker().testrunnerEntrypointClasspath(),
-        allForkEnv().view.mapValues(_.toString()).toMap,
+        allForkEnv().toStringMap,
         testSandboxWorkingDir(),
         forkWorkingDir(),
         testReportXml(),
diff --git a/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala b/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala
index 9943bed77b54..847f67abc39d 100644
--- a/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala
+++ b/libs/javalib/src/mill/javalib/bsp/BspJavaModule.scala
@@ -149,7 +149,7 @@ trait BspJavaModule extends mill.api.Module with BspJavaModuleApi {
       : Task.Simple[(
           classes: Seq[String],
           forkArgs: Opts,
-          forkEnv: Map[String, Opt]
+          forkEnv: OptMap
       )] =
     Task {
       (jm.allLocalMainClasses(), jm.forkArgs(), jm.allForkEnv())
diff --git a/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala b/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala
index 726d31b00947..e62383e77ba3 100644
--- a/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala
+++ b/libs/javalib/src/mill/javalib/bsp/BspRunModule.scala
@@ -23,7 +23,7 @@ private[mill] trait BspRunModule(runModule: RunModule) extends mill.api.Module {
         runClasspath: Seq[Path],
         forkArgs: Opts,
         forkWorkingDir: Path,
-        forkEnv: Map[String, Opt],
+        forkEnv: OptMap,
         mainClass: Option[String],
         localMainClasses: Seq[String]
     )] =
@@ -42,7 +42,7 @@ private[mill] trait BspRunModule(runModule: RunModule) extends mill.api.Module {
         runClasspath: Seq[Path],
         forkArgs: Opts,
         forkWorkingDir: Path,
-        forkEnv: Map[String, Opt],
+        forkEnv: OptMap,
         mainClass: Option[String],
         testEnvVars: Option[(
             mainClass: String,
diff --git a/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala b/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala
index 1fce4d825106..633e32e5833a 100644
--- a/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala
+++ b/libs/kotlinlib/src/mill/kotlinlib/ksp/KspModule.scala
@@ -104,8 +104,8 @@ trait KspModule extends KotlinModule { outer =>
   /**
    * Processor options to be passed to KSP.
    */
-  def kspProcessorOptions: T[Map[String, Opt]] = Task {
-    Map.empty[String, Opt]
+  def kspProcessorOptions: T[OptMap] = Task {
+    OptMap()
   }
 
   /**
diff --git a/libs/scalalib/src/mill/scalalib/ScalaModule.scala b/libs/scalalib/src/mill/scalalib/ScalaModule.scala
index aba65639de69..b40b2aea0c84 100644
--- a/libs/scalalib/src/mill/scalalib/ScalaModule.scala
+++ b/libs/scalalib/src/mill/scalalib/ScalaModule.scala
@@ -446,7 +446,7 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
               "scala.tools.nsc.MainGenericRunner",
           classPath = runClasspath().map(_.path) ++ scalaConsoleClasspath().map(_.path),
           jvmArgs = forkArgs().toStringSeq,
-          env = allForkEnv().view.mapValues(_.toString()).toMap,
+          env = allForkEnv().toStringMap,
           mainArgs =
             Seq(useJavaCp) ++ consoleScalacOptions().filterNot(Set(useJavaCp)) ++ args.value,
           cwd = forkWorkingDir(),
@@ -527,7 +527,7 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase
         mainClass = mainClass,
         classPath = ammoniteReplClasspath().map(_.path).toVector,
         jvmArgs = forkArgs().toStringSeq,
-        env = allForkEnv().view.mapValues(_.toString()).toMap,
+        env = allForkEnv().toStringMap,
         mainArgs = replOptions,
         cwd = forkWorkingDir(),
         stdin = os.Inherit,
diff --git a/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala b/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
index 16a73ff54530..299c55d31a5b 100644
--- a/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
+++ b/libs/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
@@ -390,7 +390,7 @@ trait ScalaNativeModule extends ScalaModule with ScalaNativeModuleApi { outer =>
       mainClassDefault = finalMainClassOpt(),
       nativeExe = nativeLink(),
       forkArgsDefault = forkArgs().toStringSeq,
-      forkEnvDefault = allForkEnv().view.mapValues(_.toString()).toMap,
+      forkEnvDefault = allForkEnv().toStringMap,
       propagateEnvDefault = propagateEnv()
     )
   }
@@ -400,7 +400,7 @@ trait ScalaNativeModule extends ScalaModule with ScalaNativeModuleApi { outer =>
       mainClassDefault = Right(mainClass),
       nativeExe = nativeLinkOtherMain(mainClass)(),
       forkArgsDefault = forkArgs().toStringSeq,
-      forkEnvDefault = allForkEnv().view.mapValues(_.toString()).toMap,
+      forkEnvDefault = allForkEnv().toStringMap,
       propagateEnvDefault = propagateEnv()
     )
   }
@@ -491,7 +491,7 @@ trait TestScalaNativeModule extends ScalaNativeModule with TestModule {
 
     val (close, framework) = withScalaNativeBridge.apply().apply(_.getFramework(
       nativeLink().path.toIO,
-      allForkEnv().view.mapValues(_.toString()).toMap,
+      allForkEnv().toStringMap,
       toWorkerApi(logLevel()),
       testFramework()
     ))
diff --git a/runner/bsp/worker/src/mill/bsp/worker/MillJvmBuildServer.scala b/runner/bsp/worker/src/mill/bsp/worker/MillJvmBuildServer.scala
index b9357dad0977..95a0d8612bb4 100644
--- a/runner/bsp/worker/src/mill/bsp/worker/MillJvmBuildServer.scala
+++ b/runner/bsp/worker/src/mill/bsp/worker/MillJvmBuildServer.scala
@@ -14,6 +14,7 @@ import ch.epfl.scala.bsp4j.{
   JvmTestEnvironmentResult
 }
 import mill.api.daemon.internal.{JavaModuleApi, RunModuleApi, TestModuleApi}
+import mill.api.opt.*
 import mill.bsp.worker.Utils.sanitizeUri
 import java.util.concurrent.CompletableFuture
 
@@ -75,7 +76,7 @@ private trait MillJvmBuildServer extends JvmBuildServer { this: MillBuildServer
           testEnvVars.classpath.map(sanitizeUri).asJava,
           res.forkArgs.toStringSeq.asJava,
           res.forkWorkingDir.toString(),
-          res.forkEnv.view.mapValues(_.toString()).toMap.asJava
+          res.forkEnv.toStringMap.asJava
         )
         item.setMainClasses(List(testEnvVars.mainClass).map(new JvmMainClass(
           _,
@@ -111,7 +112,7 @@ private trait MillJvmBuildServer extends JvmBuildServer { this: MillBuildServer
           classpath.asJava,
           res.forkArgs.toStringSeq.asJava,
           res.forkWorkingDir.toString(),
-          res.forkEnv.view.mapValues(_.toString()).toMap.asJava
+          res.forkEnv.toStringMap.asJava
         )
 
         val classes = res.mainClass.toList ++ res.localMainClasses