diff --git a/.changeset/stale-lemons-protect.md b/.changeset/stale-lemons-protect.md
new file mode 100644
index 00000000000..8f54eb79d5f
--- /dev/null
+++ b/.changeset/stale-lemons-protect.md
@@ -0,0 +1,5 @@
+---
+"@kilocode/cli": patch
+---
+
+Prompt before agents access files outside the active directory when a workspace boundary resolves to a filesystem root.
diff --git a/packages/opencode/src/tool/external-directory.ts b/packages/opencode/src/tool/external-directory.ts
index 88b73da5096..c6518e20d2b 100644
--- a/packages/opencode/src/tool/external-directory.ts
+++ b/packages/opencode/src/tool/external-directory.ts
@@ -3,7 +3,6 @@ import { Effect } from "effect"
 import { EffectLogger } from "@/effect"
 import { InstanceState } from "@/effect"
 import type * as Tool from "./tool"
-import { Instance } from "../project/instance"
 import { AppFileSystem } from "@opencode-ai/shared/filesystem"
 
 type Kind = "file" | "directory"
@@ -13,6 +12,16 @@ type Options = {
   kind?: Kind
 }
 
+// kilocode_change start - root boundaries must not auto-allow external_directory
+function root(dir: string) {
+  return path.parse(dir).root === dir
+}
+
+function inside(dir: string, file: string) {
+  return !root(dir) && AppFileSystem.contains(dir, file)
+}
+// kilocode_change end
+
 export const assertExternalDirectoryEffect = Effect.fn("Tool.assertExternalDirectory")(function* (
   ctx: Tool.Context,
   target?: string,
@@ -24,7 +33,9 @@ export const assertExternalDirectoryEffect = Effect.fn("Tool.assertExternalDirec
 
   const ins = yield* InstanceState.context
   const full = process.platform === "win32" ? AppFileSystem.normalizePath(target) : target
-  if (Instance.containsPath(full, ins)) return
+  // kilocode_change start - keep root-workspace behavior intact outside permission prompts
+  if (inside(ins.directory, full) || inside(ins.worktree, full)) return
+  // kilocode_change end
 
   const kind = options?.kind ?? "file"
   const dir = kind === "directory" ? full : path.dirname(full)
diff --git a/packages/opencode/src/util/filesystem.ts b/packages/opencode/src/util/filesystem.ts
index 6c4d455224e..38b7e1b7ed8 100644
--- a/packages/opencode/src/util/filesystem.ts
+++ b/packages/opencode/src/util/filesystem.ts
@@ -1,7 +1,9 @@
 import { chmod, mkdir, readFile, stat as statFile, writeFile } from "fs/promises"
 import { createWriteStream, existsSync, statSync } from "fs"
 import { realpathSync } from "fs"
-import { dirname, join, relative, resolve as pathResolve, win32 } from "path"
+// kilocode_change start - harden containment checks
+import { dirname, isAbsolute, join, relative, resolve as pathResolve, sep, win32 } from "path"
+// kilocode_change end
 import { Readable } from "stream"
 import { pipeline } from "stream/promises"
 import { Glob } from "@opencode-ai/shared/util/glob"
@@ -162,7 +164,10 @@ export function overlaps(a: string, b: string) {
 }
 
 export function contains(parent: string, child: string) {
-  return !relative(parent, child).startsWith("..")
+  // kilocode_change start - reject cross-drive and escaped relative paths
+  const rel = relative(parent, child)
+  return rel === "" || (!isAbsolute(rel) && rel !== ".." && !rel.startsWith(`..${sep}`))
+  // kilocode_change end
 }
 
 export async function findUp(
diff --git a/packages/opencode/test/kilocode/external-directory-boundary.test.ts b/packages/opencode/test/kilocode/external-directory-boundary.test.ts
new file mode 100644
index 00000000000..ea04fd71d53
--- /dev/null
+++ b/packages/opencode/test/kilocode/external-directory-boundary.test.ts
@@ -0,0 +1,95 @@
+import { describe, expect, test } from "bun:test"
+import { Effect } from "effect"
+import path from "path"
+import type { Permission } from "../../src/permission"
+import { Instance } from "../../src/project/instance"
+import { SessionID, MessageID } from "../../src/session/schema"
+import { assertExternalDirectory } from "../../src/tool/external-directory"
+import type { Tool } from "../../src/tool"
+import { Filesystem } from "../../src/util"
+import { AppFileSystem } from "@opencode-ai/shared/filesystem"
+import { tmpdir } from "../fixture/fixture"
+
+const base: Omit<Tool.Context, "ask"> = {
+  sessionID: SessionID.make("ses_test-boundary-session"),
+  messageID: MessageID.make(""),
+  callID: "",
+  agent: "code",
+  abort: AbortSignal.any([]),
+  messages: [],
+  metadata: () => Effect.void,
+}
+
+const glob = (p: string) =>
+  process.platform === "win32" ? AppFileSystem.normalizePathPattern(p) : p.replaceAll("\\", "/")
+
+const asks = () => {
+  const items: Array<Omit<Permission.Request, "id" | "sessionID" | "tool">> = []
+  const ctx: Tool.Context = {
+    ...base,
+    ask: (req) =>
+      Effect.sync(() => {
+        items.push(req)
+      }),
+  }
+  return { items, ctx }
+}
+
+describe("kilocode external directory boundaries", () => {
+  test("asks before accessing outside a repo-root session", async () => {
+    await using repo = await tmpdir({ git: true })
+    await using outer = await tmpdir()
+    const file = path.join(outer.path, "outside.txt")
+    const { items, ctx } = asks()
+
+    await Instance.provide({
+      directory: repo.path,
+      fn: async () => {
+        try {
+          await assertExternalDirectory(ctx, file)
+        } finally {
+          await Instance.dispose()
+        }
+      },
+    })
+
+    const ext = items.find((item) => item.permission === "external_directory")
+    expect(ext).toBeDefined()
+    expect(ext!.patterns).toEqual([glob(path.join(outer.path, "*"))])
+    expect(ext!.always).toEqual([glob(path.join(outer.path, "*"))])
+    expect(ext!.metadata).toMatchObject({ filepath: file, parentDir: outer.path })
+  })
+
+  test("asks when the instance directory is a filesystem root", async () => {
+    await using outer = await tmpdir()
+    const root = path.parse(outer.path).root
+    const file = path.join(outer.path, "outside-root.txt")
+    const { items, ctx } = asks()
+
+    await Instance.provide({
+      directory: root,
+      fn: async () => {
+        try {
+          await assertExternalDirectory(ctx, file)
+        } finally {
+          await Instance.dispose()
+        }
+      },
+    })
+
+    const ext = items.find((item) => item.permission === "external_directory")
+    expect(ext).toBeDefined()
+    expect(ext!.patterns).toEqual([glob(path.join(outer.path, "*"))])
+    expect(ext!.metadata).toMatchObject({ filepath: file, parentDir: outer.path })
+  })
+
+  test("contains helpers keep dot-prefixed child names internal", () => {
+    expect(Filesystem.contains("/project", "/project/..cache/file")).toBe(true)
+    expect(AppFileSystem.contains("/a/b", "/a/b/..cache/file")).toBe(true)
+  })
+
+  test("AppFileSystem.contains rejects cross-drive paths on Windows", () => {
+    if (process.platform !== "win32") return
+    expect(AppFileSystem.contains("C:\\repo", "D:\\outside\\file.txt")).toBe(false)
+  })
+})
diff --git a/packages/shared/src/filesystem.ts b/packages/shared/src/filesystem.ts
index 44346be8f94..da0af299cb3 100644
--- a/packages/shared/src/filesystem.ts
+++ b/packages/shared/src/filesystem.ts
@@ -1,5 +1,12 @@
 import { NodeFileSystem } from "@effect/platform-node"
-import { dirname, join, relative, resolve as pathResolve } from "path"
+import {
+  dirname,
+  isAbsolute,
+  join,
+  relative,
+  resolve as pathResolve,
+  sep,
+} from "path" // kilocode_change - harden containment checks
 import { realpathSync } from "fs"
 import * as NFS from "fs/promises"
 import { lookup } from "mime-types"
@@ -231,6 +238,9 @@ export namespace AppFileSystem {
   }
 
   export function contains(parent: string, child: string) {
-    return !relative(parent, child).startsWith("..")
+    // kilocode_change start - reject cross-drive and escaped relative paths
+    const rel = relative(parent, child)
+    return rel === "" || (!isAbsolute(rel) && rel !== ".." && !rel.startsWith(`..${sep}`))
+    // kilocode_change end
   }
 }
diff --git a/packages/shared/test/kilocode/filesystem-containment.test.ts b/packages/shared/test/kilocode/filesystem-containment.test.ts
new file mode 100644
index 00000000000..9f98b926002
--- /dev/null
+++ b/packages/shared/test/kilocode/filesystem-containment.test.ts
@@ -0,0 +1,13 @@
+import { describe, expect, test } from "bun:test"
+import { AppFileSystem } from "@opencode-ai/shared/filesystem"
+
+describe("kilocode filesystem containment", () => {
+  test("keeps dot-prefixed child names internal", () => {
+    expect(AppFileSystem.contains("/a/b", "/a/b/..cache/file")).toBe(true)
+  })
+
+  test("rejects cross-drive paths on Windows", () => {
+    if (process.platform !== "win32") return
+    expect(AppFileSystem.contains("C:\\repo", "D:\\outside\\file.txt")).toBe(false)
+  })
+})