fix(core): resolve Plan Mode deadlock during plan file creation due to sandbox restrictions (#24047)

packages/core/src/tools/enter-plan-mode.test.ts

@@ -11,6 +11,22 @@ import type { Config } from '../config/config.js';
 import type { MessageBus } from '../confirmation-bus/message-bus.js';
 import { ToolConfirmationOutcome } from './tools.js';
 import { ApprovalMode } from '../policy/types.js';
+import fs from 'node:fs';
+
+vi.mock('node:fs', async () => {
+  const actual = await vi.importActual<typeof import('node:fs')>('node:fs');
+  return {
+    ...actual,
+    default: {
+      // @ts-expect-error - Property 'default' does not exist on type 'typeof import("node:fs")'
+      ...actual.default,
+      existsSync: vi.fn(),
+      mkdirSync: vi.fn(),
+    },
+    existsSync: vi.fn(),
+    mkdirSync: vi.fn(),
+  };
+});
 
 describe('EnterPlanModeTool', () => {
   let tool: EnterPlanModeTool;
@@ -103,6 +119,7 @@ describe('EnterPlanModeTool', () => {
   describe('execute', () => {
     it('should set approval mode to PLAN and return message', async () => {
       const invocation = tool.build({});
+      vi.mocked(fs.existsSync).mockReturnValue(true);
 
       const result = await invocation.execute(new AbortController().signal);
 
@@ -113,9 +130,21 @@ describe('EnterPlanModeTool', () => {
       expect(result.returnDisplay).toBe('Switching to Plan mode');
     });
 
+    it('should create plans directory if it does not exist', async () => {
+      const invocation = tool.build({});
+      vi.mocked(fs.existsSync).mockReturnValue(false);
+
+      await invocation.execute(new AbortController().signal);
+
+      expect(fs.mkdirSync).toHaveBeenCalledWith('/mock/plans/dir', {
+        recursive: true,
+      });
+    });
+
     it('should include optional reason in output display but not in llmContent', async () => {
       const reason = 'Design new database schema';
       const invocation = tool.build({ reason });
+      vi.mocked(fs.existsSync).mockReturnValue(true);
 
       const result = await invocation.execute(new AbortController().signal);
 

packages/core/src/tools/enter-plan-mode.ts

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+import fs from 'node:fs';
 import {
   BaseDeclarativeTool,
   BaseToolInvocation,
@@ -18,6 +19,7 @@ import { ENTER_PLAN_MODE_TOOL_NAME } from './tool-names.js';
 import { ApprovalMode } from '../policy/types.js';
 import { ENTER_PLAN_MODE_DEFINITION } from './definitions/coreTools.js';
 import { resolveToolDeclaration } from './definitions/resolver.js';
+import { debugLogger } from '../utils/debugLogger.js';
 
 export interface EnterPlanModeParams {
   reason?: string;
@@ -122,6 +124,19 @@ export class EnterPlanModeInvocation extends BaseToolInvocation<
 
     this.config.setApprovalMode(ApprovalMode.PLAN);
 
+    // Ensure plans directory exists so that the agent can write the plan file.
+    // In sandboxed environments, the plans directory must exist on the host
+    // before it can be bound/allowed in the sandbox.
+    const plansDir = this.config.storage.getPlansDir();
+    if (!fs.existsSync(plansDir)) {
+      try {
+        fs.mkdirSync(plansDir, { recursive: true });
+      } catch (e) {
+        // Log error but don't fail; write_file will try again later
+        debugLogger.error(`Failed to create plans directory: ${plansDir}`, e);
+      }
+    }
+
     return {
       llmContent: 'Switching to Plan mode.',
       returnDisplay: this.params.reason