diff --git a/index.html b/index.html
index 5d578b6..6e51dd4 100644
--- a/index.html
+++ b/index.html
@@ -188,11 +188,6 @@
     <button id="chat-send">&gt;</button>
   </div>
   <script type="module" src="./js/main.js"></script>
-  <script>
-    // Register service worker for PWA / offline support
-    if ('serviceWorker' in navigator) {
-      navigator.serviceWorker.register('/sw.js').catch(() => {});
-    }
-  </script>
+  <!-- SW registration is handled by main.js in production builds only -->
 </body>
 </html>
diff --git a/js/main.js b/js/main.js
index 79f70d3..a9185e2 100644
--- a/js/main.js
+++ b/js/main.js
@@ -135,3 +135,8 @@ function main() {
 }
 
 main();
+
+// Register service worker only in production builds
+if (import.meta.env.PROD && 'serviceWorker' in navigator) {
+  navigator.serviceWorker.register('/sw.js').catch(() => {});
+}
diff --git a/sw.js b/sw.js
new file mode 100644
index 0000000..28d2aca
--- /dev/null
+++ b/sw.js
@@ -0,0 +1,39 @@
+/* sw.js — Matrix PWA service worker
+ * PRECACHE_URLS is replaced at build time by the generate-sw Vite plugin.
+ * Registration is gated to import.meta.env.PROD in main.js, so this template
+ * file is never evaluated by browsers during development.
+ */
+const CACHE_NAME   = 'timmy-matrix-v1';
+const PRECACHE_URLS = __PRECACHE_URLS__;
+
+self.addEventListener('install', event => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then(cache => cache.addAll(PRECACHE_URLS))
+  );
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', event => {
+  event.waitUntil(
+    caches.keys().then(keys =>
+      Promise.all(keys.filter(k => k !== CACHE_NAME).map(k => caches.delete(k)))
+    )
+  );
+  self.clients.claim();
+});
+
+self.addEventListener('fetch', event => {
+  if (event.request.method !== 'GET') return;
+  event.respondWith(
+    caches.match(event.request).then(cached => {
+      if (cached) return cached;
+      return fetch(event.request).then(response => {
+        if (!response || response.status !== 200 || response.type !== 'basic') {
+          return response;
+        }
+        caches.open(CACHE_NAME).then(cache => cache.put(event.request, response.clone()));
+        return response;
+      });
+    })
+  );
+});
diff --git a/vite.config.js b/vite.config.js
index 165bcfb..4ff1b15 100644
--- a/vite.config.js
+++ b/vite.config.js
@@ -1,4 +1,35 @@
 import { defineConfig } from 'vite';
+import { readFileSync, writeFileSync } from 'fs';
+
+/** Vite plugin: generates dist/sw.js with precache URLs from the build manifest. */
+function generateSW() {
+  return {
+    name: 'generate-sw',
+    apply: 'build',
+    closeBundle() {
+      const staticAssets = [
+        '/',
+        '/manifest.json',
+        '/icons/icon-192.svg',
+        '/icons/icon-512.svg',
+      ];
+
+      try {
+        const manifest = JSON.parse(readFileSync('dist/.vite/manifest.json', 'utf-8'));
+        for (const entry of Object.values(manifest)) {
+          staticAssets.push('/' + entry.file);
+          if (entry.css) entry.css.forEach(f => staticAssets.push('/' + f));
+        }
+      } catch { /* manifest may not exist in dev */ }
+
+      const template = readFileSync('sw.js', 'utf-8');
+      const out = template.replace('__PRECACHE_URLS__', JSON.stringify(staticAssets, null, 4));
+      writeFileSync('dist/sw.js', out);
+
+      console.log('[generate-sw] wrote dist/sw.js with', staticAssets.length, 'precache URLs');
+    },
+  };
+}
 
 export default defineConfig({
   root: '.',
@@ -6,6 +37,7 @@ export default defineConfig({
     outDir: 'dist',
     assetsDir: 'assets',
     target: 'esnext',
+    manifest: true,
     rollupOptions: {
       output: {
         manualChunks: {
@@ -14,6 +46,7 @@ export default defineConfig({
       },
     },
   },
+  plugins: [generateSW()],
   server: {
     host: true,
   },