diff --git a/docs/sw.js b/docs/sw.js
index fceffa2..46bc6e6 100644
--- a/docs/sw.js
+++ b/docs/sw.js
@@ -1,18 +1,68 @@
-self.addEventListener("install", (event) => {
+const DB_NAME = "sune-sw-db";
+const STORE = "kv";
+const KEY = "lastSession";
+function idbOpen() {
+  return new Promise((resolve, reject) => {
+    const r = indexedDB.open(DB_NAME, 1);
+    r.onupgradeneeded = () => {
+      r.result.createObjectStore(STORE);
+    };
+    r.onsuccess = () => resolve(r.result);
+    r.onerror = () => reject(r.error || new Error("idb open error"));
+  });
+}
+function idbGet(key) {
+  return idbOpen().then((db) => new Promise((res, rej) => {
+    const tx = db.transaction(STORE, "readonly");
+    const req = tx.objectStore(STORE).get(key);
+    req.onsuccess = () => res(req.result);
+    req.onerror = () => rej(req.error);
+  }));
+}
+function idbSet(key, val) {
+  return idbOpen().then((db) => new Promise((res, rej) => {
+    const tx = db.transaction(STORE, "readwrite");
+    const req = tx.objectStore(STORE).put(val, key);
+    req.onsuccess = () => res(true);
+    req.onerror = () => rej(req.error);
+  }));
+}
+const SESSION_ID = Date.now().toString(36) + "-" + Math.floor(Math.random() * 1e9).toString(36);
+const STARTED_AT = Date.now();
+self.addEventListener("install", (ev) => {
   self.skipWaiting();
 });
-self.addEventListener("activate", (event) => {
-  event.waitUntil(self.clients.claim());
+self.addEventListener("activate", (ev) => {
+  ev.waitUntil(self.clients.claim());
 });
-self.addEventListener("message", (event) => {
-  const data = event.data || {};
-  if (data?.type === "PING") {
-    if (event.ports && event.ports[0]) {
-      event.ports[0].postMessage({ type: "PONG", ts: Date.now(), ok: true });
-    } else {
-      self.clients.matchAll().then((clients) => {
-        clients.forEach((c) => c.postMessage({ type: "PONG", ts: Date.now(), ok: true }));
-      });
+self.addEventListener("message", (ev) => {
+  const data = ev.data || {};
+  if (data.type !== "PING") return;
+  const respond = async () => {
+    try {
+      const last = await idbGet(KEY);
+      const restarted = !!last && last !== SESSION_ID;
+      await idbSet(KEY, SESSION_ID);
+      const payload = {
+        type: "PONG",
+        ts: Date.now(),
+        sessionId: SESSION_ID,
+        lastSessionId: last || null,
+        restarted,
+        uptimeMs: Date.now() - STARTED_AT,
+        ok: true
+      };
+      if (ev.ports && ev.ports[0]) {
+        ev.ports[0].postMessage(payload);
+      } else {
+        const clients = await self.clients.matchAll({ includeUncontrolled: true });
+        clients.forEach((c) => c.postMessage(payload));
+      }
+    } catch (err) {
+      const errPayload = { type: "PONG", ok: false, error: String(err), ts: Date.now() };
+      if (ev.ports && ev.ports[0]) ev.ports[0].postMessage(errPayload);
     }
-  }
+  };
+  if (ev.waitUntil) ev.waitUntil(Promise.resolve(respond()));
+  else respond();
 });