diff --git a/packages/core/src/filesystem.ts b/packages/core/src/filesystem.ts
index 7f0c047de0..477122f12f 100644
--- a/packages/core/src/filesystem.ts
+++ b/packages/core/src/filesystem.ts
@@ -1,5 +1,5 @@
 import { NodeFileSystem } from "@effect/platform-node"
-import { dirname, isAbsolute, join, relative, resolve as pathResolve } from "path"
+import { dirname, isAbsolute, join, relative, resolve as pathResolve, sep } from "path"
 import { realpathSync } from "fs"
 import * as NFS from "fs/promises"
 import { lookup } from "mime-types"
@@ -233,13 +233,11 @@ export namespace AppFileSystem {
   }
 
   export function overlaps(a: string, b: string) {
-    const relA = relative(a, b)
-    const relB = relative(b, a)
-    return !relA || !relA.startsWith("..") || !relB || !relB.startsWith("..")
+    return contains(a, b) || contains(b, a)
   }
 
   export function contains(parent: string, child: string) {
-    const rel = relative(parent, child)
-    return rel === "" || (!rel.startsWith("..") && !isAbsolute(rel))
+    const result = relative(parent, child)
+    return result === "" || (!isAbsolute(result) && result !== ".." && !result.startsWith(`..${sep}`))
   }
 }
diff --git a/packages/core/test/filesystem/filesystem.test.ts b/packages/core/test/filesystem/filesystem.test.ts
index 1d9405333d..973d2eacd5 100644
--- a/packages/core/test/filesystem/filesystem.test.ts
+++ b/packages/core/test/filesystem/filesystem.test.ts
@@ -354,13 +354,21 @@ describe("AppFileSystem", () => {
 
     test("contains checks path containment", () => {
       expect(AppFileSystem.contains("/a/b", "/a/b/c")).toBe(true)
+      expect(AppFileSystem.contains("/a/b", "/a/b")).toBe(true)
+      expect(AppFileSystem.contains("/a/b", "/a/b/..c")).toBe(true)
       expect(AppFileSystem.contains("/a/b", "/a/c")).toBe(false)
+      expect(AppFileSystem.contains("/a/b", "/a")).toBe(false)
+      expect(AppFileSystem.contains("/a/b", "/a/bc")).toBe(false)
     })
 
     test("overlaps detects overlapping paths", () => {
       expect(AppFileSystem.overlaps("/a/b", "/a/b/c")).toBe(true)
+      expect(AppFileSystem.overlaps("/a/b", "/a/b")).toBe(true)
+      expect(AppFileSystem.overlaps("/a/b", "/a/b/..c")).toBe(true)
       expect(AppFileSystem.overlaps("/a/b/c", "/a/b")).toBe(true)
+      expect(AppFileSystem.overlaps("/a/b", "/a")).toBe(true)
       expect(AppFileSystem.overlaps("/a", "/b")).toBe(false)
+      expect(AppFileSystem.overlaps("/a/b", "/a/bc")).toBe(false)
     })
   })
 })
diff --git a/packages/opencode/src/util/filesystem.ts b/packages/opencode/src/util/filesystem.ts
index 696603adbb..6317cf1155 100644
--- a/packages/opencode/src/util/filesystem.ts
+++ b/packages/opencode/src/util/filesystem.ts
@@ -1,7 +1,7 @@
 import { chmod, mkdir, readFile, stat as statFile, writeFile } from "fs/promises"
 import { createWriteStream, existsSync, statSync } from "fs"
 import { realpathSync } from "fs"
-import { dirname, isAbsolute, join, relative, resolve as pathResolve, win32 } from "path"
+import { dirname, isAbsolute, join, relative, resolve as pathResolve, sep, win32 } from "path"
 import { Readable } from "stream"
 import { pipeline } from "stream/promises"
 import { Glob } from "@opencode-ai/core/util/glob"
@@ -163,13 +163,12 @@ export function windowsPath(p: string): string {
   )
 }
 export function overlaps(a: string, b: string) {
-  const relA = relative(a, b)
-  const relB = relative(b, a)
-  return !relA || !relA.startsWith("..") || !relB || !relB.startsWith("..")
+  return contains(a, b) || contains(b, a)
 }
 
 export function contains(parent: string, child: string) {
-  return !relative(parent, child).startsWith("..")
+  const result = relative(parent, child)
+  return result === "" || (!isAbsolute(result) && result !== ".." && !result.startsWith(`..${sep}`))
 }
 
 export async function findUp(
diff --git a/packages/opencode/test/file/path-traversal.test.ts b/packages/opencode/test/file/path-traversal.test.ts
index 1b0dec5aad..6850339e9c 100644
--- a/packages/opencode/test/file/path-traversal.test.ts
+++ b/packages/opencode/test/file/path-traversal.test.ts
@@ -25,6 +25,7 @@ describe("Filesystem.contains", () => {
       expect(Filesystem.contains("/project", "/project/src")).toBe(true)
       expect(Filesystem.contains("/project", "/project/src/file.ts")).toBe(true)
       expect(Filesystem.contains("/project", "/project")).toBe(true)
+      expect(Filesystem.contains("/project", "/project/..config")).toBe(true)
     }),
   )
 
@@ -33,6 +34,7 @@ describe("Filesystem.contains", () => {
       expect(Filesystem.contains("/project", "/project/../etc")).toBe(false)
       expect(Filesystem.contains("/project", "/project/src/../../etc")).toBe(false)
       expect(Filesystem.contains("/project", "/etc/passwd")).toBe(false)
+      expect(Filesystem.contains("/project", "/")).toBe(false)
     }),
   )
 
@@ -52,6 +54,29 @@ describe("Filesystem.contains", () => {
   )
 })
 
+describe("Filesystem.overlaps", () => {
+  it.effect("detects containment in either direction", () =>
+    Effect.sync(() => {
+      expect(Filesystem.overlaps("/project", "/project/src")).toBe(true)
+      expect(Filesystem.overlaps("/project/src", "/project")).toBe(true)
+      expect(Filesystem.overlaps("/project", "/project")).toBe(true)
+    }),
+  )
+
+  it.effect("keeps child names beginning with .. inside the parent", () =>
+    Effect.sync(() => {
+      expect(Filesystem.overlaps("/project", "/project/..config")).toBe(true)
+    }),
+  )
+
+  it.effect("rejects unrelated sibling and parent-only paths", () =>
+    Effect.sync(() => {
+      expect(Filesystem.overlaps("/project", "/project-other")).toBe(false)
+      expect(Filesystem.overlaps("/project/src", "/etc")).toBe(false)
+    }),
+  )
+})
+
 /*
  * Integration tests for read() and list() path traversal protection.
  *