diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4961ad0..4340ce9 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -73,8 +73,13 @@ jobs:
       - name: Check pkg-config path
         if: runner.os == 'Linux'
         run: |
-          nix develop --command cargo build -p libghostty-vt-sys
-          PKG_CONFIG_PATH=$(find target/debug/build -path '*/out/ghostty-install/share/pkgconfig' | head -1)
-          test -n "$PKG_CONFIG_PATH"
-          export PKG_CONFIG_PATH
-          nix develop --command cargo check -p libghostty-vt-sys --features pkg-config
+          test -f result/share/pkgconfig/libghostty-vt-static.pc
+          PKG_CONFIG_PATH="$PWD/result/share/pkgconfig" \
+            nix develop --command env \
+              -u GHOSTTY_SOURCE_DIR \
+              -u GHOSTTY_ZIG_SYSTEM_DIR \
+              cargo check -p libghostty-vt-sys --no-default-features --features pkg-config
+
+      - name: Run libghostty-vt tests under Valgrind
+        if: matrix.name == 'Linux x86_64'
+        run: nix build .#checks.x86_64-linux.ci-valgrind --print-build-logs
diff --git a/README.md b/README.md
index 0b4771f..ef10716 100644
--- a/README.md
+++ b/README.md
@@ -67,6 +67,9 @@ Vendored builds derive Zig's optimize mode from Cargo's profile: dev builds use
 `Debug`, size-optimized builds use `ReleaseSmall`, and other release builds use
 `ReleaseFast`. Set `LIBGHOSTTY_VT_SYS_OPTIMIZE` to `Debug`, `ReleaseSafe`,
 `ReleaseFast`, or `ReleaseSmall` to override that choice explicitly.
+Set `LIBGHOSTTY_VT_SYS_SIMD=false` to pass Ghostty's `-Dsimd=false` option and
+build scalar code paths only. The Nix output `.#libghostty-vt-non-simd` uses
+that mode, and the Valgrind CI check combines it with Zig `Debug` builds.
 
 The `pkg-config` path is opt-in. If you enable `libghostty-vt-sys/pkg-config`,
 the build will prefer an installed `libghostty-vt` discovered through
diff --git a/crates/libghostty-vt-sys/build.rs b/crates/libghostty-vt-sys/build.rs
index 8600438..8fca46d 100644
--- a/crates/libghostty-vt-sys/build.rs
+++ b/crates/libghostty-vt-sys/build.rs
@@ -72,6 +72,7 @@ fn main() {
     let link_mode = LinkMode::current();
 
     println!("cargo:rerun-if-env-changed=LIBGHOSTTY_VT_SYS_OPTIMIZE");
+    println!("cargo:rerun-if-env-changed=LIBGHOSTTY_VT_SYS_SIMD");
     println!("cargo:rerun-if-env-changed=GHOSTTY_SOURCE_DIR");
     println!("cargo:rerun-if-env-changed=GHOSTTY_ZIG_SYSTEM_DIR");
     println!("cargo:rerun-if-env-changed=TARGET");
@@ -126,12 +127,14 @@ fn build_vendored(link_mode: LinkMode) {
     let zig_global_cache_dir = out_dir.join("zig-global-cache");
 
     let optimize = zig_optimize_mode();
+    let simd = zig_simd_enabled();
 
     let mut build = Command::new("zig");
     build
         .arg("build")
         .arg("-Demit-lib-vt")
         .arg(format!("-Doptimize={optimize}"))
+        .arg(format!("-Dsimd={simd}"))
         .arg("-Demit-xcframework=false")
         .arg("-Dapp-runtime=none")
         .arg("--prefix")
@@ -316,6 +319,24 @@ fn zig_optimize_mode() -> &'static str {
     }
 }
 
+/// Decide whether to build Ghostty's SIMD-accelerated code paths.
+///
+/// Ghostty exposes this as `zig build -Dsimd=...`. Valgrind builds should turn
+/// it off so the generated library uses scalar fallbacks instead of pulling in
+/// the C++ highway/simdutf implementation block.
+fn zig_simd_enabled() -> bool {
+    match env::var("LIBGHOSTTY_VT_SYS_SIMD") {
+        Ok(value) => match value.as_str() {
+            "1" | "true" | "True" | "TRUE" => true,
+            "0" | "false" | "False" | "FALSE" => false,
+            other => {
+                panic!("LIBGHOSTTY_VT_SYS_SIMD must be one of true, false, 1, 0 (got '{other}')")
+            }
+        },
+        Err(_) => true,
+    }
+}
+
 /// Clone ghostty at the pinned commit into OUT_DIR/ghostty-src.
 /// Reuses an existing clone if the commit matches.
 fn fetch_ghostty(out_dir: &Path) -> PathBuf {
diff --git a/flake.nix b/flake.nix
index 6dfd9a7..fc6fec4 100644
--- a/flake.nix
+++ b/flake.nix
@@ -114,17 +114,90 @@
           };
 
         cargoArtifacts = craneLib.buildDepsOnly commonArgs;
+        nonSimdCargoArtifacts = craneLib.buildDepsOnly (
+          commonArgs
+          // {
+            pname = "libghostty-rs-non-simd-deps";
+            LIBGHOSTTY_VT_SYS_SIMD = "false";
+          }
+        );
+        ciValgrindCargoArtifacts = craneLib.buildDepsOnly (
+          commonArgs
+          // {
+            pname = "libghostty-rs-ci-valgrind-deps";
+            cargoExtraArgs = "--locked -p libghostty-vt";
+            cargoTestExtraArgs = "--no-run";
+            CARGO_PROFILE = "";
+
+            # The Valgrind check must not inherit ReleaseFast Zig artifacts from
+            # the normal package build. Debug keeps both Rust and Zig codegen in
+            # Valgrind's supported instruction set and gives better reports.
+            LIBGHOSTTY_VT_SYS_OPTIMIZE = "Debug";
+            LIBGHOSTTY_VT_SYS_SIMD = "false";
+          }
+        );
 
         application = craneLib.buildPackage (
           commonArgs
           // {
             inherit cargoArtifacts;
+            postInstall = ''
+              ghostty_install=$(find target -path '*/out/ghostty-install' -type d | head -1)
+              if [ -z "$ghostty_install" ]; then
+                echo "expected Cargo build script to install libghostty-vt into OUT_DIR" >&2
+                exit 1
+              fi
+              ghostty_install=$(realpath "$ghostty_install")
+
+              cp -R "$ghostty_install"/. "$out"/
+              substituteInPlace "$out"/share/pkgconfig/*.pc \
+                --replace-fail "prefix=$ghostty_install" "prefix=$out"
+            '';
+          }
+        );
+
+        libghosttyVtNonSimd = craneLib.buildPackage (
+          commonArgs
+          // {
+            cargoArtifacts = nonSimdCargoArtifacts;
+            pname = "libghostty-vt-non-simd";
+            LIBGHOSTTY_VT_SYS_SIMD = "false";
+          }
+        );
+
+        ciValgrind = craneLib.buildPackage (
+          commonArgs
+          // {
+            cargoArtifacts = ciValgrindCargoArtifacts;
+            pname = "libghostty-rs-ci-valgrind";
+            cargoBuildCommand = "";
+            cargoTestCommand = "cargo valgrind test";
+            cargoExtraArgs = "--locked -p libghostty-vt";
+            cargoTestExtraArgs = "-- --test-threads=1";
+            CARGO_PROFILE = "";
+            LIBGHOSTTY_VT_SYS_OPTIMIZE = "Debug";
+            LIBGHOSTTY_VT_SYS_SIMD = "false";
+            nativeBuildInputs = commonArgs.nativeBuildInputs ++ [
+              pkgs.cargo-valgrind
+              pkgs.valgrind
+            ];
+
+            installPhaseCommand = "mkdir -p $out";
           }
         );
       in {
-        packages.default = application;
+        packages = {
+          default = application;
+          libghostty-vt-non-simd = libghosttyVtNonSimd;
+        };
 
-        checks.default = application;
+        checks =
+          {
+            default = application;
+          }
+          // pkgs.lib.optionalAttrs (system == "x86_64-linux") {
+            ci-valgrind = ciValgrind;
+          };
 
         devShells.default = craneLib.devShell {
           packages = [
@@ -137,6 +210,9 @@
             pkgs.cmake
             pkgs.ninja
           ] ++ pkgs.lib.optionals pkgs.stdenv.hostPlatform.isLinux [
+            # Valgrind is Linux-only here. Keep it in the development shell so
+            # CI and local Linux users exercise the same memory-checking path.
+            pkgs.valgrind
             pkgs.libx11
             pkgs.libxcursor
             pkgs.libxrandr
@@ -145,6 +221,8 @@
             pkgs.libGL
             pkgs.libxkbcommon
             pkgs.wayland
+          ] ++ pkgs.lib.optionals (system == "x86_64-linux") [
+            pkgs.cargo-valgrind
           ];
 
           shellHook = ''