diff --git a/frontend/src/hooks.server.test.ts b/frontend/src/hooks.server.test.ts
index b24f2e16..37423ded 100644
--- a/frontend/src/hooks.server.test.ts
+++ b/frontend/src/hooks.server.test.ts
@@ -6,11 +6,22 @@ vi.mock('@sentry/sveltekit', () => ({
 	lastEventId: vi.fn(() => 'sentry-event-id-abc123')
 }));
 
-vi.mock('@sveltejs/kit', () => ({ redirect: vi.fn() }));
+class RedirectMarker {
+	constructor(
+		public status: number,
+		public location: string
+	) {}
+}
+
+vi.mock('@sveltejs/kit', () => ({
+	redirect: vi.fn((status: number, location: string) => new RedirectMarker(status, location)),
+	isRedirect: (e: unknown) => e instanceof RedirectMarker
+}));
 vi.mock('@sveltejs/kit/hooks', () => ({ sequence: vi.fn((...fns: unknown[]) => fns[0]) }));
 vi.mock('$lib/paraglide/server', () => ({ paraglideMiddleware: vi.fn() }));
 vi.mock('$lib/paraglide/runtime', () => ({ cookieName: 'locale', cookieMaxAge: 86400 }));
 vi.mock('$lib/shared/server/locale', () => ({ detectLocale: vi.fn(() => 'de') }));
+vi.mock('process', () => ({ env: { API_INTERNAL_URL: 'http://backend:8080' } }));
 
 const makeEvent = () => ({
 	url: { pathname: '/documents/123' },
@@ -56,3 +67,86 @@ describe('hooks.server handleError', () => {
 		expect(result.message).toBe('An unexpected error occurred');
 	});
 });
+
+interface UserGroupEvent {
+	url: URL;
+	cookies: { get: ReturnType<typeof vi.fn>; delete: ReturnType<typeof vi.fn> };
+	locals: { user?: unknown };
+	request: Request;
+}
+
+function makeUserGroupEvent(pathname: string, sessionId?: string): UserGroupEvent {
+	return {
+		url: new URL(`http://localhost${pathname}`),
+		cookies: {
+			get: vi.fn((name: string) => (name === 'fa_session' ? sessionId : undefined)),
+			delete: vi.fn()
+		},
+		locals: {},
+		request: new Request(`http://localhost${pathname}`)
+	};
+}
+
+describe('hooks.server userGroup (session lookup + 401 handling)', () => {
+	beforeEach(() => {
+		vi.resetModules();
+		vi.stubGlobal('fetch', vi.fn());
+	});
+
+	it('redirects to /login?reason=expired when backend rejects the session on a non-public path', async () => {
+		vi.mocked(fetch).mockResolvedValue(new Response(null, { status: 401 }));
+
+		const { handle } = await import('./hooks.server');
+		const event = makeUserGroupEvent('/documents/123', 'stale-session');
+		const resolve = vi.fn();
+
+		await expect((handle as (a: unknown) => unknown)({ event, resolve })).rejects.toMatchObject({
+			status: 302,
+			location: '/login?reason=expired'
+		});
+
+		expect(event.cookies.delete).toHaveBeenCalledWith('fa_session', { path: '/' });
+		expect(resolve).not.toHaveBeenCalled();
+	});
+
+	it('does not redirect when backend 401 fires on a public path (no /login → /login loop)', async () => {
+		vi.mocked(fetch).mockResolvedValue(new Response(null, { status: 401 }));
+
+		const { handle } = await import('./hooks.server');
+		const event = makeUserGroupEvent('/login', 'stale-session');
+		const resolve = vi.fn().mockResolvedValue(new Response());
+
+		await (handle as (a: unknown) => Promise<unknown>)({ event, resolve });
+
+		expect(event.cookies.delete).toHaveBeenCalledWith('fa_session', { path: '/' });
+		expect(resolve).toHaveBeenCalledWith(event);
+	});
+
+	it('passes through when no fa_session cookie is present', async () => {
+		const { handle } = await import('./hooks.server');
+		const event = makeUserGroupEvent('/documents/123', undefined);
+		const resolve = vi.fn().mockResolvedValue(new Response());
+
+		await (handle as (a: unknown) => Promise<unknown>)({ event, resolve });
+
+		expect(fetch).not.toHaveBeenCalled();
+		expect(resolve).toHaveBeenCalledWith(event);
+	});
+
+	it('attaches the user to locals when backend returns 200', async () => {
+		vi.mocked(fetch).mockResolvedValue(
+			new Response(JSON.stringify({ id: 'u1', email: 'a@b.de' }), {
+				status: 200,
+				headers: { 'Content-Type': 'application/json' }
+			})
+		);
+
+		const { handle } = await import('./hooks.server');
+		const event = makeUserGroupEvent('/documents/123', 'valid-session');
+		const resolve = vi.fn().mockResolvedValue(new Response());
+
+		await (handle as (a: unknown) => Promise<unknown>)({ event, resolve });
+
+		expect((event.locals as { user: { email: string } }).user.email).toBe('a@b.de');
+	});
+});