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

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