diff --git a/packages/opencode/src/lsp/client.ts b/packages/opencode/src/lsp/client.ts
index 8704b65acb5..447211b75a3 100644
--- a/packages/opencode/src/lsp/client.ts
+++ b/packages/opencode/src/lsp/client.ts
@@ -12,8 +12,11 @@ import { NamedError } from "@opencode-ai/util/error"
 import { withTimeout } from "../util/timeout"
 import { Instance } from "../project/instance"
 import { Filesystem } from "../util/filesystem"
+import { LRUMap } from "../util/lru-map"
 
 const DIAGNOSTICS_DEBOUNCE_MS = 150
+// Maximum number of files to track per LSP client to prevent unbounded memory growth
+const MAX_TRACKED_FILES = 1000
 
 export namespace LSPClient {
   const log = Log.create({ service: "lsp.client" })
@@ -48,7 +51,7 @@ export namespace LSPClient {
       new StreamMessageWriter(input.server.process.stdin as any),
     )
 
-    const diagnostics = new Map<string, Diagnostic[]>()
+    const diagnostics = new LRUMap<string, Diagnostic[]>(MAX_TRACKED_FILES)
     connection.onNotification("textDocument/publishDiagnostics", (params) => {
       const filePath = Filesystem.normalizePath(fileURLToPath(params.uri))
       l.info("textDocument/publishDiagnostics", {
@@ -132,9 +135,8 @@ export namespace LSPClient {
       })
     }
 
-    const files: {
-      [path: string]: number
-    } = {}
+    // Track file versions for LSP synchronization with LRU eviction
+    const files = new LRUMap<string, number>(MAX_TRACKED_FILES)
 
     const result = {
       root: input.root,
@@ -152,7 +154,7 @@ export namespace LSPClient {
           const extension = path.extname(input.path)
           const languageId = LANGUAGE_EXTENSIONS[extension] ?? "plaintext"
 
-          const version = files[input.path]
+          const version = files.get(input.path)
           if (version !== undefined) {
             log.info("workspace/didChangeWatchedFiles", input)
             await connection.sendNotification("workspace/didChangeWatchedFiles", {
@@ -165,7 +167,7 @@ export namespace LSPClient {
             })
 
             const next = version + 1
-            files[input.path] = next
+            files.set(input.path, next)
             log.info("textDocument/didChange", {
               path: input.path,
               version: next,
@@ -200,7 +202,7 @@ export namespace LSPClient {
               text,
             },
           })
-          files[input.path] = 0
+          files.set(input.path, 0)
           return
         },
       },
diff --git a/packages/opencode/src/util/lru-map.ts b/packages/opencode/src/util/lru-map.ts
new file mode 100644
index 00000000000..af192566e0f
--- /dev/null
+++ b/packages/opencode/src/util/lru-map.ts
@@ -0,0 +1,72 @@
+/**
+ * A Map with LRU (Least Recently Used) eviction policy.
+ * Uses the fact that JavaScript Maps maintain insertion order.
+ * When capacity is exceeded, the oldest (first) entries are evicted.
+ *
+ * Calling {@link get} on an existing key moves that entry to the most recently
+ * used position and therefore affects the eviction order. In contrast,
+ * {@link has} only checks for the presence of a key and does not change the
+ * recency or eviction order of entries.
+ */
+export class LRUMap<K, V> {
+  private map = new Map<K, V>()
+  private readonly capacity: number
+
+  constructor(capacity: number) {
+    if (!Number.isFinite(capacity) || !Number.isInteger(capacity) || capacity <= 0) {
+      throw new RangeError(`LRUMap capacity must be a positive integer, got: ${capacity}`)
+    }
+    this.capacity = capacity
+  }
+
+  get(key: K): V | undefined {
+    if (!this.map.has(key)) {
+      return undefined
+    }
+    const value = this.map.get(key)!
+    // Move to end (most recently used)
+    this.map.delete(key)
+    this.map.set(key, value)
+    return value
+  }
+
+  set(key: K, value: V): this {
+    // Delete first to ensure it goes to end if it exists
+    this.map.delete(key)
+    this.map.set(key, value)
+    // Evict oldest entry if over capacity (can only exceed by 1 since we add one at a time)
+    if (this.map.size > this.capacity) {
+      const oldest = this.map.keys().next().value as K
+      this.map.delete(oldest)
+    }
+    return this
+  }
+
+  has(key: K): boolean {
+    return this.map.has(key)
+  }
+
+  delete(key: K): boolean {
+    return this.map.delete(key)
+  }
+
+  get size(): number {
+    return this.map.size
+  }
+
+  keys(): IterableIterator<K> {
+    return this.map.keys()
+  }
+
+  values(): IterableIterator<V> {
+    return this.map.values()
+  }
+
+  entries(): IterableIterator<[K, V]> {
+    return this.map.entries()
+  }
+
+  clear(): void {
+    this.map.clear()
+  }
+}
diff --git a/packages/opencode/test/util/lru-map.test.ts b/packages/opencode/test/util/lru-map.test.ts
new file mode 100644
index 00000000000..ab81cddb27b
--- /dev/null
+++ b/packages/opencode/test/util/lru-map.test.ts
@@ -0,0 +1,176 @@
+import { describe, expect, test } from "bun:test"
+import { LRUMap } from "../../src/util/lru-map"
+
+describe("LRUMap", () => {
+  test("basic get and set operations", () => {
+    const map = new LRUMap<string, number>(10)
+    map.set("a", 1)
+    map.set("b", 2)
+    expect(map.get("a")).toBe(1)
+    expect(map.get("b")).toBe(2)
+    expect(map.size).toBe(2)
+  })
+
+  test("has returns correct value", () => {
+    const map = new LRUMap<string, number>(10)
+    map.set("a", 1)
+    expect(map.has("a")).toBe(true)
+    expect(map.has("b")).toBe(false)
+  })
+
+  test("delete removes entry", () => {
+    const map = new LRUMap<string, number>(10)
+    map.set("a", 1)
+    expect(map.has("a")).toBe(true)
+    map.delete("a")
+    expect(map.has("a")).toBe(false)
+    expect(map.size).toBe(0)
+  })
+
+  test("evicts oldest entries when over capacity", () => {
+    const map = new LRUMap<string, number>(3)
+    map.set("a", 1)
+    map.set("b", 2)
+    map.set("c", 3)
+    expect(map.size).toBe(3)
+
+    // Adding a 4th entry should evict the oldest ("a")
+    map.set("d", 4)
+    expect(map.size).toBe(3)
+    expect(map.has("a")).toBe(false)
+    expect(map.has("b")).toBe(true)
+    expect(map.has("c")).toBe(true)
+    expect(map.has("d")).toBe(true)
+  })
+
+  test("get moves entry to end (most recently used)", () => {
+    const map = new LRUMap<string, number>(3)
+    map.set("a", 1)
+    map.set("b", 2)
+    map.set("c", 3)
+
+    // Access "a" to make it most recently used
+    map.get("a")
+
+    // Adding "d" should now evict "b" (oldest after "a" was accessed)
+    map.set("d", 4)
+    expect(map.has("a")).toBe(true)
+    expect(map.has("b")).toBe(false)
+    expect(map.has("c")).toBe(true)
+    expect(map.has("d")).toBe(true)
+  })
+
+  test("set updates value and moves to end", () => {
+    const map = new LRUMap<string, number>(3)
+    map.set("a", 1)
+    map.set("b", 2)
+    map.set("c", 3)
+
+    // Update "a" to make it most recently used
+    map.set("a", 100)
+    expect(map.get("a")).toBe(100)
+
+    // Adding "d" should evict "b"
+    map.set("d", 4)
+    expect(map.has("a")).toBe(true)
+    expect(map.has("b")).toBe(false)
+    expect(map.has("c")).toBe(true)
+    expect(map.has("d")).toBe(true)
+  })
+
+  test("clear removes all entries", () => {
+    const map = new LRUMap<string, number>(10)
+    map.set("a", 1)
+    map.set("b", 2)
+    map.clear()
+    expect(map.size).toBe(0)
+    expect(map.has("a")).toBe(false)
+  })
+
+  test("iterators work correctly", () => {
+    const map = new LRUMap<string, number>(10)
+    map.set("a", 1)
+    map.set("b", 2)
+
+    expect([...map.keys()]).toEqual(["a", "b"])
+    expect([...map.values()]).toEqual([1, 2])
+    expect([...map.entries()]).toEqual([
+      ["a", 1],
+      ["b", 2],
+    ])
+  })
+
+  test("handles capacity of 1", () => {
+    const map = new LRUMap<string, number>(1)
+    map.set("a", 1)
+    expect(map.get("a")).toBe(1)
+    map.set("b", 2)
+    expect(map.has("a")).toBe(false)
+    expect(map.get("b")).toBe(2)
+    expect(map.size).toBe(1)
+  })
+
+  test("handles falsy but valid values (0, false, empty string)", () => {
+    // Test with 0
+    const numMap = new LRUMap<string, number>(3)
+    numMap.set("a", 0)
+    numMap.set("b", 1)
+    numMap.set("c", 2)
+    expect(numMap.get("a")).toBe(0)
+
+    // After accessing "a", it should be most recently used
+    // Adding "d" should evict "b" (not "a")
+    numMap.set("d", 3)
+    expect(numMap.has("a")).toBe(true)
+    expect(numMap.has("b")).toBe(false)
+
+    // Test with false
+    const boolMap = new LRUMap<string, boolean>(3)
+    boolMap.set("a", false)
+    boolMap.set("b", true)
+    boolMap.set("c", true)
+    expect(boolMap.get("a")).toBe(false)
+
+    boolMap.set("d", true)
+    expect(boolMap.has("a")).toBe(true)
+    expect(boolMap.has("b")).toBe(false)
+
+    // Test with empty string
+    const strMap = new LRUMap<string, string>(3)
+    strMap.set("a", "")
+    strMap.set("b", "x")
+    strMap.set("c", "y")
+    expect(strMap.get("a")).toBe("")
+
+    strMap.set("d", "z")
+    expect(strMap.has("a")).toBe(true)
+    expect(strMap.has("b")).toBe(false)
+  })
+
+  test("throws on invalid capacity values", () => {
+    expect(() => new LRUMap<string, number>(0)).toThrow(RangeError)
+    expect(() => new LRUMap<string, number>(-1)).toThrow(RangeError)
+    expect(() => new LRUMap<string, number>(1.5)).toThrow(RangeError)
+    expect(() => new LRUMap<string, number>(NaN)).toThrow(RangeError)
+    expect(() => new LRUMap<string, number>(Infinity)).toThrow(RangeError)
+  })
+
+  test("handles large number of entries", () => {
+    const capacity = 100
+    const map = new LRUMap<number, number>(capacity)
+
+    // Add more entries than capacity
+    for (let i = 0; i < 200; i++) {
+      map.set(i, i * 2)
+    }
+
+    expect(map.size).toBe(capacity)
+    // First 100 entries should be evicted
+    expect(map.has(0)).toBe(false)
+    expect(map.has(99)).toBe(false)
+    // Last 100 entries should remain
+    expect(map.has(100)).toBe(true)
+    expect(map.has(199)).toBe(true)
+    expect(map.get(150)).toBe(300)
+  })
+})