diff --git a/src/process/extensions/sandbox/permissions.ts b/src/process/extensions/sandbox/permissions.ts
index c6171db90..e6df8e838 100644
--- a/src/process/extensions/sandbox/permissions.ts
+++ b/src/process/extensions/sandbox/permissions.ts
@@ -78,6 +78,17 @@ export interface PermissionSummary {
   granted: boolean;
 }
 
+export function hasSandboxStoragePermission(permissions?: ExtPermissions): boolean {
+  return permissions?.storage === true;
+}
+
+export function getSandboxPermissionDeniedError(method: string, permissions?: ExtPermissions): string | null {
+  if (method.startsWith('storage.') && !hasSandboxStoragePermission(permissions)) {
+    return 'Permission denied: storage access requires "storage: true" in manifest';
+  }
+  return null;
+}
+
 // ============ Permission Analysis ============
 
 /**
diff --git a/src/process/extensions/sandbox/sandbox.ts b/src/process/extensions/sandbox/sandbox.ts
index 8050a50be..c6e602b1a 100644
--- a/src/process/extensions/sandbox/sandbox.ts
+++ b/src/process/extensions/sandbox/sandbox.ts
@@ -4,9 +4,9 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import { Worker, type MessagePort } from 'worker_threads';
+import { Worker } from 'worker_threads';
 import * as path from 'path';
-import type { ExtPermissions } from './permissions';
+import { getSandboxPermissionDeniedError, type ExtPermissions } from './permissions';
 import { extensionEventBus } from '../lifecycle/ExtensionEventBus';
 
 /**
@@ -282,6 +282,16 @@ export class SandboxHost {
    * Handle an api-call from the Worker and send back an api-response.
    */
   private handleWorkerApiCall(id: string, method: string, args: unknown[]): void {
+    const permissionError = getSandboxPermissionDeniedError(method, this.options.permissions);
+    if (permissionError) {
+      this.worker?.postMessage({
+        type: 'api-response',
+        id,
+        error: permissionError,
+      } satisfies SandboxMessage);
+      return;
+    }
+
     const handler = this.options.apiHandlers?.[method];
     if (!handler) {
       this.worker?.postMessage({
diff --git a/src/process/extensions/sandbox/sandboxWorker.ts b/src/process/extensions/sandbox/sandboxWorker.ts
index 28db1165d..2e892d6f2 100644
--- a/src/process/extensions/sandbox/sandboxWorker.ts
+++ b/src/process/extensions/sandbox/sandboxWorker.ts
@@ -17,17 +17,19 @@
  * - Extension code runs with full Worker Thread privileges (Node.js built-ins accessible)
  * - Electron main-process APIs are not directly accessible (different process/thread)
  * - The `aion` proxy provides a structured communication channel to the main thread
- * - Declared permissions in the manifest are NOT enforced at runtime — they are
- *   informational only and used for UI display purposes
+ * - Declared permissions in the manifest are partially enforced at runtime:
+ *   storage access is gated by the storage permission flag on both the worker
+ *   and host sides. Other permissions remain informational only.
  *
- * TODO: Enforce declared permissions at runtime (e.g. via vm.runInNewContext +
- * custom require proxy, or Node.js --experimental-permission flag) to prevent
- * extensions from accessing undeclared Node.js APIs.
+ * TODO: Enforce remaining declared permissions at runtime (e.g. via
+ * vm.runInNewContext + custom require proxy, or Node.js --experimental-permission
+ * flag) to prevent extensions from accessing undeclared Node.js APIs.
  */
 
 import { parentPort, workerData } from 'worker_threads';
 import * as path from 'path';
 import type { SandboxMessage } from './sandbox';
+import { hasSandboxStoragePermission } from './permissions';
 
 if (!parentPort) {
   throw new Error('This script must be run as a Worker Thread');
@@ -78,16 +80,19 @@ const aionProxy = {
     port.postMessage({ type: 'event', name: `ext:${eventName}`, payload } satisfies SandboxMessage);
   },
 
-  /** Storage API (if permission granted) */
-  storage: {
-    get: async (key: string): Promise<unknown> => callMainThread('storage.get', [key]),
-    set: async (key: string, value: unknown): Promise<void> => {
-      await callMainThread('storage.set', [key, value]);
-    },
-    delete: async (key: string): Promise<void> => {
-      await callMainThread('storage.delete', [key]);
-    },
-  },
+  ...(hasSandboxStoragePermission(config.permissions)
+    ? {
+        storage: {
+          get: async (key: string): Promise<unknown> => callMainThread('storage.get', [key]),
+          set: async (key: string, value: unknown): Promise<void> => {
+            await callMainThread('storage.set', [key, value]);
+          },
+          delete: async (key: string): Promise<void> => {
+            await callMainThread('storage.delete', [key]);
+          },
+        },
+      }
+    : {}),
 };
 
 let callIdCounter = 0;
diff --git a/tests/unit/extensions/sandboxHost.test.ts b/tests/unit/extensions/sandboxHost.test.ts
index 1c6208427..1026cfd1d 100644
--- a/tests/unit/extensions/sandboxHost.test.ts
+++ b/tests/unit/extensions/sandboxHost.test.ts
@@ -40,9 +40,36 @@ function createHost(overrides: Partial<ConstructorParameters<typeof SandboxHost>
 
 describe('extensions/SandboxHost — handleMessage', () => {
   describe('api-call routing (Bug #4 fix)', () => {
+    it('should reject storage api-call before routing when storage permission is missing', () => {
+      const handler = vi.fn();
+      const host = createHost({
+        apiHandlers: { 'storage.get': handler },
+        permissions: undefined,
+      });
+      const mock = createMockWorker();
+
+      (host as unknown as { worker: unknown }).worker = mock.instance;
+      (host as unknown as { _running: boolean })._running = true;
+
+      const msg: SandboxMessage = { type: 'api-call', id: 'w-0', method: 'storage.get', args: ['myKey'] };
+      (host as unknown as { handleMessage: (m: SandboxMessage) => void }).handleMessage(msg);
+
+      expect(handler).not.toHaveBeenCalled();
+      expect(mock.posted).toEqual([
+        {
+          type: 'api-response',
+          id: 'w-0',
+          error: 'Permission denied: storage access requires "storage: true" in manifest',
+        },
+      ]);
+    });
+
     it('should route Worker api-call to registered apiHandler and respond with result', () => {
       const handler = vi.fn().mockReturnValue('hello');
-      const host = createHost({ apiHandlers: { 'storage.get': handler } });
+      const host = createHost({
+        apiHandlers: { 'storage.get': handler },
+        permissions: { storage: true, events: true },
+      });
       const mock = createMockWorker();
 
       // Inject mock worker
@@ -60,7 +87,10 @@ describe('extensions/SandboxHost — handleMessage', () => {
 
     it('should handle async apiHandler and respond with resolved value', async () => {
       const handler = vi.fn().mockResolvedValue({ items: [1, 2] });
-      const host = createHost({ apiHandlers: { 'storage.get': handler } });
+      const host = createHost({
+        apiHandlers: { 'storage.get': handler },
+        permissions: { storage: true, events: true },
+      });
       const mock = createMockWorker();
       (host as unknown as { worker: unknown }).worker = mock.instance;
       (host as unknown as { _running: boolean })._running = true;
@@ -79,7 +109,10 @@ describe('extensions/SandboxHost — handleMessage', () => {
       const handler = vi.fn().mockImplementation(() => {
         throw new Error('boom');
       });
-      const host = createHost({ apiHandlers: { 'storage.set': handler } });
+      const host = createHost({
+        apiHandlers: { 'storage.set': handler },
+        permissions: { storage: true, events: true },
+      });
       const mock = createMockWorker();
       (host as unknown as { worker: unknown }).worker = mock.instance;
       (host as unknown as { _running: boolean })._running = true;
@@ -93,7 +126,10 @@ describe('extensions/SandboxHost — handleMessage', () => {
 
     it('should respond with error when async apiHandler rejects', async () => {
       const handler = vi.fn().mockRejectedValue(new Error('async boom'));
-      const host = createHost({ apiHandlers: { 'storage.delete': handler } });
+      const host = createHost({
+        apiHandlers: { 'storage.delete': handler },
+        permissions: { storage: true, events: true },
+      });
       const mock = createMockWorker();
       (host as unknown as { worker: unknown }).worker = mock.instance;
       (host as unknown as { _running: boolean })._running = true;
@@ -125,7 +161,9 @@ describe('extensions/SandboxHost — handleMessage', () => {
     });
 
     it('should respond with error when apiHandlers option is not provided', () => {
-      const host = createHost(); // no apiHandlers
+      const host = createHost({
+        permissions: { storage: true, events: true },
+      }); // no apiHandlers
       const mock = createMockWorker();
       (host as unknown as { worker: unknown }).worker = mock.instance;
       (host as unknown as { _running: boolean })._running = true;
diff --git a/tests/unit/extensions/sandboxPermission.test.ts b/tests/unit/extensions/sandboxPermission.test.ts
new file mode 100644
index 000000000..2d9340a3d
--- /dev/null
+++ b/tests/unit/extensions/sandboxPermission.test.ts
@@ -0,0 +1,33 @@
+import { describe, expect, it } from 'vitest';
+import {
+  getSandboxPermissionDeniedError,
+  hasSandboxStoragePermission,
+} from '../../../src/process/extensions/sandbox/permissions';
+
+describe('extensions/sandbox permissions', () => {
+  it('treats omitted permissions as no storage access', () => {
+    expect(hasSandboxStoragePermission(undefined)).toBe(false);
+  });
+
+  it('treats storage permission false as no storage access', () => {
+    expect(hasSandboxStoragePermission({ storage: false, events: true })).toBe(false);
+  });
+
+  it('treats storage permission true as storage access enabled', () => {
+    expect(hasSandboxStoragePermission({ storage: true, events: true })).toBe(true);
+  });
+
+  it('returns a permission error for storage api calls without storage permission', () => {
+    expect(getSandboxPermissionDeniedError('storage.get', undefined)).toBe(
+      'Permission denied: storage access requires "storage: true" in manifest'
+    );
+  });
+
+  it('does not return a permission error for non-storage api calls without storage permission', () => {
+    expect(getSandboxPermissionDeniedError('custom.method', undefined)).toBeNull();
+  });
+
+  it('does not return a permission error for storage api calls when storage permission is granted', () => {
+    expect(getSandboxPermissionDeniedError('storage.get', { storage: true, events: true })).toBeNull();
+  });
+});