diff --git a/src/components/AuthProvider.test.tsx b/src/components/AuthProvider.test.tsx
new file mode 100644
index 0000000..49e31ac
--- /dev/null
+++ b/src/components/AuthProvider.test.tsx
@@ -0,0 +1,86 @@
+import { act, useEffect, type ReactNode } from 'react';
+import { createRoot } from 'react-dom/client';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+const mocks = vi.hoisted(() => ({
+  session: { data: null as null | { user: { id: string; email: string } }, isPending: true },
+  social: vi.fn(),
+  signOut: vi.fn(),
+  oneTap: vi.fn(),
+}));
+vi.mock('@/lib/auth-client', () => ({
+  useSession: () => mocks.session,
+  signIn: { social: mocks.social },
+  signOut: mocks.signOut,
+  startGoogleOneTap: mocks.oneTap,
+}));
+vi.mock('@/lib/analytics', () => ({ trackReturnedOnce: vi.fn(), trackSignupOnce: vi.fn() }));
+vi.mock('@/lib/foundry-monitoring', () => ({ captureAuthFailure: vi.fn() }));
+import { AuthProvider, useAuth } from './AuthProvider';
+import LoginClient from './LoginClient';
+let host: HTMLDivElement;
+let root: ReturnType<typeof createRoot>;
+beforeEach(() => {
+  mocks.session = { data: null, isPending: true };
+  mocks.oneTap.mockReset().mockResolvedValue(undefined);
+  mocks.social.mockReset().mockResolvedValue({});
+  host = document.createElement('div');
+  document.body.append(host);
+  root = createRoot(host);
+});
+afterEach(async () => {
+  await act(async () => root.unmount());
+  host.remove();
+});
+async function render(child: ReactNode) {
+  await act(async () => {
+    root.render(<AuthProvider>{child}</AuthProvider>);
+  });
+}
+describe('auth loading preserves its subtree', () => {
+  it('retains DOM and local state through pending, signed-out and account changes', async () => {
+    let mounts = 0;
+    function Child() {
+      const { user, loading } = useAuth();
+      useEffect(() => {
+        mounts += 1;
+      }, []);
+      return (
+        <>
+          <input defaultValue="draft" />
+          <output>{loading ? 'pending' : (user?.id ?? 'signed-out')}</output>
+        </>
+      );
+    }
+    await render(<Child />);
+    const input = host.querySelector('input')!;
+    input.value = 'retained draft';
+    expect(host.querySelector('output')?.textContent).toBe('pending');
+    mocks.session = { data: null, isPending: false };
+    await render(<Child />);
+    expect(host.querySelector('output')?.textContent).toBe('signed-out');
+    mocks.session = {
+      data: { user: { id: 'synthetic-account', email: 'audit@example.invalid' } },
+      isPending: false,
+    };
+    await render(<Child />);
+    expect(host.querySelector('output')?.textContent).toBe('synthetic-account');
+    expect(host.querySelector('input')).toBe(input);
+    expect(input.value).toBe('retained draft');
+    expect(mounts).toBe(1);
+  });
+  it('initializes Login once and shows NetworkError while keeping standard sign-in', async () => {
+    mocks.oneTap.mockRejectedValue(
+      new DOMException('Synthetic FedCM network failure', 'NetworkError')
+    );
+    await render(<LoginClient />);
+    expect(mocks.oneTap).toHaveBeenCalledTimes(1);
+    expect(mocks.oneTap).toHaveBeenCalledWith('/library');
+    expect(host.querySelector('[role=alert]')?.textContent).toBe(
+      'One-tap sign-in is unavailable. Sign in with Google to continue.'
+    );
+    const button = host.querySelector('button')!;
+    expect(button.disabled).toBe(false);
+    await act(async () => button.click());
+    expect(mocks.social).toHaveBeenCalledWith({ provider: 'google', callbackURL: '/' });
+  });
+});
diff --git a/src/components/AuthProvider.tsx b/src/components/AuthProvider.tsx
index 8e1e712..5ae49e6 100644
--- a/src/components/AuthProvider.tsx
+++ b/src/components/AuthProvider.tsx
@@ -34,7 +34,13 @@ function AnalyticsTracker({ useSession }: { useSession: AuthClientModule['useSes
   return null;
 }
 
-function AuthProviderReady({ auth, children }: { auth: AuthClientModule; children: ReactNode }) {
+function AuthProviderReady({
+  auth,
+  onValue,
+}: {
+  auth: AuthClientModule;
+  onValue: (value: AuthContextValue) => void;
+}) {
   const { data: session, isPending } = auth.useSession();
   const sessionUser = session?.user;
 
@@ -63,12 +69,9 @@ function AuthProviderReady({ auth, children }: { auth: AuthClientModule; childre
     [auth, isPending, sessionUser]
   );
 
-  return (
-    <AuthContext.Provider value={value}>
-      <AnalyticsTracker useSession={auth.useSession} />
-      {children}
-    </AuthContext.Provider>
-  );
+  useEffect(() => onValue(value), [onValue, value]);
+
+  return <AnalyticsTracker useSession={auth.useSession} />;
 }
 
 export function AuthProvider({ children }: { children: ReactNode }) {
@@ -101,11 +104,14 @@ export function AuthProvider({ children }: { children: ReactNode }) {
     []
   );
 
-  if (!auth) {
-    return <AuthContext.Provider value={bootValue}>{children}</AuthContext.Provider>;
-  }
+  const [value, setValue] = useState<AuthContextValue>(bootValue);
 
-  return <AuthProviderReady auth={auth}>{children}</AuthProviderReady>;
+  return (
+    <AuthContext.Provider value={value}>
+      {auth && <AuthProviderReady auth={auth} onValue={setValue} />}
+      {children}
+    </AuthContext.Provider>
+  );
 }
 
 export function useAuth(): AuthContextValue {
diff --git a/src/components/LoginClient.tsx b/src/components/LoginClient.tsx
index 6df8cd1..1a75f9a 100644
--- a/src/components/LoginClient.tsx
+++ b/src/components/LoginClient.tsx
@@ -13,7 +13,11 @@ export default function LoginClient() {
   const [redirecting, setRedirecting] = useState(false);
 
   useEffect(() => {
+    let active = true;
     void startGoogleOneTap('/library').catch((error) => {
+      if (active) {
+        setError('One-tap sign-in is unavailable. Sign in with Google to continue.');
+      }
       captureAuthFailure({
         provider: 'google',
         stage: 'one-tap',
@@ -21,6 +25,9 @@ export default function LoginClient() {
         source: 'login-client',
       });
     });
+    return () => {
+      active = false;
+    };
   }, []);
 
   const handleSignIn = async () => {
@@ -94,7 +101,11 @@ export default function LoginClient() {
           )}
         </Button>
 
-        {error && <p className="mt-4 text-sm text-red-400">{error}</p>}
+        {error && (
+          <p role="alert" className="mt-4 text-sm text-red-400">
+            {error}
+          </p>
+        )}
       </div>
     </div>
   );
diff --git a/src/lib/auth-client.test.ts b/src/lib/auth-client.test.ts
new file mode 100644
index 0000000..1351438
--- /dev/null
+++ b/src/lib/auth-client.test.ts
@@ -0,0 +1,55 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+const mocks = vi.hoisted(() => ({ oneTap: vi.fn(), vanilla: vi.fn() }));
+vi.mock('better-auth/react', () => ({
+  createAuthClient: () => ({ useSession: vi.fn(), signIn: {}, signOut: vi.fn() }),
+}));
+vi.mock('better-auth/client', () => ({ createAuthClient: mocks.vanilla }));
+vi.mock('better-auth/client/plugins', () => ({ oneTapClient: vi.fn() }));
+afterEach(() => {
+  vi.unstubAllGlobals();
+  vi.resetModules();
+  vi.clearAllMocks();
+});
+describe('concurrent One Tap initialization', () => {
+  it('shares initialization, propagates failure, then permits retry', async () => {
+    let reject!: (error: Error) => void;
+    mocks.oneTap.mockImplementationOnce(
+      () =>
+        new Promise<void>((_, fail) => {
+          reject = fail;
+        })
+    );
+    mocks.oneTap.mockResolvedValue(undefined);
+    mocks.vanilla.mockReturnValue({ oneTap: mocks.oneTap });
+    const fetch = vi
+      .fn()
+      .mockImplementation(
+        async () => new Response(JSON.stringify({ googleClientId: 'synthetic-only' }))
+      );
+    vi.stubGlobal('fetch', fetch);
+    const { startGoogleOneTap } = await import('./auth-client');
+    const first = startGoogleOneTap('/library');
+    const second = startGoogleOneTap('/library');
+    expect(second).toBe(first);
+    const results = Promise.allSettled([first, second]);
+    await vi.waitFor(() => expect(mocks.oneTap).toHaveBeenCalledTimes(1));
+    const error = new DOMException('Synthetic network failure', 'NetworkError');
+    reject(error);
+    expect(await results).toEqual([
+      { status: 'rejected', reason: error },
+      { status: 'rejected', reason: error },
+    ]);
+    expect(fetch).toHaveBeenCalledTimes(1);
+    await startGoogleOneTap('/library');
+    expect(mocks.oneTap).toHaveBeenCalledTimes(2);
+  });
+  it('uses null config without invoking a provider', async () => {
+    vi.stubGlobal(
+      'fetch',
+      vi.fn().mockResolvedValue(new Response(JSON.stringify({ googleClientId: null })))
+    );
+    const { startGoogleOneTap } = await import('./auth-client');
+    await startGoogleOneTap('/library');
+    expect(mocks.vanilla).not.toHaveBeenCalled();
+  });
+});
diff --git a/src/lib/auth-client.ts b/src/lib/auth-client.ts
index 4f4c78e..83d16c8 100644
--- a/src/lib/auth-client.ts
+++ b/src/lib/auth-client.ts
@@ -15,7 +15,18 @@ type AuthClientConfig = {
   googleClientId: string | null;
 };
 
-export async function startGoogleOneTap(callbackPath = '/library'): Promise<void> {
+let oneTapInFlight: Promise<void> | null = null;
+
+export function startGoogleOneTap(callbackPath = '/library'): Promise<void> {
+  if (oneTapInFlight) return oneTapInFlight;
+  const pending = initializeGoogleOneTap(callbackPath).finally(() => {
+    if (oneTapInFlight === pending) oneTapInFlight = null;
+  });
+  oneTapInFlight = pending;
+  return pending;
+}
+
+async function initializeGoogleOneTap(callbackPath: string): Promise<void> {
   const response = await fetch('/api/auth/client-config', {
     credentials: 'same-origin',
     cache: 'no-store',
diff --git a/tests/one-tap.spec.ts b/tests/one-tap.spec.ts
new file mode 100644
index 0000000..1524469
--- /dev/null
+++ b/tests/one-tap.spec.ts
@@ -0,0 +1,82 @@
+import { expect, test } from './fixtures';
+
+test('one synthetic initialization across auth loading; errors retain standard sign-in', async ({
+  page,
+  browserName,
+}) => {
+  test.setTimeout(60000);
+  if (browserName === 'chromium') {
+    const cdp = await page.context().newCDPSession(page);
+    await cdp.send('Network.enable');
+    await cdp.send('Network.emulateNetworkConditions', {
+      offline: false,
+      latency: 150,
+      downloadThroughput: 1_600_000 / 8,
+      uploadThroughput: 750_000 / 8,
+    });
+    await cdp.send('Emulation.setCPUThrottlingRate', { rate: 4 });
+  }
+  let configRequests = 0;
+  await page.addInitScript(() => {
+    Object.assign(window, {
+      googleScriptInitialized: true,
+      syntheticOneTapInitializations: 0,
+      google: {
+        accounts: {
+          id: {
+            initialize: () => {
+              const state = window as unknown as { syntheticOneTapInitializations: number };
+              state.syntheticOneTapInitializations += 1;
+              throw new DOMException('Synthetic FedCM network failure', 'NetworkError');
+            },
+          },
+        },
+      },
+    });
+  });
+  await page.route('**/api/auth/get-session', async (route) => {
+    await new Promise((resolve) => setTimeout(resolve, 500));
+    await route.fulfill({ json: null });
+  });
+  await page.route('**/api/auth/client-config', (route) => {
+    configRequests += 1;
+    return route.fulfill({ json: { googleClientId: 'synthetic-local-only' } });
+  });
+  const consoleErrors: string[] = [];
+  page.on('console', (message) => {
+    if (message.type() === 'error') consoleErrors.push(message.text());
+  });
+  const sessionResponse = page.waitForResponse((response) =>
+    response.url().includes('/api/auth/get-session')
+  );
+  await page.goto('/login');
+  await expect(page.getByRole('alert')).toHaveText(
+    'One-tap sign-in is unavailable. Sign in with Google to continue.'
+  );
+  await sessionResponse;
+  expect(configRequests).toBe(1);
+  expect(
+    await page.evaluate(
+      () =>
+        (window as unknown as { syntheticOneTapInitializations: number })
+          .syntheticOneTapInitializations
+    )
+  ).toBe(1);
+  expect(
+    consoleErrors.some((message) => message.includes('Error during Google One Tap flow'))
+  ).toBe(true);
+  expect(await page.evaluate(() => document.documentElement.scrollWidth <= innerWidth)).toBe(true);
+  await page.screenshot({ path: test.info().outputPath('one-tap-error.png'), fullPage: true });
+  await page.route('**/api/auth/sign-in/social', async (route) => {
+    expect(route.request().postDataJSON()).toMatchObject({ provider: 'google', callbackURL: '/' });
+    await route.fulfill({
+      status: 503,
+      json: { message: 'Synthetic standard sign-in unavailable' },
+    });
+  });
+  await page.getByRole('button', { name: 'Sign in with Google', exact: true }).click();
+  await expect(page.getByRole('alert')).toHaveText('Failed to sign in. Please try again.');
+  await expect(
+    page.getByRole('button', { name: 'Sign in with Google', exact: true })
+  ).toBeEnabled();
+});