diff --git a/__tests__/lib/highlight-sync-manager.test.ts b/__tests__/lib/highlight-sync-manager.test.ts
index a5093eb..2c3a966 100644
--- a/__tests__/lib/highlight-sync-manager.test.ts
+++ b/__tests__/lib/highlight-sync-manager.test.ts
@@ -76,4 +76,31 @@ describe('HighlightSyncManager', () => {
     const syncing = await manager.getSyncingItems()
     expect(syncing.length).toBe(1)
   })
+
+  it('should perform sync and mark items as synced', async () => {
+    const highlight: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: Date.now(),
+      updatedAt: Date.now(),
+      syncStatus: 'pending'
+    }
+
+    await manager.queueHighlight(highlight)
+    await manager.init()
+
+    // Mock fetch
+    global.fetch = jest.fn(() =>
+      Promise.resolve({
+        ok: true,
+        json: () => Promise.resolve({ synced: 1, errors: [] })
+      })
+    ) as jest.Mock
+
+    const result = await manager.performSync()
+
+    expect(result.synced).toBe(1)
+    expect(result.errors).toBe(0)
+  })
 })
diff --git a/lib/highlight-sync-manager.ts b/lib/highlight-sync-manager.ts
index 7f628e0..1988e2c 100644
--- a/lib/highlight-sync-manager.ts
+++ b/lib/highlight-sync-manager.ts
@@ -108,11 +108,69 @@ export class HighlightSyncManager {
     }
   }
 
-  startAutoSync(intervalMs: number = 30000, onSyncNeeded?: () => void) {
-    this.syncInterval = setInterval(async () => {
+  async performSync(): Promise<{ synced: number; errors: number }> {
+    if (!this.db) await this.init()
+
+    try {
       const pending = await this.getPendingSyncItems()
-      if (pending.length > 0 && onSyncNeeded) {
-        onSyncNeeded()
+      if (pending.length === 0) return { synced: 0, errors: 0 }
+
+      // Mark as syncing
+      await this.markSyncing(pending.map(h => h.id))
+
+      // POST to backend
+      const response = await fetch('/api/highlights/bulk', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ highlights: pending })
+      })
+
+      if (!response.ok) {
+        // Mark all as error
+        const errorIds = pending.map(h => h.id)
+        await this.markError(errorIds, `HTTP ${response.status}`)
+        return { synced: 0, errors: pending.length }
+      }
+
+      const result = await response.json()
+
+      // Mark successfully synced items
+      if (result.synced > 0) {
+        const syncedIds = pending
+          .filter(h => !result.errors.some((e: any) => e.verseId === h.verseId))
+          .map(h => h.id)
+        await this.markSynced(syncedIds)
+      }
+
+      // Mark errored items
+      if (result.errors && result.errors.length > 0) {
+        for (const error of result.errors) {
+          const h = pending.find(item => item.verseId === error.verseId)
+          if (h) {
+            await this.markError([h.id], error.error)
+          }
+        }
+      }
+
+      return { synced: result.synced, errors: result.errors?.length || 0 }
+    } catch (error) {
+      console.error('Sync failed:', error)
+      const pending = await this.getPendingSyncItems()
+      if (pending.length > 0) {
+        await this.markError(
+          pending.map(h => h.id),
+          'Network error'
+        )
+      }
+      return { synced: 0, errors: pending.length }
+    }
+  }
+
+  startAutoSync(intervalMs: number = 30000, onSyncNeeded?: (result: { synced: number; errors: number }) => void) {
+    this.syncInterval = setInterval(async () => {
+      const result = await this.performSync()
+      if (result.synced > 0 || result.errors > 0) {
+        onSyncNeeded?.(result)
       }
     }, intervalMs)
   }