diff --git a/src/app/state/sessions.ts b/src/app/state/sessions.ts
index a9c34ce9..19f2212d 100644
--- a/src/app/state/sessions.ts
+++ b/src/app/state/sessions.ts
@@ -40,17 +40,17 @@ export function setFallbackSession(
   localStorage.setItem('cinny_user_id', userId);
   localStorage.setItem('cinny_hs_base_url', baseUrl);
 }
-export const removeFallbackSession = () => {
-  localStorage.removeItem('cinny_hs_base_url');
-  localStorage.removeItem('cinny_user_id');
-  localStorage.removeItem('cinny_device_id');
-  localStorage.removeItem('cinny_access_token');
+export const removeFallbackSession = (storage: Storage = localStorage) => {
+  storage.removeItem('cinny_hs_base_url');
+  storage.removeItem('cinny_user_id');
+  storage.removeItem('cinny_device_id');
+  storage.removeItem('cinny_access_token');
 };
-export const getFallbackSession = (): Session | undefined => {
-  const baseUrl = localStorage.getItem('cinny_hs_base_url');
-  const userId = localStorage.getItem('cinny_user_id');
-  const deviceId = localStorage.getItem('cinny_device_id');
-  const accessToken = localStorage.getItem('cinny_access_token');
+export const getFallbackSession = (storage: Storage = localStorage): Session | undefined => {
+  const baseUrl = storage.getItem('cinny_hs_base_url');
+  const userId = storage.getItem('cinny_user_id');
+  const deviceId = storage.getItem('cinny_device_id');
+  const accessToken = storage.getItem('cinny_access_token');
 
   if (baseUrl && userId && deviceId && accessToken) {
     const session: Session = {
diff --git a/src/index.tsx b/src/index.tsx
index 6c7a3bc2..51e9760f 100644
--- a/src/index.tsx
+++ b/src/index.tsx
@@ -30,29 +30,32 @@ import { getFallbackSession } from './app/state/sessions';
 
 document.body.classList.add(configClass, varsClass);
 
-// Register Service Worker
-if ('serviceWorker' in navigator) {
-  const swUrl =
-    import.meta.env.MODE === 'production'
-      ? `${trimTrailingSlash(import.meta.env.BASE_URL)}/sw.js`
-      : `/dev-sw.js?dev-sw`;
+// Register Service Worker. After the preview demo has settled the stored session: the worker is
+// handed that session, and the demo may drop one it cannot serve.
+const registerServiceWorker = () => {
+  if ('serviceWorker' in navigator) {
+    const swUrl =
+      import.meta.env.MODE === 'production'
+        ? `${trimTrailingSlash(import.meta.env.BASE_URL)}/sw.js`
+        : `/dev-sw.js?dev-sw`;
 
-  const sendSessionToSW = () => {
-    const session = getFallbackSession();
-    pushSessionToSW(session?.baseUrl, session?.accessToken);
-  };
+    const sendSessionToSW = () => {
+      const session = getFallbackSession();
+      pushSessionToSW(session?.baseUrl, session?.accessToken);
+    };
 
-  navigator.serviceWorker.register(swUrl).then(sendSessionToSW);
-  navigator.serviceWorker.ready.then(sendSessionToSW);
+    navigator.serviceWorker.register(swUrl).then(sendSessionToSW);
+    navigator.serviceWorker.ready.then(sendSessionToSW);
 
-  navigator.serviceWorker.addEventListener('message', (ev) => {
-    const { type } = ev.data ?? {};
+    navigator.serviceWorker.addEventListener('message', (ev) => {
+      const { type } = ev.data ?? {};
 
-    if (type === 'requestSession') {
-      sendSessionToSW();
-    }
-  });
-}
+      if (type === 'requestSession') {
+        sendSessionToSW();
+      }
+    });
+  }
+};
 
 // The pretend homeserver of the dev preview: built in only with VITE_PREVIEW_DEMO=1 (the dev
 // workflow) and switched on by `previewDemo: true` in config.json. Any other build drops it.
@@ -76,4 +79,7 @@ const mountApp = () => {
   root.render(<App />);
 };
 
-installPreviewDemo().finally(mountApp);
+installPreviewDemo().finally(() => {
+  registerServiceWorker();
+  mountApp();
+});
diff --git a/src/preview-demo/install.ts b/src/preview-demo/install.ts
index a7b66de9..10b9a3ee 100644
--- a/src/preview-demo/install.ts
+++ b/src/preview-demo/install.ts
@@ -4,6 +4,7 @@
 // only by a build with VITE_PREVIEW_DEMO=1 whose config.json says previewDemo: true (src/index.tsx).
 // Own-origin requests (the app's static files) pass; the demo origin is answered in-page; any
 // other origin is refused, so no real homeserver can be contacted while this is on.
+import { removeFallbackSession } from '../app/state/sessions';
 import { DEMO_ORIGIN } from './fixtures';
 import { createDemoServer, DemoServer } from './server';
 
@@ -27,8 +28,6 @@ const refusal = (what: string, url: string) =>
   new Error(`preview: pretend server only; ${what} to ${url} refused`);
 
 const BANNER_TEXT = 'preview: pretend server. Any password works.';
-const FOREIGN_SESSION_TEXT =
-  'preview: pretend server. A login for another server is stored here and cannot be used.';
 
 // In flow, above the app: body becomes a column and the app root takes the rest, so the banner
 // never covers the room header or the connection strip.
@@ -78,7 +77,16 @@ export const guardedFetch =
     });
   };
 
+// Every preview of this site shares one origin, so one localStorage: a login another preview left
+// (a real homeserver) would make the app connect to that server, which the guard refuses. The
+// demo has one server, so a stored login for any other is dropped before the app reads it.
+export const dropForeignSession = (storage: Storage) => {
+  const base = storage.getItem('cinny_hs_base_url');
+  if (base !== null && base !== DEMO_ORIGIN) removeFallbackSession(storage);
+};
+
 export const installDemo = (win: Window & typeof globalThis = window) => {
+  dropForeignSession(win.localStorage);
   const server = createDemoServer();
   const { origin } = win.location;
   const realFetch = win.fetch.bind(win);
@@ -111,7 +119,6 @@ export const installDemo = (win: Window & typeof globalThis = window) => {
       routeOf(String(url), origin) === 'own' ? realBeacon(url, data) : false;
   }
 
-  const stored = win.localStorage.getItem('cinny_hs_base_url');
-  showBanner(stored !== null && stored !== DEMO_ORIGIN ? FOREIGN_SESSION_TEXT : BANNER_TEXT);
+  showBanner(BANNER_TEXT);
   return server;
 };
diff --git a/src/preview-demo/server.test.ts b/src/preview-demo/server.test.ts
index d277b97b..ec9031c1 100644
--- a/src/preview-demo/server.test.ts
+++ b/src/preview-demo/server.test.ts
@@ -2,9 +2,10 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
 import { ClientEvent, MatrixClient, SyncState, createClient } from 'matrix-js-sdk';
 import { DEMO_ORIGIN, ME, ROOMS, GENERAL, COMMONS } from './fixtures';
-import { guardedFetch, installDemo, routeOf } from './install';
+import { dropForeignSession, guardedFetch, installDemo, routeOf } from './install';
 import { createDemoServer, DemoServer } from './server';
 import { seedDemoSettings } from './seed';
+import { getFallbackSession } from '../app/state/sessions';
 import { mindOfSender, POSE_KEY, resolvePose } from '../app/components/message/pose/poses';
 import { parseRoomNotice, roomTrainingLine } from '../app/utils/trainingNotice';
 
@@ -233,6 +234,7 @@ describe('the guard', () => {
       localStorage: {
         getItem: (k: string) => store.get(k) ?? null,
         setItem: (k: string, v: string) => void store.set(k, v),
+        removeItem: (k: string) => void store.delete(k),
       },
     } as unknown as Window & typeof globalThis;
     vi.stubGlobal('document', {
@@ -254,6 +256,59 @@ describe('the guard', () => {
     vi.unstubAllGlobals();
   });
 
+  it('drops a stored login for a real homeserver before the app reads it, keeps the demo one', () => {
+    const SESSION = {
+      cinny_access_token: 'tok',
+      cinny_device_id: 'DEV',
+      cinny_user_id: '@a:b',
+      cinny_hs_base_url: 'https://chat.gamecult.org',
+    };
+    const store = new Map<string, string>(Object.entries({ ...SESSION, settings: '{}' }));
+    const storage = {
+      getItem: (k: string) => store.get(k) ?? null,
+      setItem: (k: string, v: string) => void store.set(k, v),
+      removeItem: (k: string) => void store.delete(k),
+    } as Storage;
+    dropForeignSession(storage);
+    expect(getFallbackSession(storage)).toBeUndefined();
+    expect([...store.keys()]).toEqual(['settings']);
+
+    Object.entries({ ...SESSION, cinny_hs_base_url: DEMO_ORIGIN }).forEach(([k, v]) =>
+      store.set(k, v)
+    );
+    dropForeignSession(storage);
+    expect(getFallbackSession(storage)?.baseUrl).toBe(DEMO_ORIGIN);
+  });
+
+  it('installDemo clears a foreign login so the app starts at the login screen', () => {
+    const store = new Map<string, string>([
+      ['cinny_hs_base_url', 'https://chat.gamecult.org'],
+      ['cinny_access_token', 'tok'],
+      ['cinny_device_id', 'DEV'],
+      ['cinny_user_id', '@a:b'],
+    ]);
+    const win = {
+      location: { origin: PAGE },
+      fetch: never,
+      WebSocket: class {},
+      XMLHttpRequest: { prototype: { open: vi.fn() } },
+      navigator: {},
+      localStorage: {
+        getItem: (k: string) => store.get(k) ?? null,
+        removeItem: (k: string) => void store.delete(k),
+      },
+    } as unknown as Window & typeof globalThis;
+    vi.stubGlobal('document', {
+      createElement: () => ({ style: {}, setAttribute: vi.fn() }),
+      getElementById: () => null,
+      body: { style: {}, insertBefore: vi.fn() },
+    });
+    const server = installDemo(win);
+    live.push({ server, mx: clientFor(server) });
+    expect(store.size).toBe(0);
+    vi.unstubAllGlobals();
+  });
+
   it('seeds pose portraits on for a first visit and keeps what a visitor already chose', () => {
     const fresh = new Map<string, string>();
     seedDemoSettings({