diff --git a/packages/core/src/tools/enter-plan-mode.test.ts b/packages/core/src/tools/enter-plan-mode.test.ts
index 0b477d7be6..91d08ccdff 100644
--- a/packages/core/src/tools/enter-plan-mode.test.ts
+++ b/packages/core/src/tools/enter-plan-mode.test.ts
@@ -39,6 +39,8 @@ describe('EnterPlanModeTool', () => {
 
     mockConfig = {
       setApprovalMode: vi.fn(),
+      getExtensions: vi.fn().mockReturnValue([]),
+      getExtensionSetting: vi.fn(),
       storage: {
         getPlansDir: vi.fn().mockReturnValue('/mock/plans/dir'),
       } as unknown as Config['storage'],
@@ -132,15 +134,71 @@ 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({});
+    it('should create custom plan directories for active extensions', async () => {
+      vi.mocked(mockConfig.getExtensions!).mockReturnValue([
+        {
+          name: 'ext-a',
+          isActive: true,
+        } as import('../config/config.js').GeminiCLIExtension,
+        {
+          name: 'ext-b',
+          isActive: false,
+        } as import('../config/config.js').GeminiCLIExtension,
+      ]);
+      vi.mocked(mockConfig.getExtensionSetting!).mockImplementation(
+        (name, setting) => {
+          if (name === 'ext-a' && setting === 'plan.directory')
+            return '.ext-a-plans';
+          return undefined;
+        },
+      );
+      vi.mocked(mockConfig.storage!.getPlansDir).mockImplementation(
+        (customDir?: string) => {
+          if (customDir === '.ext-a-plans') return '/mock/plans/ext-a-plans';
+          return '/mock/plans/dir';
+        },
+      );
       vi.mocked(fs.existsSync).mockReturnValue(false);
 
+      const invocation = tool.build({});
       await invocation.execute({ abortSignal: new AbortController().signal });
 
       expect(fs.mkdirSync).toHaveBeenCalledWith('/mock/plans/dir', {
         recursive: true,
       });
+      expect(fs.mkdirSync).toHaveBeenCalledWith('/mock/plans/ext-a-plans', {
+        recursive: true,
+      });
+      expect(fs.mkdirSync).toHaveBeenCalledTimes(2);
+    });
+
+    it('should ignore validation failures for extension-specific plan directories', async () => {
+      vi.mocked(mockConfig.getExtensions!).mockReturnValue([
+        {
+          name: 'ext-a',
+          isActive: true,
+        } as import('../config/config.js').GeminiCLIExtension,
+      ]);
+      vi.mocked(mockConfig.getExtensionSetting!).mockReturnValue(
+        '../outside-workspace',
+      );
+      vi.mocked(mockConfig.storage!.getPlansDir).mockImplementation(
+        (customDir?: string) => {
+          if (customDir === '../outside-workspace')
+            throw new Error('Path traversal detected');
+          return '/mock/plans/dir';
+        },
+      );
+      vi.mocked(fs.existsSync).mockReturnValue(false);
+
+      const invocation = tool.build({});
+      await invocation.execute({ abortSignal: new AbortController().signal });
+
+      // Should only create the default one
+      expect(fs.mkdirSync).toHaveBeenCalledWith('/mock/plans/dir', {
+        recursive: true,
+      });
+      expect(fs.mkdirSync).toHaveBeenCalledTimes(1);
     });
 
     it('should include optional reason in output display but not in llmContent', async () => {
diff --git a/packages/core/src/tools/enter-plan-mode.ts b/packages/core/src/tools/enter-plan-mode.ts
index 81c3f095ce..34ece3c067 100644
--- a/packages/core/src/tools/enter-plan-mode.ts
+++ b/packages/core/src/tools/enter-plan-mode.ts
@@ -125,16 +125,46 @@ export class EnterPlanModeInvocation extends BaseToolInvocation<
 
     this.config.setApprovalMode(ApprovalMode.PLAN);
 
-    // Ensure plans directory exists so that the agent can write the plan file.
+    // Ensure plans directories exist so that the agent can write plan files.
     // 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: ${plansDir}`, e);
+    const dirsToCreate = new Set<string>();
+
+    // Always ensure the default plans directory exists
+    try {
+      dirsToCreate.add(this.config.storage.getPlansDir(undefined));
+    } catch {
+      // Ignore if default somehow throws (unlikely)
+    }
+
+    // Ensure extension-specific plan directories exist
+    for (const ext of this.config.getExtensions()) {
+      if (!ext.isActive) continue;
+
+      const customDir = this.config.getExtensionSetting<string>(
+        ext.name,
+        'plan.directory',
+      );
+      if (customDir) {
+        try {
+          dirsToCreate.add(this.config.storage.getPlansDir(customDir));
+        } catch (e) {
+          debugLogger.warn(
+            `Invalid custom plan directory '${customDir}' for extension '${ext.name}':`,
+            e,
+          );
+        }
+      }
+    }
+
+    for (const dir of dirsToCreate) {
+      if (!fs.existsSync(dir)) {
+        try {
+          fs.mkdirSync(dir, { recursive: true });
+        } catch (e) {
+          // Log error but don't fail; write_file will try again later
+          debugLogger.error(`Failed to create plans directory: ${dir}`, e);
+        }
       }
     }