diff --git a/frontend/src/api/client.ts b/frontend/src/api/client.ts
index 8e42a32..ea23d61 100644
--- a/frontend/src/api/client.ts
+++ b/frontend/src/api/client.ts
@@ -7,12 +7,27 @@ export class ApiError extends Error {
   }
 }
 
+// Thrown when fetch() itself fails (e.g. offline with no cache entry for this
+// request) -- distinct from ApiError, which means the server was reachable
+// and responded with an error status. Callers use this to show a quiet
+// "not available offline" state instead of a real-error message.
+export class NetworkError extends Error {
+  constructor() {
+    super('Network unreachable')
+  }
+}
+
 export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T> {
-  const response = await fetch(path, {
-    credentials: 'include',
-    headers: { 'Content-Type': 'application/json' },
-    ...init,
-  })
+  let response: Response
+  try {
+    response = await fetch(path, {
+      credentials: 'include',
+      headers: { 'Content-Type': 'application/json' },
+      ...init,
+    })
+  } catch {
+    throw new NetworkError()
+  }
 
   if (!response.ok) {
     let detail = response.statusText
diff --git a/frontend/src/components/ChatPane.css b/frontend/src/components/ChatPane.css
index 49546fe..26e1a67 100644
--- a/frontend/src/components/ChatPane.css
+++ b/frontend/src/components/ChatPane.css
@@ -70,3 +70,7 @@
   padding: var(--sp-2) var(--sp-4) 0;
   margin: 0;
 }
+
+.chat-pane-note {
+  color: var(--ds-muted);
+}
diff --git a/frontend/src/components/ChatPane.tsx b/frontend/src/components/ChatPane.tsx
index 6de84a3..a206ddf 100644
--- a/frontend/src/components/ChatPane.tsx
+++ b/frontend/src/components/ChatPane.tsx
@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useState } from 'react'
 import { useNavigate } from 'react-router-dom'
+import { NetworkError } from '../api/client'
 import { getRoomMessages } from '../api/rooms'
 import { useChatSocket } from '../ws/useChatSocket'
 import type { ChatMessageEnvelope, Message, MyRoomItem, RoomMember, ServerEnvelope } from '../types'
@@ -21,12 +22,22 @@ export function ChatPane({ room, members, isMobile, onBack, onToggleInfo, infoOp
   const [history, setHistory] = useState<Message[]>([])
   const [live, setLive] = useState<ChatMessageEnvelope[]>([])
   const [wsError, setWsError] = useState<string | null>(null)
+  const [historyUnavailableOffline, setHistoryUnavailableOffline] = useState(false)
 
   useEffect(() => {
     setHistory([])
     setLive([])
     setWsError(null)
-    getRoomMessages(room.id).then(setHistory).catch((err) => setWsError(String(err)))
+    setHistoryUnavailableOffline(false)
+    getRoomMessages(room.id)
+      .then(setHistory)
+      .catch((err) => {
+        if (err instanceof NetworkError) {
+          setHistoryUnavailableOffline(true)
+        } else {
+          setWsError(String(err))
+        }
+      })
   }, [room.id])
 
   const onMessage = useCallback((envelope: ServerEnvelope) => {
@@ -71,6 +82,11 @@ export function ChatPane({ room, members, isMobile, onBack, onToggleInfo, infoOp
       </header>
 
       {wsError && <p className="chat-pane-error">{wsError}</p>}
+      {historyUnavailableOffline && (
+        <p className="chat-pane-error chat-pane-note">
+          Message history for this room isn't available offline yet.
+        </p>
+      )}
 
       <MessageList messages={[...history, ...live]} members={members} />
       <Composer roomName={room.name} disabled={!connected} onSend={send} />
diff --git a/frontend/src/components/Composer.css b/frontend/src/components/Composer.css
index 8bfee81..58435c2 100644
--- a/frontend/src/components/Composer.css
+++ b/frontend/src/components/Composer.css
@@ -2,12 +2,18 @@
   padding: var(--sp-3) var(--sp-4);
   border-top: 1px solid var(--ds-border);
   background: var(--ds-surface);
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
+.composer-box {
   display: flex;
   gap: var(--sp-2);
   align-items: flex-end;
 }
 
-.composer textarea {
+.composer-box textarea {
   flex: 1;
   resize: none;
   background: var(--ds-surface-2);
@@ -20,7 +26,7 @@
   max-height: 120px;
 }
 
-.composer textarea:focus {
+.composer-box textarea:focus {
   border-color: var(--ds-accent);
 }
 
@@ -42,3 +48,9 @@
   opacity: 0.5;
   cursor: not-allowed;
 }
+
+.composer-status {
+  font-size: 0.76rem;
+  color: var(--ds-muted);
+  padding-left: 2px;
+}
diff --git a/frontend/src/components/Composer.tsx b/frontend/src/components/Composer.tsx
index 6eb948d..248d97a 100644
--- a/frontend/src/components/Composer.tsx
+++ b/frontend/src/components/Composer.tsx
@@ -1,4 +1,5 @@
 import { useRef, useState, type KeyboardEvent } from 'react'
+import { useOnlineStatus } from '../hooks/useOnlineStatus'
 import './Composer.css'
 
 interface ComposerProps {
@@ -10,6 +11,7 @@ interface ComposerProps {
 export function Composer({ roomName, disabled, onSend }: ComposerProps) {
   const [value, setValue] = useState('')
   const textareaRef = useRef<HTMLTextAreaElement>(null)
+  const online = useOnlineStatus()
 
   function autoGrow() {
     const el = textareaRef.current
@@ -35,29 +37,34 @@ export function Composer({ roomName, disabled, onSend }: ComposerProps) {
 
   return (
     <div className="composer">
-      <textarea
-        ref={textareaRef}
-        rows={1}
-        value={value}
-        disabled={disabled}
-        onChange={(e) => {
-          setValue(e.target.value)
-          autoGrow()
-        }}
-        onKeyDown={handleKeyDown}
-        placeholder={`Message #${roomName}`}
-      />
-      <button
-        type="button"
-        className="composer-send"
-        onClick={handleSend}
-        disabled={disabled || !value.trim()}
-        aria-label="Send message"
-      >
-        <svg width="15" height="15" viewBox="0 0 20 20" aria-hidden="true">
-          <polygon points="2,2 18,10 2,18 6,10" fill="currentColor" />
-        </svg>
-      </button>
+      <div className="composer-box">
+        <textarea
+          ref={textareaRef}
+          rows={1}
+          value={value}
+          disabled={disabled}
+          onChange={(e) => {
+            setValue(e.target.value)
+            autoGrow()
+          }}
+          onKeyDown={handleKeyDown}
+          placeholder={disabled ? (online ? 'Connecting…' : "You're offline") : `Message #${roomName}`}
+        />
+        <button
+          type="button"
+          className="composer-send"
+          onClick={handleSend}
+          disabled={disabled || !value.trim()}
+          aria-label="Send message"
+        >
+          <svg width="15" height="15" viewBox="0 0 20 20" aria-hidden="true">
+            <polygon points="2,2 18,10 2,18 6,10" fill="currentColor" />
+          </svg>
+        </button>
+      </div>
+      {disabled && (
+        <div className="composer-status">{online ? 'Connecting…' : "You're offline — messages can't be sent right now"}</div>
+      )}
     </div>
   )
 }
diff --git a/frontend/src/components/OfflineBanner.css b/frontend/src/components/OfflineBanner.css
new file mode 100644
index 0000000..c5f2c08
--- /dev/null
+++ b/frontend/src/components/OfflineBanner.css
@@ -0,0 +1,10 @@
+.offline-banner {
+  flex: none;
+  background: color-mix(in srgb, var(--ds-highlight) 16%, var(--ds-surface));
+  color: var(--ds-text);
+  border-bottom: 1px solid var(--ds-border);
+  font-size: 0.8rem;
+  font-weight: 600;
+  text-align: center;
+  padding: 6px var(--sp-4);
+}
diff --git a/frontend/src/components/OfflineBanner.tsx b/frontend/src/components/OfflineBanner.tsx
new file mode 100644
index 0000000..881bcd2
--- /dev/null
+++ b/frontend/src/components/OfflineBanner.tsx
@@ -0,0 +1,14 @@
+import { useOnlineStatus } from '../hooks/useOnlineStatus'
+import './OfflineBanner.css'
+
+export function OfflineBanner() {
+  const online = useOnlineStatus()
+
+  if (online) return null
+
+  return (
+    <div className="offline-banner" role="status">
+      You're offline — showing cached data.
+    </div>
+  )
+}
diff --git a/frontend/src/components/Sidebar.css b/frontend/src/components/Sidebar.css
index 97fcbcd..df3542f 100644
--- a/frontend/src/components/Sidebar.css
+++ b/frontend/src/components/Sidebar.css
@@ -103,3 +103,11 @@
   color: var(--ds-muted);
   text-transform: uppercase;
 }
+
+.sidebar-offline-note {
+  color: var(--ds-muted);
+  font-size: 0.82rem;
+  padding: var(--sp-4);
+  line-height: 1.5;
+  margin: 0;
+}
diff --git a/frontend/src/components/Sidebar.tsx b/frontend/src/components/Sidebar.tsx
index db52f42..db1661f 100644
--- a/frontend/src/components/Sidebar.tsx
+++ b/frontend/src/components/Sidebar.tsx
@@ -11,6 +11,7 @@ interface SidebarProps {
   onOpenBrowse: () => void
   onOpenInvites: () => void
   inviteCount: number
+  unavailableOffline?: boolean
 }
 
 export function Sidebar({
@@ -22,6 +23,7 @@ export function Sidebar({
   onOpenBrowse,
   onOpenInvites,
   inviteCount,
+  unavailableOffline,
 }: SidebarProps) {
   const query = searchQuery.trim().toLowerCase()
   const filtered = query ? rooms.filter((r) => r.name.toLowerCase().includes(query)) : rooms
@@ -67,17 +69,25 @@ export function Sidebar({
           Browse rooms
         </button>
 
-        {filtered.length > 0 && <div className="sidebar-section-label">Rooms</div>}
-        <nav>
-          {filtered.map((room, i) => (
-            <RoomRow
-              key={room.id}
-              room={room}
-              colorIndex={i}
-              active={room.id === activeRoomId}
-            />
-          ))}
-        </nav>
+        {unavailableOffline ? (
+          <p className="sidebar-offline-note">
+            Your rooms aren't available offline yet. Reconnect to load them.
+          </p>
+        ) : (
+          <>
+            {filtered.length > 0 && <div className="sidebar-section-label">Rooms</div>}
+            <nav>
+              {filtered.map((room, i) => (
+                <RoomRow
+                  key={room.id}
+                  room={room}
+                  colorIndex={i}
+                  active={room.id === activeRoomId}
+                />
+              ))}
+            </nav>
+          </>
+        )}
       </div>
     </aside>
   )
diff --git a/frontend/src/context/AuthContext.tsx b/frontend/src/context/AuthContext.tsx
index 197d61e..46c2243 100644
--- a/frontend/src/context/AuthContext.tsx
+++ b/frontend/src/context/AuthContext.tsx
@@ -1,11 +1,13 @@
 import { createContext, useContext, useEffect, useState, type ReactNode } from 'react'
 import * as authApi from '../api/auth'
-import { ApiError } from '../api/client'
+import { ApiError, NetworkError } from '../api/client'
+import { clearLastUser, loadLastUser, saveLastUser } from '../lib/lastUser'
 import type { User } from '../types'
 
 interface AuthContextValue {
   user: User | null
   loading: boolean
+  offline: boolean
   login: (usernameOrEmail: string, password: string) => Promise<void>
   logout: () => Promise<void>
 }
@@ -15,30 +17,58 @@ const AuthContext = createContext<AuthContextValue | undefined>(undefined)
 export function AuthProvider({ children }: { children: ReactNode }) {
   const [user, setUser] = useState<User | null>(null)
   const [loading, setLoading] = useState(true)
+  const [offline, setOffline] = useState(false)
 
   useEffect(() => {
     authApi
       .me()
-      .then(setUser)
+      .then((u) => {
+        setUser(u)
+        setOffline(false)
+        saveLastUser(u)
+      })
       .catch((err) => {
-        if (!(err instanceof ApiError && err.status === 401)) {
+        // A confirmed 401 means the server is reachable and says "not
+        // logged in" -- that's the only case that should actually clear the
+        // cached identity. Everything else (a real network failure, or the
+        // backend being down behind a reverse proxy that answers with its
+        // own 502/503/504 -- both happen in production, not just literal
+        // offline) means we simply couldn't get a confirmed answer.
+        // /api/auth/me is NetworkOnly by design (never trust a stale
+        // "who am I" as the *source of truth*), but treating "couldn't
+        // check" the same as "confirmed logged out" would lock users out of
+        // the cached rooms/messages entirely whenever the backend is
+        // unreachable. Every real action still re-checks the actual session
+        // cookie server-side, so falling back here can't grant anything.
+        if (err instanceof ApiError && err.status === 401) {
+          clearLastUser()
+          return
+        }
+        if (!(err instanceof NetworkError)) {
           console.error('Failed to load current user', err)
         }
+        const cached = loadLastUser()
+        setUser(cached)
+        setOffline(cached !== null)
       })
       .finally(() => setLoading(false))
   }, [])
 
   async function login(usernameOrEmail: string, password: string) {
-    setUser(await authApi.login(usernameOrEmail, password))
+    const u = await authApi.login(usernameOrEmail, password)
+    setUser(u)
+    setOffline(false)
+    saveLastUser(u)
   }
 
   async function logout() {
     await authApi.logout()
     setUser(null)
+    clearLastUser()
   }
 
   return (
-    <AuthContext.Provider value={{ user, loading, login, logout }}>
+    <AuthContext.Provider value={{ user, loading, offline, login, logout }}>
       {children}
     </AuthContext.Provider>
   )
diff --git a/frontend/src/hooks/useOnlineStatus.ts b/frontend/src/hooks/useOnlineStatus.ts
new file mode 100644
index 0000000..a46a7f0
--- /dev/null
+++ b/frontend/src/hooks/useOnlineStatus.ts
@@ -0,0 +1,23 @@
+import { useEffect, useState } from 'react'
+
+// navigator.onLine reflects the OS network interface, not whether our
+// backend is actually reachable -- it can be true when the API is down. This
+// is still a useful fast/cheap signal for a banner; the WebSocket
+// `connected` state (see useChatSocket) is the more reliable one for
+// whether live messaging actually works right now.
+export function useOnlineStatus(): boolean {
+  const [online, setOnline] = useState(() => navigator.onLine)
+
+  useEffect(() => {
+    const goOnline = () => setOnline(true)
+    const goOffline = () => setOnline(false)
+    window.addEventListener('online', goOnline)
+    window.addEventListener('offline', goOffline)
+    return () => {
+      window.removeEventListener('online', goOnline)
+      window.removeEventListener('offline', goOffline)
+    }
+  }, [])
+
+  return online
+}
diff --git a/frontend/src/lib/lastUser.ts b/frontend/src/lib/lastUser.ts
new file mode 100644
index 0000000..0be977c
--- /dev/null
+++ b/frontend/src/lib/lastUser.ts
@@ -0,0 +1,35 @@
+import type { User } from '../types'
+
+// A minimal, non-sensitive "who was I last logged in as" cache, so the app
+// shell can render offline (ProtectedRoute has something to show) even
+// though GET /api/auth/me is intentionally NetworkOnly and can't confirm the
+// session while offline. This never grants access to anything real -- every
+// server-side action still re-checks the actual session cookie, so a stale
+// or wrong cached user here can, at worst, show cached UI; it can't act.
+const KEY = 'kit_last_user'
+
+export function saveLastUser(user: User): void {
+  try {
+    localStorage.setItem(KEY, JSON.stringify(user))
+  } catch {
+    // storage unavailable (private browsing, quota) -- offline fallback
+    // just won't work this session, not fatal.
+  }
+}
+
+export function loadLastUser(): User | null {
+  try {
+    const raw = localStorage.getItem(KEY)
+    return raw ? (JSON.parse(raw) as User) : null
+  } catch {
+    return null
+  }
+}
+
+export function clearLastUser(): void {
+  try {
+    localStorage.removeItem(KEY)
+  } catch {
+    // ignore
+  }
+}
diff --git a/frontend/src/pages/ChatShellPage.tsx b/frontend/src/pages/ChatShellPage.tsx
index 25a2c0d..3bde037 100644
--- a/frontend/src/pages/ChatShellPage.tsx
+++ b/frontend/src/pages/ChatShellPage.tsx
@@ -1,11 +1,13 @@
 import { useCallback, useEffect, useState } from 'react'
 import { useNavigate, useParams } from 'react-router-dom'
+import { NetworkError } from '../api/client'
 import { listMyInvites } from '../api/invites'
 import { listMyRooms, listRoomMembers } from '../api/rooms'
 import { BrowseRoomsModal } from '../components/BrowseRoomsModal'
 import { ChatPane } from '../components/ChatPane'
 import { InvitesModal } from '../components/InvitesModal'
 import { NewRoomModal } from '../components/NewRoomModal'
+import { OfflineBanner } from '../components/OfflineBanner'
 import { RoomInfoPanel } from '../components/RoomInfoPanel'
 import { Sidebar } from '../components/Sidebar'
 import { TopBar } from '../components/TopBar'
@@ -27,13 +29,23 @@ export function ChatShellPage() {
   const [inviteCount, setInviteCount] = useState(0)
   const [infoOpen, setInfoOpen] = useState(false)
   const [modal, setModal] = useState<ModalKind>(null)
+  const [roomsUnavailableOffline, setRoomsUnavailableOffline] = useState(false)
 
   const activeRoom = rooms.find((r) => r.id === roomId)
 
   const refreshRooms = useCallback(async () => {
-    const list = await listMyRooms()
-    setRooms(list)
-    return list
+    try {
+      const list = await listMyRooms()
+      setRooms(list)
+      setRoomsUnavailableOffline(false)
+      return list
+    } catch (err) {
+      if (err instanceof NetworkError) {
+        setRoomsUnavailableOffline(true)
+        return []
+      }
+      throw err
+    }
   }, [])
 
   const refreshMembers = useCallback(() => {
@@ -62,6 +74,7 @@ export function ChatShellPage() {
   return (
     <div className="chat-shell">
       <TopBar />
+      <OfflineBanner />
       <div className="chat-shell-body">
         {(!isMobile || !roomId) && (
           <Sidebar
@@ -73,6 +86,7 @@ export function ChatShellPage() {
             onOpenBrowse={() => setModal('browse')}
             onOpenInvites={() => setModal('invites')}
             inviteCount={inviteCount}
+            unavailableOffline={roomsUnavailableOffline && rooms.length === 0}
           />
         )}
 
diff --git a/frontend/vite.config.ts b/frontend/vite.config.ts
index 18df443..8a16b7c 100644
--- a/frontend/vite.config.ts
+++ b/frontend/vite.config.ts
@@ -2,6 +2,11 @@ import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
 import { VitePWA } from 'vite-plugin-pwa'
 
+const READ_CACHE_EXPIRATION = {
+  maxEntries: 50,
+  maxAgeSeconds: 7 * 24 * 60 * 60, // 7 days
+}
+
 // https://vite.dev/config/
 export default defineConfig({
   plugins: [
@@ -13,8 +18,8 @@ export default defineConfig({
         short_name: 'Talking',
         start_url: '/',
         display: 'standalone',
-        background_color: '#ffffff',
-        theme_color: '#111111',
+        background_color: '#07080f', // --ds-void
+        theme_color: '#101030', // --ds-surface
         icons: [
           { src: '/icons/icon-192.png', sizes: '192x192', type: 'image/png' },
           { src: '/icons/icon-512.png', sizes: '512x512', type: 'image/png' },
@@ -28,9 +33,71 @@ export default defineConfig({
       },
       workbox: {
         navigateFallbackDenylist: [/^\/api/, /^\/ws/],
+        // urlPattern uses function matchers against url.pathname rather than
+        // RegExp (which Workbox tests against the *full href*, origin
+        // included -- a `^/api/` anchor would silently never match).
         runtimeCaching: [
+          // Never serve a stale cached "who am I" response.
           {
-            urlPattern: /^\/api\//,
+            urlPattern: ({ url }) => url.pathname.startsWith('/api/auth/'),
+            handler: 'NetworkOnly',
+          },
+          // Cache-and-refresh: show the last known list/history immediately,
+          // update from the network in the background. Routes default to
+          // matching GET only, so mutations to these same paths are
+          // untouched and still go straight to network.
+          {
+            urlPattern: ({ url }) => url.pathname === '/api/rooms/mine',
+            handler: 'StaleWhileRevalidate',
+            options: {
+              cacheName: 'api-rooms-mine',
+              expiration: READ_CACHE_EXPIRATION,
+              cacheableResponse: { statuses: [0, 200] },
+            },
+          },
+          {
+            urlPattern: ({ url }) => url.pathname === '/api/rooms',
+            handler: 'StaleWhileRevalidate',
+            options: {
+              cacheName: 'api-rooms-open',
+              expiration: READ_CACHE_EXPIRATION,
+              cacheableResponse: { statuses: [0, 200] },
+            },
+          },
+          {
+            urlPattern: ({ url }) =>
+              /^\/api\/rooms\/[^/]+\/messages$/.test(url.pathname),
+            handler: 'StaleWhileRevalidate',
+            options: {
+              cacheName: 'api-room-messages',
+              expiration: READ_CACHE_EXPIRATION,
+              cacheableResponse: { statuses: [0, 200] },
+            },
+          },
+          {
+            urlPattern: ({ url }) =>
+              /^\/api\/rooms\/[^/]+\/members$/.test(url.pathname),
+            handler: 'StaleWhileRevalidate',
+            options: {
+              cacheName: 'api-room-members',
+              expiration: READ_CACHE_EXPIRATION,
+              cacheableResponse: { statuses: [0, 200] },
+            },
+          },
+          {
+            urlPattern: ({ url }) => url.pathname === '/api/invites/mine',
+            handler: 'StaleWhileRevalidate',
+            options: {
+              cacheName: 'api-invites-mine',
+              expiration: READ_CACHE_EXPIRATION,
+              cacheableResponse: { statuses: [0, 200] },
+            },
+          },
+          // Defensive default: anything else under /api/ (including any
+          // future GET endpoint) stays network-only until explicitly opted
+          // in above.
+          {
+            urlPattern: ({ url }) => url.pathname.startsWith('/api/'),
             handler: 'NetworkOnly',
           },
         ],
@@ -49,4 +116,19 @@ export default defineConfig({
       },
     },
   },
+  // `vite preview` doesn't inherit `server.proxy` -- needed to exercise the
+  // real production service worker (only registered against a built
+  // bundle, not `vite dev`) against the actual backend.
+  preview: {
+    proxy: {
+      '/api': {
+        target: 'http://localhost:8000',
+        changeOrigin: true,
+      },
+      '/ws': {
+        target: 'ws://localhost:8000',
+        ws: true,
+      },
+    },
+  },
 })