diff --git a/frontend/src/db.js b/frontend/src/db.js
index cbc0d38..bfddf9f 100644
--- a/frontend/src/db.js
+++ b/frontend/src/db.js
@@ -51,7 +51,16 @@ db.version(5).stores({
   participants: 'id, email, preferred_name, email_confirmed, updated_at, deleted_at',
 })
 
-db.version(6).stores({}).upgrade(tx => tx.table('session').clear())
+db.version(6).stores({}).upgrade(async tx => {
+  await tx.table('session').clear()
+  await tx.table('meta').clear()
+  await tx.table('participants').clear()
+  await tx.table('tickets').clear()
+  await tx.table('departments').clear()
+  await tx.table('volunteers').clear()
+  await tx.table('shifts').clear()
+  await tx.table('volunteer_shifts').clear()
+})
 
 export async function getLastSync() {
   const m = await db.meta.get('last_sync')
diff --git a/frontend/src/sync.js b/frontend/src/sync.js
index fa3b641..f59fb27 100644
--- a/frontend/src/sync.js
+++ b/frontend/src/sync.js
@@ -4,10 +4,36 @@ import { api } from './api.js'
 let syncing = false
 let sseSource = null
 
+async function checkBuildChanged() {
+  try {
+    const res = await fetch('/api/version')
+    const { build } = await res.json()
+    if (!build) return
+    const stored = await db.meta.get('build')
+    if (stored && stored.value !== build) {
+      await db.transaction('rw',
+        [db.meta, db.event, db.participants, db.tickets, db.departments, db.volunteers, db.shifts, db.volunteer_shifts],
+        async () => {
+          await db.meta.clear()
+          await db.event.clear()
+          await db.participants.clear()
+          await db.tickets.clear()
+          await db.departments.clear()
+          await db.volunteers.clear()
+          await db.shifts.clear()
+          await db.volunteer_shifts.clear()
+        }
+      )
+    }
+    await db.meta.put({ key: 'build', value: build })
+  } catch {}
+}
+
 export async function syncPull() {
   if (syncing) return
   syncing = true
   try {
+    await checkBuildChanged()
     const since = await getLastSync()
     const data = await api.sync.pull(since)
 
@@ -51,7 +77,7 @@ export async function syncPull() {
       }
     )
 
-    await setLastSync(data.server_time)
+    if (data.server_time) await setLastSync(data.server_time)
     return true
   } catch (err) {
     console.warn('Sync pull failed:', err.message)
@@ -97,7 +123,7 @@ export function startSSE(onEvent) {
           syncPull()
         }, 5000)
       }
-    })
+    }).catch(() => {})
   }
 
   connect()
@@ -108,18 +134,23 @@ export function stopSSE() {
   sseSource = null
 }
 
-// Poll for sync when online, with exponential backoff on failure
 let syncInterval = null
+let onlineHandler = null
 
 export function startSyncLoop(intervalMs = 30000) {
   if (syncInterval) return
   syncInterval = setInterval(() => {
     if (navigator.onLine) syncPull()
   }, intervalMs)
-  window.addEventListener('online', () => syncPull())
+  onlineHandler = () => syncPull()
+  window.addEventListener('online', onlineHandler)
 }
 
 export function stopSyncLoop() {
   clearInterval(syncInterval)
   syncInterval = null
+  if (onlineHandler) {
+    window.removeEventListener('online', onlineHandler)
+    onlineHandler = null
+  }
 }