diff --git a/frontend/src/App.test.tsx b/frontend/src/App.test.tsx
index a49322e..3a14683 100644
--- a/frontend/src/App.test.tsx
+++ b/frontend/src/App.test.tsx
@@ -179,6 +179,49 @@ describe('App', () => {
     expect(calls.find((c) => c.path === '/api/v1/auth/logout')?.headers['X-CSRF-Token']).toBe('token-1');
   });
 
+  describe('a sign-out that does not complete keeps the user signed in and says so', () => {
+    const failures: [string, () => Response][] = [
+      ['the connection fails', () => {
+        throw new TypeError('Failed to fetch');
+      }],
+      ['the API answers 500', () => problem(500, 'internal-error')],
+      ['the edge answers 502', () => new Response(null, { status: 502 })],
+      ['the CSRF token is refused again after reading the session', () => problem(403, 'csrf-invalid')],
+    ];
+    it.each(failures)('when %s', async (_, answer) => {
+      fakeApi({
+        'GET /api/v1/auth/session': json(200, session()),
+        'GET /api/v1/organizations/org-a': organization(['organization.view', 'membership.list']),
+        'GET /api/v1/organizations/org-a/members?page=1': members,
+        'POST /api/v1/auth/logout': answer,
+      });
+      renderAt('/o/org-a');
+
+      await userEvent.click(await screen.findByRole('button', { name: 'Sign out' }));
+
+      expect(await screen.findByRole('alert')).toHaveTextContent('Signing out did not complete. You may still be signed in, so try again.');
+      expect(screen.getByRole('button', { name: 'Sign out' })).toBeEnabled();
+      expect(screen.getByRole('heading', { name: 'Alpha' })).toBeInTheDocument();
+      expect(screen.queryByRole('heading', { name: 'Sign in' })).not.toBeInTheDocument();
+    });
+  });
+
+  it('treats a 401 to the sign-out as a session that has already ended', async () => {
+    fakeApi({
+      'GET /api/v1/auth/session': json(200, session()),
+      'GET /api/v1/organizations/org-a': organization(['organization.view', 'membership.list']),
+      'GET /api/v1/organizations/org-a/members?page=1': members,
+      'POST /api/v1/auth/logout': problem(401, 'unauthenticated'),
+      'GET /api/v1/system/status': status,
+    });
+    renderAt('/o/org-a');
+
+    await userEvent.click(await screen.findByRole('button', { name: 'Sign out' }));
+
+    expect(await screen.findByRole('heading', { name: 'Sign in' })).toBeInTheDocument();
+    expect(screen.queryByText('Signing out did not complete', { exact: false })).not.toBeInTheDocument();
+  });
+
   it('shows a forbidden answer without data', async () => {
     fakeApi({
       'GET /api/v1/auth/session': json(200, session()),
diff --git a/frontend/src/auth/AuthProvider.tsx b/frontend/src/auth/AuthProvider.tsx
index a485f01..c5b280f 100644
--- a/frontend/src/auth/AuthProvider.tsx
+++ b/frontend/src/auth/AuthProvider.tsx
@@ -1,12 +1,14 @@
 import { type ReactNode, useCallback, useEffect, useMemo, useState } from 'react';
 import { getSession, login as apiLogin, logout as apiLogout, type Session } from '../api/auth.ts';
-import { onUnauthenticated, setCsrfRefresher, setCsrfToken } from '../api/client.ts';
+import { ApiProblem, onUnauthenticated, setCsrfRefresher, setCsrfToken } from '../api/client.ts';
 import { type Auth, AuthContext, type AuthState } from './context.ts';
 
 /**
  * The authentication state of the SPA (ADR-019): read from
  * GET /api/v1/auth/session on load, set by login and logout, and reset to
  * anonymous whenever the API answers 401 (the server ended the session).
+ * A logout that does not complete leaves the state signed in and rejects:
+ * the session may still be valid on the server.
  * The CSRF token is kept in memory only. Nothing is written to browser
  * storage.
  */
@@ -62,10 +64,15 @@ export function AuthProvider({ children }: { children: ReactNode }) {
   const logout = useCallback(async () => {
     try {
       await apiLogout();
-    } finally {
-      setCsrfToken(null);
-      setState({ status: 'anonymous', ended: false });
+    } catch (error) {
+      // A 401: the session had already ended on the server. Anything else
+      // (no answer, a server error, a refused CSRF token) did not end it.
+      if (!(error instanceof ApiProblem && error.status === 401)) {
+        throw error;
+      }
     }
+    setCsrfToken(null);
+    setState({ status: 'anonymous', ended: false });
   }, []);
 
   const auth = useMemo<Auth>(() => ({ state, login, logout, refresh }), [state, login, logout, refresh]);
diff --git a/frontend/src/index.css b/frontend/src/index.css
index 55a95fd..10c1d71 100644
--- a/frontend/src/index.css
+++ b/frontend/src/index.css
@@ -30,6 +30,11 @@ body {
   padding: 1.5rem;
 }
 
+.shell-alert {
+  max-width: 48rem;
+  margin: 1rem 1.5rem 0;
+}
+
 .status dl {
   display: grid;
   grid-template-columns: max-content 1fr;
diff --git a/frontend/src/pages/AppShell.tsx b/frontend/src/pages/AppShell.tsx
index d0de0af..e41648c 100644
--- a/frontend/src/pages/AppShell.tsx
+++ b/frontend/src/pages/AppShell.tsx
@@ -7,19 +7,25 @@ export function AppShell() {
   const { state, logout } = useAuth();
   const navigate = useNavigate();
   const [pending, setPending] = useState(false);
+  const [failed, setFailed] = useState(false);
   if (state.status !== 'authenticated') {
     return null;
   }
   const { user, memberships } = state.session;
 
+  /** Goes to the login only when the session has ended; otherwise says that it may not have. */
   async function signOut() {
     setPending(true);
+    setFailed(false);
     try {
       await logout();
-    } finally {
+    } catch {
       setPending(false);
-      await navigate('/login', { replace: true });
+      setFailed(true);
+      return;
     }
+    setPending(false);
+    await navigate('/login', { replace: true });
   }
 
   return (
@@ -42,6 +48,11 @@ export function AppShell() {
           Sign out
         </button>
       </header>
+      {failed && (
+        <p role="alert" className="form-error shell-alert">
+          Signing out did not complete. You may still be signed in, so try again.
+        </p>
+      )}
       <main className="shell-main">
         <Outlet />
       </main>