diff --git a/public/manifest.webmanifest b/public/manifest.webmanifest
new file mode 100644
index 0000000..591f199
--- /dev/null
+++ b/public/manifest.webmanifest
@@ -0,0 +1,19 @@
+{
+  "name": "MCMA — Music",
+  "short_name": "MCMA",
+  "description": "Self-hosted music — control center and offline-capable player.",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#0b0b0b",
+  "theme_color": "#0b0b0b",
+  "icons": [
+    {
+      "src": "/favicon.png",
+      "sizes": "128x128",
+      "type": "image/png",
+      "purpose": "any"
+    }
+  ]
+}
diff --git a/public/sw-core.js b/public/sw-core.js
new file mode 100644
index 0000000..46d0613
--- /dev/null
+++ b/public/sw-core.js
@@ -0,0 +1,98 @@
+/*
+ * Service-worker core: pure helpers with NO side effects (no `self`, no
+ * `caches`, no `fetch`). Split out from `sw.js` so the tricky bits — HTTP Range
+ * parsing, LRU eviction, cache-key normalization — can be unit-tested in Node.
+ *
+ * This is an ES module: `sw.js` imports it (the SW is registered with
+ * { type: 'module' }) and tests import it natively — one source, no drift.
+ */
+
+// Bump the version to invalidate every cached blob on a breaking change.
+export const AUDIO_CACHE = 'mcma-audio-v1';
+// Synthetic same-origin key holding the LRU index ({ key: {size, lastAccess} }).
+export const INDEX_URL = '/__mcma_audio_index__';
+// Soft cap on total cached audio. LRU eviction keeps us under this.
+export const MAX_BYTES = 500 * 1024 * 1024; // 500 MB
+
+// Cover art (album/artist/playlist images) gets its own cache with a simple
+// count cap — covers are small and stable, so no per-byte LRU is needed.
+export const COVER_CACHE = 'mcma-covers-v1';
+export const MAX_COVERS = 600;
+
+// Backend stream route: /api/v1/streaming/tracks/<trackId>?token=...
+const STREAM_RE = /\/streaming\/tracks\/([^/?#]+)/;
+
+/** The track (content) id from a stream URL, or null if it isn't one. */
+export function trackIdFromUrl(url) {
+  const m = STREAM_RE.exec(url);
+  return m ? m[1] : null;
+}
+
+/**
+ * Canonical cache key for a stream URL: origin + path, query dropped. The auth
+ * token rides in the query and rotates on refresh, so keying by content path
+ * (origin keeps two backends from colliding) makes the cache token-stable —
+ * the same track is one entry regardless of which token fetched it.
+ */
+export function cacheKeyFor(url) {
+  try {
+    const u = new URL(url);
+    return u.origin + u.pathname;
+  } catch {
+    return String(url).split('?')[0];
+  }
+}
+
+/**
+ * Parse an HTTP `Range` header against a known resource size. Returns inclusive
+ * { start, end } byte offsets, or null for "no range / serve the whole thing".
+ * Handles `bytes=a-b`, `bytes=a-` (open-ended) and `bytes=-n` (last n bytes).
+ */
+export function parseRangeHeader(rangeHeader, size) {
+  if (!rangeHeader) return null;
+  const m = /^bytes=(\d*)-(\d*)$/.exec(String(rangeHeader).trim());
+  if (!m) return null;
+  let start = m[1] === '' ? null : parseInt(m[1], 10);
+  let end = m[2] === '' ? null : parseInt(m[2], 10);
+  if (start === null && end === null) return null;
+  if (start === null) {
+    // suffix range: the final `end` bytes
+    start = Math.max(0, size - end);
+    end = size - 1;
+  } else if (end === null) {
+    end = size - 1;
+  }
+  if (Number.isNaN(start) || Number.isNaN(end)) return null;
+  if (end >= size) end = size - 1;
+  if (start < 0 || start > end) return null;
+  return { start, end };
+}
+
+/**
+ * Choose which cached entries to evict (least-recently-used first) so that the
+ * existing total plus an incoming blob fits under `maxBytes`. Returns the list
+ * of keys to delete; empty when there's already room.
+ *
+ * `index` is { [key]: { size, lastAccess } }.
+ */
+export function selectEvictions(index, incomingSize, maxBytes) {
+  let total = incomingSize;
+  for (const k in index) total += index[k].size || 0;
+  if (total <= maxBytes) return [];
+
+  const entries = Object.keys(index)
+    .map((k) => ({
+      key: k,
+      lastAccess: index[k].lastAccess || 0,
+      size: index[k].size || 0,
+    }))
+    .sort((a, b) => a.lastAccess - b.lastAccess); // oldest first
+
+  const evict = [];
+  for (const e of entries) {
+    if (total <= maxBytes) break;
+    evict.push(e.key);
+    total -= e.size;
+  }
+  return evict;
+}
diff --git a/public/sw.js b/public/sw.js
new file mode 100644
index 0000000..f514c38
--- /dev/null
+++ b/public/sw.js
@@ -0,0 +1,256 @@
+/*
+ * MCMA service worker — Tier 3 offline support: audio blob cache.
+ *
+ * It sits between the app and the network for audio-stream requests only
+ * (`/streaming/tracks/<id>`). The first time a track is streamed it's copied
+ * into the Cache API (keyed by content id, token stripped); afterwards — or
+ * whenever the backend is unreachable — playback is served straight from the
+ * cache, so already-heard tracks play with no network at all.
+ *
+ * Pure helpers (range parsing, LRU, key normalization) live in sw-core.js so
+ * they can be unit-tested; this file owns the side-effectful cache/network I/O.
+ * Registered as a module worker ({ type: 'module' }), so it uses ES imports.
+ */
+import {
+  AUDIO_CACHE,
+  INDEX_URL,
+  MAX_BYTES,
+  COVER_CACHE,
+  MAX_COVERS,
+  trackIdFromUrl,
+  cacheKeyFor,
+  parseRangeHeader,
+  selectEvictions,
+} from './sw-core.js';
+
+self.addEventListener('install', () => {
+  // Activate immediately on first install / update — no stale SW lingering.
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', (event) => {
+  event.waitUntil(self.clients.claim());
+});
+
+self.addEventListener('fetch', (event) => {
+  const req = event.request;
+  if (req.method !== 'GET') return;
+  if (trackIdFromUrl(req.url)) {
+    event.respondWith(handleAudio(event)); // audio stream → range-aware cache
+    return;
+  }
+  if (req.destination === 'image') {
+    event.respondWith(handleImage(event)); // cover art → stale-while-revalidate
+  }
+});
+
+async function handleAudio(event) {
+  const req = event.request;
+  const key = cacheKeyFor(req.url);
+  const range = req.headers.get('range');
+  const cache = await caches.open(AUDIO_CACHE);
+
+  // 1) Serve from cache when we have it (works fully offline).
+  const cached = await cache.match(key);
+  if (cached) {
+    event.waitUntil(touch(key));
+    return range ? buildRangeResponse(cached, range) : cached.clone();
+  }
+
+  // 2) Cache miss → fetch the WHOLE file (strip Range) so we can store a
+  //    complete copy, then satisfy the original request (range-sliced if asked).
+  try {
+    const fullReq = new Request(req.url, { headers: withoutRange(req.headers) });
+    const resp = await fetch(fullReq);
+    if (isCacheable(resp)) {
+      event.waitUntil(storeInCache(key, resp.clone()));
+    }
+    return range ? buildRangeResponse(resp, range) : resp;
+  } catch {
+    // Offline and never cached — nothing we can do for this one.
+    return new Response('Offline and not cached', {
+      status: 504,
+      statusText: 'Offline',
+    });
+  }
+}
+
+/**
+ * Cover art: stale-while-revalidate. Serve the cached image instantly (so the
+ * library renders offline), refresh it in the background, and cache fresh hits.
+ * Cover URLs are token-free and stable, and `<img>` is happy with opaque
+ * cross-origin responses — so unlike audio, these cache even cross-origin.
+ */
+async function handleImage(event) {
+  const req = event.request;
+  const cache = await caches.open(COVER_CACHE);
+  const cached = await cache.match(req);
+
+  const fromNetwork = fetch(req)
+    .then((resp) => {
+      if (resp && (resp.status === 200 || resp.type === 'opaque')) {
+        return cache
+          .put(req, resp.clone())
+          .then(() => trimCovers(cache))
+          .then(() => resp);
+      }
+      return resp;
+    })
+    .catch(() => null);
+
+  if (cached) {
+    event.waitUntil(fromNetwork); // revalidate without blocking the response
+    return cached;
+  }
+  const resp = await fromNetwork;
+  return resp || new Response('', { status: 504, statusText: 'Offline' });
+}
+
+/** Keep the cover cache bounded — drop the oldest entries past the cap. */
+async function trimCovers(cache) {
+  const keys = await cache.keys();
+  const excess = keys.length - MAX_COVERS;
+  for (let i = 0; i < excess; i++) await cache.delete(keys[i]);
+}
+
+/** Only cache readable, complete 200 responses — opaque/partial are useless. */
+function isCacheable(resp) {
+  return (
+    resp.status === 200 &&
+    resp.type !== 'opaque' &&
+    resp.type !== 'opaqueredirect'
+  );
+}
+
+function withoutRange(headers) {
+  const out = new Headers();
+  for (const [k, v] of headers.entries()) {
+    if (k.toLowerCase() !== 'range') out.set(k, v);
+  }
+  return out;
+}
+
+/** Build a 206 Partial Content response by slicing a full cached/fetched body. */
+async function buildRangeResponse(response, rangeHeader) {
+  const buf = await response.clone().arrayBuffer();
+  const size = buf.byteLength;
+  const r = parseRangeHeader(rangeHeader, size);
+  const type = response.headers.get('content-type') || 'application/octet-stream';
+
+  if (!r) {
+    return new Response(buf, {
+      status: 200,
+      headers: {
+        'content-type': type,
+        'content-length': String(size),
+        'accept-ranges': 'bytes',
+      },
+    });
+  }
+
+  const sliced = buf.slice(r.start, r.end + 1);
+  return new Response(sliced, {
+    status: 206,
+    statusText: 'Partial Content',
+    headers: {
+      'content-type': type,
+      'content-range': `bytes ${r.start}-${r.end}/${size}`,
+      'content-length': String(sliced.byteLength),
+      'accept-ranges': 'bytes',
+    },
+  });
+}
+
+async function responseSize(resp) {
+  const len = resp.headers.get('content-length');
+  if (len) return Number(len);
+  const blob = await resp.blob();
+  return blob.size;
+}
+
+// --- LRU index -------------------------------------------------------------
+// The index lives as a JSON entry in the same cache. All mutations go through a
+// single serialized chain so concurrent fetch handlers can't clobber it.
+let indexChain = Promise.resolve();
+
+async function readIndex(cache) {
+  try {
+    const res = await cache.match(INDEX_URL);
+    if (!res) return {};
+    return (await res.json()) || {};
+  } catch {
+    return {};
+  }
+}
+
+function writeIndex(mutator) {
+  indexChain = indexChain
+    .then(async () => {
+      const cache = await caches.open(AUDIO_CACHE);
+      const index = await readIndex(cache);
+      await mutator(cache, index);
+      await cache.put(
+        INDEX_URL,
+        new Response(JSON.stringify(index), {
+          headers: { 'content-type': 'application/json' },
+        }),
+      );
+    })
+    .catch(() => {
+      /* keep the chain alive even if one write fails */
+    });
+  return indexChain;
+}
+
+async function storeInCache(key, resp) {
+  const size = await responseSize(resp.clone());
+  await writeIndex(async (cache, index) => {
+    for (const ek of selectEvictions(index, size, MAX_BYTES)) {
+      await cache.delete(ek);
+      delete index[ek];
+    }
+    await cache.put(key, resp);
+    index[key] = { size, lastAccess: Date.now() };
+  });
+}
+
+function touch(key) {
+  return writeIndex((_cache, index) => {
+    if (index[key]) index[key].lastAccess = Date.now();
+  });
+}
+
+// --- client messaging ------------------------------------------------------
+// The app talks to the SW over a MessageChannel: it sends a port, we reply on
+// it. Used for the offline-cache stats/controls in the UI.
+self.addEventListener('message', (event) => {
+  const data = event.data || {};
+  const reply = (result) => {
+    if (event.ports && event.ports[0]) event.ports[0].postMessage(result);
+  };
+  event.waitUntil(
+    (async () => {
+      try {
+        const cache = await caches.open(AUDIO_CACHE);
+        const index = await readIndex(cache);
+        if (data.type === 'STATS') {
+          const keys = Object.keys(index);
+          const bytes = keys.reduce((s, k) => s + (index[k].size || 0), 0);
+          reply({ count: keys.length, bytes, maxBytes: MAX_BYTES });
+        } else if (data.type === 'HAS') {
+          reply({ cached: !!index[cacheKeyFor(data.url)] });
+        } else if (data.type === 'CLEAR') {
+          await Promise.all([
+            caches.delete(AUDIO_CACHE),
+            caches.delete(COVER_CACHE),
+          ]);
+          reply({ ok: true });
+        } else {
+          reply({ error: 'unknown-message' });
+        }
+      } catch (e) {
+        reply({ error: String(e) });
+      }
+    })(),
+  );
+});
diff --git a/rsbuild.config.ts b/rsbuild.config.ts
index 35fdd33..794272d 100644
--- a/rsbuild.config.ts
+++ b/rsbuild.config.ts
@@ -32,5 +32,16 @@ export default defineConfig({
   // no manual source.define needed. See src/config/env.ts.
   html: {
     title: 'MCMA',
+    // PWA: link the manifest + declare theme/icon so the browser offers
+    // "Install app". The service worker (audio offline cache) is registered
+    // from src/index.tsx, not here.
+    tags: [
+      {
+        tag: 'link',
+        attrs: { rel: 'manifest', href: '/manifest.webmanifest' },
+      },
+      { tag: 'meta', attrs: { name: 'theme-color', content: '#0b0b0b' } },
+      { tag: 'link', attrs: { rel: 'apple-touch-icon', href: '/favicon.png' } },
+    ],
   },
 });
diff --git a/src/api/index.ts b/src/api/index.ts
index a872493..f9a4bdd 100644
--- a/src/api/index.ts
+++ b/src/api/index.ts
@@ -1,5 +1,6 @@
 import { createApi } from '@reduxjs/toolkit/query/react';
 import { baseQueryWithReauth } from './baseQuery';
+import { REHYDRATE_API, type RehydrateApiPayload } from './rehydrate';
 
 export const api = createApi({
   reducerPath: 'api',
@@ -14,5 +15,16 @@ export const api = createApi({
     'User',
     'Storage',
   ],
+  // Tier 2 offline: seed the cache from the persisted snapshot dispatched at
+  // startup (see `store/rtkqPersist.ts`). Returning the saved queries/mutations
+  // lets the last-seen library render before — or instead of — any network call.
+  extractRehydrationInfo(action) {
+    if (action.type === REHYDRATE_API) {
+      // The api reducer reads `queries`/`mutations` off this and restores any
+      // fulfilled entries; pending/rejected ones are ignored automatically.
+      return action.payload as RehydrateApiPayload as never;
+    }
+    return undefined;
+  },
   endpoints: () => ({}),
 });
diff --git a/src/api/rehydrate.ts b/src/api/rehydrate.ts
new file mode 100644
index 0000000..fd8e706
--- /dev/null
+++ b/src/api/rehydrate.ts
@@ -0,0 +1,20 @@
+import { createAction } from '@reduxjs/toolkit';
+
+/*
+ * Tier 2 offline support contract. RTK Query can seed its cache from a
+ * persisted snapshot via `extractRehydrationInfo` (see `api/index.ts`). We use
+ * a single action whose payload is the previously-saved api slice state; the
+ * api reducer pulls `queries`/`mutations` out of it on startup so last-seen
+ * library data renders read-only while the backend is unreachable.
+ *
+ * The type string lives here (not in the store) so `api/index.ts` can match it
+ * without importing from the store layer (which would create a cycle).
+ */
+export const REHYDRATE_API = 'api/rehydrate';
+
+export interface RehydrateApiPayload {
+  queries: Record<string, unknown>;
+  mutations: Record<string, unknown>;
+}
+
+export const rehydrateApi = createAction<RehydrateApiPayload>(REHYDRATE_API);
diff --git a/src/components/common/ConnectionStatus.tsx b/src/components/common/ConnectionStatus.tsx
index bcbd2d0..974afc9 100644
--- a/src/components/common/ConnectionStatus.tsx
+++ b/src/components/common/ConnectionStatus.tsx
@@ -22,9 +22,15 @@ export function ConnectionStatus() {
   const status = useConnectionStatus();
   const baseUrl = getApiBaseUrl();
   const label = t(STATUS_KEY[status]);
+  // When the backend is unreachable the UI falls back to the persisted RTKQ
+  // cache (Tier 2), so flag that the data on screen is last-seen, not live.
+  const offline = status === 'disconnected' || status === 'error';
+  const tip = offline
+    ? `${label} · ${baseUrl} · ${t('conn.cached')}`
+    : `${label} · ${baseUrl}`;
 
   return (
-    <Tooltip content={`${label} · ${baseUrl}`}>
+    <Tooltip content={tip}>
       <Badge variant={STATUS_VARIANTS[status]} dot>
         {label}
       </Badge>
diff --git a/src/components/player/PersistentPlayer.tsx b/src/components/player/PersistentPlayer.tsx
index ce5a188..cc1c34d 100644
--- a/src/components/player/PersistentPlayer.tsx
+++ b/src/components/player/PersistentPlayer.tsx
@@ -14,6 +14,7 @@ import {
   toggleQueue,
 } from '../../store/slices/player';
 import { useAudioPlayer } from '../../hooks/useAudioPlayer';
+import { useStreamCached } from '../../hooks/useStreamCached';
 import { formatDuration } from '../../lib/format';
 import { getCoverUrl } from '../../api/endpoints/streaming';
 
@@ -24,6 +25,8 @@ export function PersistentPlayer() {
   const player = useAppSelector((s) => s.player);
   const queue = useAppSelector((s) => s.queue);
   const currentEntry = queue.entries[queue.currentIndex];
+  // Source indicator: cached → playing locally, otherwise streaming.
+  const cached = useStreamCached(currentEntry?.trackId);
 
   if (!currentEntry && !player.currentTrackId) {
     return <div className="player empty">{t('player.nothingPlaying')}</div>;
@@ -31,7 +34,7 @@ export function PersistentPlayer() {
 
   const artUrl = getCoverUrl(currentEntry?.albumArtUrl);
   const seedLabel = currentEntry?.albumTitle ?? currentEntry?.title ?? '';
-  const onStream = true;
+  const onStream = !cached;
 
   return (
     <div className="player">
diff --git a/src/hooks/useStreamCached.ts b/src/hooks/useStreamCached.ts
new file mode 100644
index 0000000..ac213af
--- /dev/null
+++ b/src/hooks/useStreamCached.ts
@@ -0,0 +1,30 @@
+import { useEffect, useState } from 'react';
+import { useAppSelector } from './useAppDispatch';
+import { isStreamCached } from '../lib/sw';
+import { getStreamUrl } from '../api/endpoints/streaming';
+
+/**
+ * Whether the given track is available from the offline audio cache (Tier 3).
+ * Drives the player-bar source indicator (local vs streaming). Returns false
+ * until the service worker is controlling and confirms a hit.
+ */
+export function useStreamCached(trackId: string | undefined): boolean {
+  const token = useAppSelector((s) => s.auth.accessToken);
+  const [cached, setCached] = useState(false);
+
+  useEffect(() => {
+    if (!trackId || !token) {
+      setCached(false);
+      return;
+    }
+    let active = true;
+    void isStreamCached(getStreamUrl(trackId, token)).then((hit) => {
+      if (active) setCached(hit);
+    });
+    return () => {
+      active = false;
+    };
+  }, [trackId, token]);
+
+  return cached;
+}
diff --git a/src/i18n/locales/en.ts b/src/i18n/locales/en.ts
index ca753de..d63a315 100644
--- a/src/i18n/locales/en.ts
+++ b/src/i18n/locales/en.ts
@@ -17,6 +17,7 @@ const en = {
     disconnected: 'Offline',
     error: 'Unreachable',
     manage: 'Connection — manage instances',
+    cached: 'Showing last-seen data',
   },
   user: {
     online: 'online',
diff --git a/src/i18n/locales/ru.ts b/src/i18n/locales/ru.ts
index 9b7506a..9f22a97 100644
--- a/src/i18n/locales/ru.ts
+++ b/src/i18n/locales/ru.ts
@@ -19,6 +19,7 @@ const ru: Translations = {
     disconnected: 'Нет связи',
     error: 'Недоступно',
     manage: 'Соединение — управление экземплярами',
+    cached: 'Показаны последние данные',
   },
   user: {
     online: 'онлайн',
diff --git a/src/index.tsx b/src/index.tsx
index 5de2225..73853df 100644
--- a/src/index.tsx
+++ b/src/index.tsx
@@ -10,6 +10,7 @@ import { BrowserRouter } from 'react-router';
 import { ThemeProvider, TooltipProvider } from '@olly/modern-sk';
 import { store } from './store';
 import { AppRoutes } from './routes';
+import { registerServiceWorker } from './lib/sw';
 
 // Import all endpoint injections to ensure they are registered
 import './api/endpoints/auth';
@@ -21,6 +22,10 @@ import './api/endpoints/storage';
 import './api/endpoints/admin';
 import './api/endpoints/upload';
 
+// Tier 3 offline: register the audio-caching service worker (no-op if the
+// browser/origin doesn't support it).
+registerServiceWorker();
+
 const rootEl = document.getElementById('root');
 if (rootEl) {
   // grained black-ish background + base text color from modern-sk
diff --git a/src/lib/sw.ts b/src/lib/sw.ts
new file mode 100644
index 0000000..4f5c404
--- /dev/null
+++ b/src/lib/sw.ts
@@ -0,0 +1,80 @@
+/*
+ * Service-worker client: registration + a typed bridge to the audio offline
+ * cache (Tier 3). The SW itself lives in `public/sw.js`; this is the app side.
+ *
+ * Messaging uses a MessageChannel — we hand the SW a port and await its reply —
+ * so each call resolves with that request's result rather than a global event.
+ */
+
+export interface AudioCacheStats {
+  count: number;
+  bytes: number;
+  maxBytes: number;
+}
+
+/** Register the service worker. No-op when unsupported (e.g. plain http host). */
+export function registerServiceWorker(): void {
+  if (typeof navigator === 'undefined' || !('serviceWorker' in navigator)) {
+    return;
+  }
+  window.addEventListener('load', () => {
+    // Module worker: sw.js uses ES imports (see public/sw.js + sw-core.js).
+    navigator.serviceWorker.register('/sw.js', { type: 'module' }).catch(() => {
+      /* SW unavailable (insecure origin, blocked, …) — app still works online */
+    });
+  });
+}
+
+function controller(): ServiceWorker | null {
+  if (typeof navigator === 'undefined' || !('serviceWorker' in navigator)) {
+    return null;
+  }
+  return navigator.serviceWorker.controller;
+}
+
+/** Round-trip a message to the SW; rejects if no controlling SW is present. */
+function send<T>(message: Record<string, unknown>): Promise<T> {
+  const sw = controller();
+  if (!sw) return Promise.reject(new Error('no-service-worker'));
+  return new Promise<T>((resolve, reject) => {
+    const channel = new MessageChannel();
+    channel.port1.onmessage = (event) => resolve(event.data as T);
+    try {
+      sw.postMessage(message, [channel.port2]);
+    } catch (err) {
+      reject(err as Error);
+    }
+  });
+}
+
+/** Total size + count of cached audio, or null when the SW isn't controlling. */
+export async function getAudioCacheStats(): Promise<AudioCacheStats | null> {
+  try {
+    return await send<AudioCacheStats>({ type: 'STATS' });
+  } catch {
+    return null;
+  }
+}
+
+/** Whether a given stream URL is already cached for offline playback. */
+export async function isStreamCached(streamUrl: string): Promise<boolean> {
+  try {
+    const { cached } = await send<{ cached: boolean }>({
+      type: 'HAS',
+      url: streamUrl,
+    });
+    return cached;
+  } catch {
+    return false;
+  }
+}
+
+/** Drop the entire audio cache. Resolves false if the SW isn't controlling. */
+export async function clearAudioCache(): Promise<boolean> {
+  try {
+    const { ok } = await send<{ ok: boolean }>({ type: 'CLEAR' });
+    return ok;
+  } catch {
+    return false;
+  }
+}
diff --git a/src/store/index.ts b/src/store/index.ts
index 31aa86b..90f0286 100644
--- a/src/store/index.ts
+++ b/src/store/index.ts
@@ -4,6 +4,8 @@ import authReducer from './slices/auth';
 import playerReducer from './slices/player';
 import queueReducer from './slices/queue';
 import uiReducer from './slices/ui';
+import { loadPlayerState, loadQueueState, startPersistence } from './persist';
+import { rehydrateApiCache, startApiPersistence } from './rtkqPersist';
 
 export const store = configureStore({
   reducer: {
@@ -13,9 +15,23 @@ export const store = configureStore({
     queue: queueReducer,
     ui: uiReducer,
   },
+  // Tier 1 offline: rehydrate queue/player from the active backend's namespace
+  // so a reload (even with no backend) restores exactly where the user left off.
+  preloadedState: {
+    queue: loadQueueState(),
+    player: loadPlayerState(),
+  },
   middleware: (getDefaultMiddleware) =>
     getDefaultMiddleware().concat(api.middleware),
 });
 
+// Flush queue/player changes back to localStorage (throttled).
+startPersistence(store);
+
+// Tier 2 offline: replay the last-seen RTKQ cache, then keep snapshotting it.
+// Rehydrate first so cached server data is present before any component mounts.
+rehydrateApiCache(store.dispatch);
+startApiPersistence(store);
+
 export type RootState = ReturnType<typeof store.getState>;
 export type AppDispatch = typeof store.dispatch;
diff --git a/src/store/persist.ts b/src/store/persist.ts
new file mode 100644
index 0000000..1603b40
--- /dev/null
+++ b/src/store/persist.ts
@@ -0,0 +1,123 @@
+/*
+ * Tier 1 offline support: persist client state (queue + player) to the active
+ * backend's localStorage namespace, mirroring the auth slice. This is what lets
+ * the UI come back exactly as the user left it after a reload with no backend
+ * reachable — no server data is duplicated (the queue stores track IDs + minimal
+ * display fields only; full track records still live in the RTKQ cache).
+ *
+ * Server data (library/albums/…) is Tier 2 (see `rtkqPersist.ts`).
+ */
+import { instanceStorage } from '../config/instances';
+import {
+  queueInitialState,
+  type QueueState,
+} from './slices/queue';
+import {
+  playerInitialState,
+  type PlayerState,
+} from './slices/player';
+import type { RootState } from './index';
+
+const QUEUE_KEY = 'queue';
+const PLAYER_KEY = 'player';
+
+// Only persist fields that make sense to restore. `duration`/`isPlaying` are
+// derived from the <audio> element on next load, and the panel toggles are
+// transient UI, so they are intentionally left out.
+type PersistedQueue = Pick<
+  QueueState,
+  'entries' | 'currentIndex' | 'source' | 'sourceId' | 'sourceName'
+>;
+type PersistedPlayer = Pick<
+  PlayerState,
+  'currentTrackId' | 'position' | 'volume' | 'muted' | 'repeat' | 'shuffle'
+>;
+
+function pickQueue(state: QueueState): PersistedQueue {
+  return {
+    entries: state.entries,
+    currentIndex: state.currentIndex,
+    source: state.source,
+    sourceId: state.sourceId,
+    sourceName: state.sourceName,
+  };
+}
+
+function pickPlayer(state: PlayerState): PersistedPlayer {
+  return {
+    currentTrackId: state.currentTrackId,
+    position: state.position,
+    volume: state.volume,
+    muted: state.muted,
+    repeat: state.repeat,
+    shuffle: state.shuffle,
+  };
+}
+
+function read<T>(key: string): Partial<T> | null {
+  try {
+    const raw = instanceStorage.get(key);
+    return raw ? (JSON.parse(raw) as Partial<T>) : null;
+  } catch {
+    return null;
+  }
+}
+
+/** Build the queue slice's initial state, restoring any persisted queue. */
+export function loadQueueState(): QueueState {
+  const persisted = read<PersistedQueue>(QUEUE_KEY);
+  if (!persisted) return queueInitialState;
+  const merged: QueueState = { ...queueInitialState, ...persisted };
+  // Guard the index against a corrupted/short entries array.
+  if (
+    merged.currentIndex >= merged.entries.length ||
+    merged.currentIndex < -1
+  ) {
+    merged.currentIndex = merged.entries.length ? 0 : -1;
+  }
+  return merged;
+}
+
+/** Build the player slice's initial state, restoring any persisted player. */
+export function loadPlayerState(): PlayerState {
+  const persisted = read<PersistedPlayer>(PLAYER_KEY);
+  if (!persisted) return playerInitialState;
+  // Never auto-resume playback on load: browsers block autoplay and the
+  // <audio> element starts paused regardless. isPlaying stays false.
+  return { ...playerInitialState, ...persisted, isPlaying: false };
+}
+
+/**
+ * Subscribe a store so queue/player changes are flushed to localStorage. The
+ * write is throttled because `setPosition` fires several times a second during
+ * playback — without throttling we'd hammer localStorage on every tick.
+ */
+export function startPersistence(store: {
+  getState: () => RootState;
+  subscribe: (listener: () => void) => () => void;
+}): () => void {
+  const initial = store.getState();
+  let lastQueue = JSON.stringify(pickQueue(initial.queue));
+  let lastPlayer = JSON.stringify(pickPlayer(initial.player));
+  let timer: ReturnType<typeof setTimeout> | null = null;
+
+  const flush = () => {
+    timer = null;
+    const state = store.getState();
+    const queueSnapshot = JSON.stringify(pickQueue(state.queue));
+    if (queueSnapshot !== lastQueue) {
+      instanceStorage.set(QUEUE_KEY, queueSnapshot);
+      lastQueue = queueSnapshot;
+    }
+    const playerSnapshot = JSON.stringify(pickPlayer(state.player));
+    if (playerSnapshot !== lastPlayer) {
+      instanceStorage.set(PLAYER_KEY, playerSnapshot);
+      lastPlayer = playerSnapshot;
+    }
+  };
+
+  return store.subscribe(() => {
+    if (timer) return;
+    timer = setTimeout(flush, 1000);
+  });
+}
diff --git a/src/store/rtkqPersist.ts b/src/store/rtkqPersist.ts
new file mode 100644
index 0000000..fcd451b
--- /dev/null
+++ b/src/store/rtkqPersist.ts
@@ -0,0 +1,76 @@
+/*
+ * Tier 2 offline support: persist the RTK Query cache (last-seen server data —
+ * library/albums/artists/…) to the active backend's localStorage namespace and
+ * replay it into the cache on startup. With the backend down, components keep
+ * rendering the last-known data read-only instead of an error state.
+ *
+ * Per the architecture invariant, server data is NOT copied into a slice: this
+ * snapshots the RTKQ cache itself and feeds it back through RTKQ's own
+ * `extractRehydrationInfo` mechanism (see `api/index.ts`).
+ */
+import { instanceStorage } from '../config/instances';
+import { rehydrateApi, type RehydrateApiPayload } from '../api/rehydrate';
+import type { RootState } from './index';
+
+const CACHE_KEY = 'rtkq';
+
+type ApiState = RootState['api'];
+type QueryEntry = ApiState['queries'][string];
+
+/**
+ * Keep only successfully-fulfilled query results — pending/rejected entries
+ * carry no usable data and subscriptions are rebuilt by components on mount.
+ * Mutation results are never restored.
+ */
+function snapshot(apiState: ApiState): RehydrateApiPayload {
+  const queries: Record<string, unknown> = {};
+  for (const [key, entry] of Object.entries(apiState.queries)) {
+    const q = entry as QueryEntry | undefined;
+    if (q && q.status === 'fulfilled') queries[key] = q;
+  }
+  return { queries, mutations: {} };
+}
+
+function load(): RehydrateApiPayload | null {
+  try {
+    const raw = instanceStorage.get(CACHE_KEY);
+    if (!raw) return null;
+    const parsed = JSON.parse(raw) as Partial<RehydrateApiPayload>;
+    if (!parsed.queries) return null;
+    return { queries: parsed.queries, mutations: {} };
+  } catch {
+    return null;
+  }
+}
+
+/** Replay the persisted cache into RTKQ. Call once after the store is created. */
+export function rehydrateApiCache(dispatch: (action: unknown) => void): void {
+  const cached = load();
+  if (cached) dispatch(rehydrateApi(cached));
+}
+
+/**
+ * Subscribe a store so the RTKQ cache is flushed to localStorage. Throttled,
+ * since cache state churns on every in-flight query transition.
+ */
+export function startApiPersistence(store: {
+  getState: () => RootState;
+  subscribe: (listener: () => void) => () => void;
+}): () => void {
+  let last = '';
+  let timer: ReturnType<typeof setTimeout> | null = null;
+
+  const flush = () => {
+    timer = null;
+    const snap = JSON.stringify(snapshot(store.getState().api));
+    if (snap !== last) {
+      instanceStorage.set(CACHE_KEY, snap);
+      last = snap;
+    }
+  };
+
+  return store.subscribe(() => {
+    if (timer) return;
+    timer = setTimeout(flush, 2000);
+  });
+}
diff --git a/src/store/slices/player.ts b/src/store/slices/player.ts
index 5813dfb..e90d252 100644
--- a/src/store/slices/player.ts
+++ b/src/store/slices/player.ts
@@ -1,8 +1,8 @@
 import { createSlice, type PayloadAction } from '@reduxjs/toolkit';
 
-type RepeatMode = 'none' | 'one' | 'all';
+export type RepeatMode = 'none' | 'one' | 'all';
 
-interface PlayerState {
+export interface PlayerState {
   currentTrackId: string | null;
   isPlaying: boolean;
   position: number;
@@ -15,7 +15,7 @@ interface PlayerState {
   isQueueOpen: boolean;
 }
 
-const initialState: PlayerState = {
+export const playerInitialState: PlayerState = {
   currentTrackId: null,
   isPlaying: false,
   position: 0,
@@ -25,14 +25,12 @@ const initialState: PlayerState = {
   repeat: 'none',
   shuffle: false,
   isNowPlayingOpen: false,
-  // STUB: open by default so the queue drawer look is visible before a backend
-  // exists (pairs with DEMO_QUEUE). Default to false once real playback lands.
-  isQueueOpen: true,
+  isQueueOpen: false,
 };
 
 export const playerSlice = createSlice({
   name: 'player',
-  initialState,
+  initialState: playerInitialState,
   reducers: {
     play(state, action: PayloadAction<string>) {
       state.currentTrackId = action.payload;
diff --git a/src/store/slices/queue.ts b/src/store/slices/queue.ts
index 95fb88f..004c2e8 100644
--- a/src/store/slices/queue.ts
+++ b/src/store/slices/queue.ts
@@ -1,6 +1,6 @@
 import { createSlice, type PayloadAction } from '@reduxjs/toolkit';
 
-type QueueSource =
+export type QueueSource =
   | 'manual'
   | 'album'
   | 'playlist'
@@ -8,7 +8,7 @@ type QueueSource =
   | 'search'
   | 'radio';
 
-interface QueueEntry {
+export interface QueueEntry {
   trackId: string;
   title: string;
   artistName: string;
@@ -17,7 +17,7 @@ interface QueueEntry {
   albumArtUrl?: string;
 }
 
-interface QueueState {
+export interface QueueState {
   entries: QueueEntry[];
   currentIndex: number;
   source: QueueSource;
@@ -25,52 +25,17 @@ interface QueueState {
   sourceName: string | null;
 }
 
-// STUB demo queue — purely client-side display data so the player bar and
-// queue drawer render with content before the backend exists. Delete this
-// block (reset entries/currentIndex/source to the empty values) once real
-// playback wires tracks into the queue.
-const DEMO_QUEUE: QueueEntry[] = [
-  {
-    trackId: 'd1',
-    title: 'Quiet Storage',
-    artistName: 'Cyan Atlas',
-    albumTitle: 'Night Index',
-    durationMs: 312_000,
-  },
-  {
-    trackId: 'd2',
-    title: 'Magnetic North',
-    artistName: 'Tidal Bloom',
-    albumTitle: 'Ferric Coast',
-    durationMs: 243_000,
-  },
-  {
-    trackId: 'd3',
-    title: 'Ambergris',
-    artistName: 'Møller',
-    albumTitle: 'Warm Static',
-    durationMs: 201_000,
-  },
-  {
-    trackId: 'd4',
-    title: 'Slow Carrier',
-    artistName: 'Tidal Bloom',
-    albumTitle: 'Ferric Coast',
-    durationMs: 301_000,
-  },
-];
-
-const initialState: QueueState = {
-  entries: DEMO_QUEUE,
-  currentIndex: 0,
-  source: 'radio',
+export const queueInitialState: QueueState = {
+  entries: [],
+  currentIndex: -1,
+  source: 'manual',
   sourceId: null,
-  sourceName: 'My radio',
+  sourceName: null,
 };
 
 export const queueSlice = createSlice({
   name: 'queue',
-  initialState,
+  initialState: queueInitialState,
   reducers: {
     setQueue(
       state,
diff --git a/tests/persist.test.ts b/tests/persist.test.ts
new file mode 100644
index 0000000..fd45a56
--- /dev/null
+++ b/tests/persist.test.ts
@@ -0,0 +1,120 @@
+// @rstest-environment jsdom
+import { expect, test, beforeEach, rstest } from '@rstest/core';
+import {
+  loadQueueState,
+  loadPlayerState,
+  startPersistence,
+} from '../src/store/persist';
+import { queueInitialState, type QueueState } from '../src/store/slices/queue';
+import {
+  playerInitialState,
+  type PlayerState,
+} from '../src/store/slices/player';
+import {
+  upsertInstance,
+  setActiveInstanceId,
+  instanceStorage,
+} from '../src/config/instances';
+
+beforeEach(() => {
+  localStorage.clear();
+  const inst = upsertInstance('http://test.local');
+  setActiveInstanceId(inst.id);
+});
+
+const sampleQueue: QueueState = {
+  entries: [
+    {
+      trackId: 't1',
+      title: 'A',
+      artistName: 'X',
+      albumTitle: 'Alb',
+      durationMs: 1000,
+    },
+    {
+      trackId: 't2',
+      title: 'B',
+      artistName: 'Y',
+      albumTitle: 'Alb',
+      durationMs: 2000,
+    },
+  ],
+  currentIndex: 1,
+  source: 'album',
+  sourceId: 'alb-1',
+  sourceName: 'My Album',
+};
+
+test('loaders fall back to initial state with nothing persisted', () => {
+  expect(loadQueueState()).toEqual(queueInitialState);
+  expect(loadPlayerState()).toEqual(playerInitialState);
+});
+
+test('loadQueueState restores a persisted queue', () => {
+  instanceStorage.set('queue', JSON.stringify(sampleQueue));
+  expect(loadQueueState()).toEqual(sampleQueue);
+});
+
+test('loadQueueState guards a currentIndex past the entries array', () => {
+  instanceStorage.set(
+    'queue',
+    JSON.stringify({ ...sampleQueue, currentIndex: 99 }),
+  );
+  expect(loadQueueState().currentIndex).toBe(0);
+});
+
+test('loadPlayerState restores fields but never auto-resumes playback', () => {
+  instanceStorage.set(
+    'player',
+    JSON.stringify({
+      currentTrackId: 't2',
+      position: 42,
+      volume: 0.5,
+      muted: true,
+      repeat: 'all',
+      shuffle: true,
+      // a stale isPlaying:true must not survive a reload
+      isPlaying: true,
+    }),
+  );
+  const loaded = loadPlayerState();
+  expect(loaded.currentTrackId).toBe('t2');
+  expect(loaded.position).toBe(42);
+  expect(loaded.volume).toBe(0.5);
+  expect(loaded.repeat).toBe('all');
+  expect(loaded.isPlaying).toBe(false);
+});
+
+test('corrupt JSON falls back to initial state', () => {
+  instanceStorage.set('queue', '{not json');
+  expect(loadQueueState()).toEqual(queueInitialState);
+});
+
+test('startPersistence flushes changed state to storage after throttle', () => {
+  rstest.useFakeTimers();
+  let state = {
+    queue: queueInitialState,
+    player: playerInitialState,
+  } as { queue: QueueState; player: PlayerState };
+  let listener: (() => void) | null = null;
+  const store = {
+    getState: () => state as never,
+    subscribe: (l: () => void) => {
+      listener = l;
+      return () => {};
+    },
+  };
+
+  startPersistence(store);
+
+  // mutate + notify
+  state = { ...state, queue: sampleQueue };
+  listener!();
+  // nothing written before the throttle window elapses
+  expect(instanceStorage.get('queue')).toBeNull();
+
+  rstest.advanceTimersByTime(1000);
+  expect(JSON.parse(instanceStorage.get('queue')!).currentIndex).toBe(1);
+
+  rstest.useRealTimers();
+});
diff --git a/tests/rtkqPersist.test.ts b/tests/rtkqPersist.test.ts
new file mode 100644
index 0000000..1c11933
--- /dev/null
+++ b/tests/rtkqPersist.test.ts
@@ -0,0 +1,82 @@
+// @rstest-environment jsdom
+import { expect, test, beforeEach, rstest } from '@rstest/core';
+import {
+  rehydrateApiCache,
+  startApiPersistence,
+} from '../src/store/rtkqPersist';
+import { REHYDRATE_API } from '../src/api/rehydrate';
+import {
+  upsertInstance,
+  setActiveInstanceId,
+  instanceStorage,
+} from '../src/config/instances';
+import type { RootState } from '../src/store/index';
+
+beforeEach(() => {
+  localStorage.clear();
+  const inst = upsertInstance('http://test.local');
+  setActiveInstanceId(inst.id);
+});
+
+function apiStateWith(queries: Record<string, unknown>) {
+  return {
+    api: { queries, mutations: {}, provided: {}, subscriptions: {}, config: {} },
+  } as unknown as RootState;
+}
+
+test('rehydrateApiCache dispatches nothing when no cache is stored', () => {
+  const dispatched: unknown[] = [];
+  rehydrateApiCache((a) => dispatched.push(a));
+  expect(dispatched).toHaveLength(0);
+});
+
+test('rehydrateApiCache replays a stored cache as a rehydrate action', () => {
+  instanceStorage.set(
+    'rtkq',
+    JSON.stringify({
+      queries: { 'getLibrary(undefined)': { status: 'fulfilled', data: [1] } },
+      mutations: {},
+    }),
+  );
+  const dispatched: Array<{ type: string; payload: unknown }> = [];
+  rehydrateApiCache((a) =>
+    dispatched.push(a as { type: string; payload: unknown }),
+  );
+  expect(dispatched).toHaveLength(1);
+  expect(dispatched[0].type).toBe(REHYDRATE_API);
+  expect(dispatched[0].payload).toMatchObject({
+    queries: { 'getLibrary(undefined)': { status: 'fulfilled' } },
+  });
+});
+
+test('startApiPersistence saves only fulfilled queries after throttle', () => {
+  rstest.useFakeTimers();
+  let state = apiStateWith({});
+  let listener: (() => void) | null = null;
+  const store = {
+    getState: () => state,
+    subscribe: (l: () => void) => {
+      listener = l;
+      return () => {};
+    },
+  };
+
+  startApiPersistence(store);
+
+  state = apiStateWith({
+    'getAlbums(undefined)': { status: 'fulfilled', data: ['a'] },
+    'getArtists(undefined)': { status: 'pending' },
+    'getTracks(undefined)': { status: 'rejected', error: 'boom' },
+  });
+  listener!();
+
+  // throttled — nothing yet
+  expect(instanceStorage.get('rtkq')).toBeNull();
+
+  rstest.advanceTimersByTime(2000);
+  const saved = JSON.parse(instanceStorage.get('rtkq')!);
+  expect(Object.keys(saved.queries)).toEqual(['getAlbums(undefined)']);
+  expect(saved.mutations).toEqual({});
+
+  rstest.useRealTimers();
+});
diff --git a/tests/sw-core.test.ts b/tests/sw-core.test.ts
new file mode 100644
index 0000000..3e5b900
--- /dev/null
+++ b/tests/sw-core.test.ts
@@ -0,0 +1,79 @@
+import { expect, test } from '@rstest/core';
+import {
+  trackIdFromUrl,
+  cacheKeyFor,
+  parseRangeHeader,
+  selectEvictions,
+} from '../public/sw-core.js';
+
+test('trackIdFromUrl extracts the content id from a stream URL', () => {
+  expect(
+    trackIdFromUrl('https://host/api/v1/streaming/tracks/abc123?token=xyz'),
+  ).toBe('abc123');
+  expect(trackIdFromUrl('https://host/api/v1/library/albums')).toBeNull();
+});
+
+test('cacheKeyFor strips the token so the key is token-stable', () => {
+  const a = cacheKeyFor('https://host/api/v1/streaming/tracks/t1?token=AAA');
+  const b = cacheKeyFor('https://host/api/v1/streaming/tracks/t1?token=BBB');
+  expect(a).toBe(b);
+  expect(a).toBe('https://host/api/v1/streaming/tracks/t1');
+});
+
+test('cacheKeyFor keeps different origins distinct', () => {
+  expect(cacheKeyFor('https://a/streaming/tracks/t1?token=x')).not.toBe(
+    cacheKeyFor('https://b/streaming/tracks/t1?token=x'),
+  );
+});
+
+test('parseRangeHeader: closed range', () => {
+  expect(parseRangeHeader('bytes=0-99', 1000)).toEqual({ start: 0, end: 99 });
+});
+
+test('parseRangeHeader: open-ended range clamps to size', () => {
+  expect(parseRangeHeader('bytes=500-', 1000)).toEqual({
+    start: 500,
+    end: 999,
+  });
+});
+
+test('parseRangeHeader: suffix range (last N bytes)', () => {
+  expect(parseRangeHeader('bytes=-200', 1000)).toEqual({
+    start: 800,
+    end: 999,
+  });
+});
+
+test('parseRangeHeader: end past size is clamped', () => {
+  expect(parseRangeHeader('bytes=900-5000', 1000)).toEqual({
+    start: 900,
+    end: 999,
+  });
+});
+
+test('parseRangeHeader: invalid / no range returns null', () => {
+  expect(parseRangeHeader('', 1000)).toBeNull();
+  expect(parseRangeHeader('items=0-1', 1000)).toBeNull();
+  expect(parseRangeHeader('bytes=500-100', 1000)).toBeNull();
+});
+
+test('selectEvictions: nothing evicted when under cap', () => {
+  const index = {
+    a: { size: 100, lastAccess: 1 },
+    b: { size: 100, lastAccess: 2 },
+  };
+  expect(selectEvictions(index, 100, 1000)).toEqual([]);
+});
+
+test('selectEvictions: evicts least-recently-used first until it fits', () => {
+  const index = {
+    a: { size: 400, lastAccess: 10 }, // oldest
+    b: { size: 400, lastAccess: 30 },
+    c: { size: 400, lastAccess: 20 },
+  };
+  // total 1200 + incoming 400 = 1600, cap 1000 → must free >=600.
+  // LRU order: a (10), c (20). Evict a (1200→800... wait incl incoming)
+  const evicted = selectEvictions(index, 400, 1000);
+  // total with incoming = 1600; evict a → 1200; evict c → 800 <= 1000.
+  expect(evicted).toEqual(['a', 'c']);
+});