diff --git a/frontend/src/__tests__/crosstab-session.test.ts b/frontend/src/__tests__/crosstab-session.test.ts
new file mode 100644
index 000000000..fb03e1e1e
--- /dev/null
+++ b/frontend/src/__tests__/crosstab-session.test.ts
@@ -0,0 +1,76 @@
+/**
+ * Cross-tab logout sync regression (issue #493).
+ * Guards installStorageListener() in api/client.ts: if its key filter,
+ * newValue check, install-order, or idempotency ever regresses the
+ * SECURITY mitigation noted in client.ts goes silently false.
+ */
+
+const originalLocation = window.location;
+afterEach(() => {
+  Object.defineProperty(window, 'location', { configurable: true, value: originalLocation });
+  // initAuth() seeds in-memory auth from localStorage, so clear it between
+  // tests to stop stale tokens from one case leaking into the next.
+  localStorage.clear();
+});
+
+function loadAuth(): { api: typeof import('../api'); handler: (e: StorageEvent) => void; reload: jest.Mock; addSpy: jest.SpyInstance } {
+  let out: ReturnType<typeof loadAuth> | undefined;
+  jest.isolateModules(() => {
+    const addSpy = jest.spyOn(window, 'addEventListener');
+    const reload = jest.fn();
+    Object.defineProperty(window, 'location', { configurable: true, value: { ...window.location, reload } });
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    const api = require('../api') as typeof import('../api');
+    api.initAuth();
+    const call = addSpy.mock.calls.find(c => c[0] === 'storage');
+    if (!call) throw new Error('initAuth() did not install a storage listener');
+    out = { api, handler: call[1] as (e: StorageEvent) => void, reload, addSpy };
+  });
+  if (!out) throw new Error('loadAuth failed to capture storage listener');
+  return out;
+}
+
+// jsdom rejects the jest.fn() storage mock as storageArea, so omit it
+// (the listener never reads it).
+const evt = (key: string | null, newValue: string | null): StorageEvent =>
+  new StorageEvent('storage', { key, newValue });
+
+describe('cross-tab logout sync (issue #493)', () => {
+  test.each(['authToken', 'apiKey', 'csrfToken'])('cleared %s in another tab clears in-memory auth and reloads', key => {
+    const { api, handler, reload } = loadAuth();
+    api.setAuthToken('t'); api.setApiKey('k');
+    handler(evt(key, null));
+    expect(api.isAuthenticated()).toBe(false);
+    expect(reload).toHaveBeenCalledTimes(1);
+  });
+
+  test('ignores non-auth keys', () => {
+    const { api, handler, reload } = loadAuth();
+    api.setAuthToken('t');
+    handler(evt('someOtherKey', null));
+    expect(api.isAuthenticated()).toBe(true);
+    expect(reload).not.toHaveBeenCalled();
+  });
+
+  test('ignores partial updates such as token refresh (newValue !== null)', () => {
+    const { api, handler, reload } = loadAuth();
+    api.setAuthToken('t');
+    handler(evt('authToken', 'rotated'));
+    expect(api.isAuthenticated()).toBe(true);
+    expect(reload).not.toHaveBeenCalled();
+  });
+
+  test('ignores events with null key (e.g. localStorage.clear())', () => {
+    const { api, handler, reload } = loadAuth();
+    api.setAuthToken('t');
+    handler(evt(null, null));
+    expect(api.isAuthenticated()).toBe(true);
+    expect(reload).not.toHaveBeenCalled();
+  });
+
+  test('initAuth() is idempotent: storage listener installs exactly once', () => {
+    const { api, addSpy } = loadAuth();
+    api.initAuth(); api.initAuth();
+    expect(addSpy.mock.calls.filter(c => c[0] === 'storage')).toHaveLength(1);
+  });
+});