diff --git a/src/app/storage/sync/engine.ts b/src/app/storage/sync/engine.ts
new file mode 100644
index 000000000..37df9da96
--- /dev/null
+++ b/src/app/storage/sync/engine.ts
@@ -0,0 +1,261 @@
+import { IS_BROWSER } from '@open-pencil/core/constants'
+
+import {
+  activeStorageProviderID,
+  createActiveStorageAdapter,
+  storagePreferencesComplete
+} from '@/app/integrations/storage'
+import { getLocalCanvasStore } from '@/app/storage/local-store'
+import { getOutbox } from '@/app/storage/sync/outbox'
+import { setUploadProgress } from '@/app/storage/sync/progress'
+import { setPendingSyncCount, setSyncUi } from '@/app/storage/sync/status'
+import type { OutboxJob } from '@/app/storage/sync/types'
+
+const MAX_ATTEMPTS = 8
+const BASE_BACKOFF_MS = 1500
+const MAX_BACKOFF_MS = 60_000
+
+let pumping = false
+let wakeTimer: ReturnType<typeof setTimeout> | null = null
+let onlineBound = false
+
+function isOnline(): boolean {
+  if (typeof navigator === 'undefined') return true
+  return navigator.onLine
+}
+
+function backoffMs(attempts: number): number {
+  const exp = Math.min(MAX_BACKOFF_MS, BASE_BACKOFF_MS * 2 ** Math.max(0, attempts - 1))
+  const jitter = Math.floor(exp * 0.2 * ((crypto.getRandomValues(new Uint8Array(1))[0] ?? 0) / 255))
+  return exp + jitter
+}
+
+function isPermanentError(error: unknown): boolean {
+  if (!(error instanceof Error)) return false
+  const msg = error.message.toLowerCase()
+  return (
+    msg.includes('403') ||
+    msg.includes('401') ||
+    msg.includes('access denied') ||
+    msg.includes('invalid access key') ||
+    msg.includes('not configured')
+  )
+}
+
+async function runJob(job: OutboxJob): Promise<void> {
+  if (!storagePreferencesComplete(activeStorageProviderID.value)) {
+    throw new Error('Storage is not configured')
+  }
+  const adapter = createActiveStorageAdapter()
+  const store = getLocalCanvasStore()
+  const meta = await store.getMeta(job.canvasId)
+
+  if (job.type === 'deleteCanvas') {
+    await adapter.deleteDocument(job.canvasId)
+    // Keep the tombstoned row: reconcile purges it once the remote listing
+    // confirms the object is gone. Removing it here opened a race where a
+    // concurrent reconcile re-seeded the canvas from a stale remote listing.
+    await store.updateMeta(job.canvasId, { syncStatus: 'synced', lastSyncError: null })
+    return
+  }
+
+  if (!meta || meta.tombstoned) {
+    // Nothing to put
+    return
+  }
+
+  if (job.type === 'putCanvas') {
+    // Superseded by a newer local revision already on disk
+    if (meta.revision > job.revision) return
+    if (!meta.hasFig) return
+    const fig = await store.readFig(job.canvasId)
+    if (!fig || fig.byteLength === 0) throw new Error('Local document missing for sync')
+    setUploadProgress(job.canvasId, 0)
+    try {
+      await adapter.putDocument(
+        job.canvasId,
+        fig,
+        {
+          name: meta.name,
+          updatedAt: meta.updatedAt
+        },
+        ({ transferredBytes, totalBytes }) => {
+          if (totalBytes) setUploadProgress(job.canvasId, transferredBytes / totalBytes)
+        }
+      )
+    } finally {
+      setUploadProgress(job.canvasId, null)
+    }
+    // Only mark synced if still on this revision and no other pending work for newer rev
+    const latest = await store.getMeta(job.canvasId)
+    if (latest && latest.revision === job.revision && !latest.tombstoned) {
+      await store.updateMeta(job.canvasId, {
+        syncStatus: 'synced',
+        lastSyncedAt: new Date().toISOString(),
+        lastSyncError: null
+      })
+    }
+    return
+  }
+
+  // Remaining job type: putThumb
+  if (!adapter.putThumbnail) return
+  const thumb = await store.readThumb(job.canvasId)
+  if (!thumb) return
+  await adapter.putThumbnail(job.canvasId, thumb)
+}
+
+async function pumpOnce(): Promise<void> {
+  const outbox = getOutbox()
+  const jobs = await outbox.list()
+  setPendingSyncCount(jobs.length)
+
+  if (jobs.length === 0) {
+    if (isOnline()) setSyncUi('idle')
+    return
+  }
+
+  if (!isOnline()) {
+    setSyncUi('offline')
+    scheduleWake(5000)
+    return
+  }
+
+  setSyncUi('syncing')
+  const now = Date.now()
+  // Single-flight globally for simplicity (large figs)
+  const job = jobs.find((j) => j.nextAttemptAt <= now)
+  if (!job) {
+    const nextAt = Math.min(...jobs.map((j) => j.nextAttemptAt))
+    scheduleWake(Math.max(250, nextAt - now))
+    return
+  }
+
+  try {
+    await runJob(job)
+    await outbox.remove(job.id)
+    const remaining = await outbox.list()
+    setPendingSyncCount(remaining.length)
+    if (remaining.length === 0) setSyncUi('idle')
+    else scheduleWake(50)
+  } catch (error) {
+    const attempts = job.attempts + 1
+    const permanent = isPermanentError(error) || attempts >= MAX_ATTEMPTS
+    const message = error instanceof Error ? error.message : String(error)
+    console.warn('[Storage sync] job failed:', job.type, job.canvasId, message)
+
+    if (permanent) {
+      // A failed thumbnail upload must not poison the document's sync status —
+      // only canvas/delete jobs reflect into the meta row.
+      if (job.type !== 'putThumb') {
+        await getLocalCanvasStore().updateMeta(job.canvasId, {
+          syncStatus: 'error',
+          lastSyncError: message
+        })
+        setSyncUi('error', message.slice(0, 120))
+      } else {
+        // Keep a record without touching syncStatus so the stale remote
+        // thumbnail is at least diagnosable.
+        await getLocalCanvasStore().updateMeta(job.canvasId, { lastSyncError: message })
+      }
+      await outbox.remove(job.id)
+      const remaining = await outbox.list()
+      setPendingSyncCount(remaining.length)
+      if (remaining.length > 0) scheduleWake(1000)
+      else if (job.type === 'putThumb') setSyncUi('idle')
+      return
+    }
+
+    const updated: OutboxJob = {
+      ...job,
+      attempts,
+      nextAttemptAt: Date.now() + backoffMs(attempts)
+    }
+    await outbox.update(updated)
+    if (job.type !== 'putThumb') {
+      await getLocalCanvasStore().updateMeta(job.canvasId, {
+        syncStatus: 'pending',
+        lastSyncError: message
+      })
+    }
+    // Wake for the next ready job across the whole queue — not this job's
+    // full backoff, which starved other jobs that were ready sooner.
+    const all = await outbox.list()
+    const nextAt = Math.min(...all.map((j) => j.nextAttemptAt))
+    scheduleWake(Math.max(250, nextAt - Date.now()))
+  }
+}
+
+function scheduleWake(ms: number) {
+  if (wakeTimer != null) clearTimeout(wakeTimer)
+  wakeTimer = setTimeout(() => {
+    wakeTimer = null
+    void kickSyncEngine()
+  }, ms)
+}
+
+function ensureOnlineListeners() {
+  if (onlineBound || !IS_BROWSER) return
+  onlineBound = true
+  window.addEventListener('online', () => {
+    setSyncUi('syncing')
+    void kickSyncEngine()
+  })
+  window.addEventListener('offline', () => {
+    setSyncUi('offline')
+  })
+}
+
+/** Start or continue draining the outbox. Safe to call often. */
+export async function kickSyncEngine(): Promise<void> {
+  ensureOnlineListeners()
+  if (pumping) return
+  pumping = true
+  let pumpFailed = false
+  try {
+    // Drain a few jobs per kick to avoid long tight loops blocking the tab.
+    for (let i = 0; i < 3; i++) {
+      const before = (await getOutbox().list()).length
+      await pumpOnce()
+      const after = (await getOutbox().list()).length
+      if (after === 0 || after >= before) break
+    }
+  } catch (error) {
+    // Never let an escaped rejection strand the queue — retry shortly.
+    pumpFailed = true
+    console.warn('[Storage sync] pump failed:', error)
+    scheduleWake(5000)
+  } finally {
+    pumping = false
+  }
+  // A job enqueued mid-pump can slip past the loop's exit check while its
+  // kick was swallowed by the pumping guard — re-wake if work is already due.
+  // (Skip offline — pumpOnce owns those wakes — and errors, which keep their
+  // 5s backoff; re-waking would clobber it into a tight retry loop.)
+  if (pumpFailed || !isOnline()) return
+  const jobs = await getOutbox().list()
+  if (jobs.some((job) => job.nextAttemptAt <= Date.now())) scheduleWake(250)
+}
+
+export async function enqueuePutCanvas(canvasId: string, revision: number): Promise<void> {
+  await getOutbox().enqueue({ canvasId, type: 'putCanvas', revision })
+  void kickSyncEngine()
+}
+
+export async function enqueuePutThumb(canvasId: string, revision: number): Promise<void> {
+  await getOutbox().enqueue({ canvasId, type: 'putThumb', revision })
+  void kickSyncEngine()
+}
+
+export async function enqueueDeleteCanvas(canvasId: string): Promise<void> {
+  await getOutbox().enqueue({ canvasId, type: 'deleteCanvas', revision: 0 })
+  void kickSyncEngine()
+}
+
+/** After credentials cleared — drop local mirror + outbox (optional safety). */
+export async function clearStorageLocalMirror(): Promise<void> {
+  await getLocalCanvasStore().clearAll()
+  await getOutbox().clear()
+  setPendingSyncCount(0)
+  setSyncUi('idle')
+}
diff --git a/src/app/storage/sync/index.ts b/src/app/storage/sync/index.ts
new file mode 100644
index 000000000..678bc70ef
--- /dev/null
+++ b/src/app/storage/sync/index.ts
@@ -0,0 +1,24 @@
+export {
+  clearStorageLocalMirror,
+  enqueueDeleteCanvas,
+  enqueuePutCanvas,
+  enqueuePutThumb,
+  kickSyncEngine
+} from './engine'
+export { createMemoryOutbox, getOutbox, resetOutboxForTests } from './outbox'
+export { setUploadProgress, uploadProgressByCanvas } from './progress'
+export {
+  pendingSyncCount,
+  setPendingSyncCount,
+  setSyncUi,
+  syncStatusLabel,
+  syncUiDetail,
+  syncUiState
+} from './status'
+export {
+  makeJobId,
+  supersedePutCanvasJobs,
+  type OutboxJob,
+  type OutboxJobType,
+  type SyncUiState
+} from './types'
diff --git a/src/app/storage/sync/progress.ts b/src/app/storage/sync/progress.ts
new file mode 100644
index 000000000..faac9999c
--- /dev/null
+++ b/src/app/storage/sync/progress.ts
@@ -0,0 +1,14 @@
+import { ref } from 'vue'
+
+/**
+ * Per-canvas upload progress (0..1) while a putCanvas job is in flight.
+ * Drives the card badge fill on the Cloud Workspace grid.
+ */
+export const uploadProgressByCanvas = ref<ReadonlyMap<string, number>>(new Map())
+
+export function setUploadProgress(canvasId: string, fraction: number | null) {
+  const next = new Map(uploadProgressByCanvas.value)
+  if (fraction == null) next.delete(canvasId)
+  else next.set(canvasId, Math.max(0, Math.min(1, fraction)))
+  uploadProgressByCanvas.value = next
+}
diff --git a/src/app/storage/sync/status.ts b/src/app/storage/sync/status.ts
new file mode 100644
index 000000000..4dc0ea567
--- /dev/null
+++ b/src/app/storage/sync/status.ts
@@ -0,0 +1,30 @@
+import { computed, ref } from 'vue'
+
+import type { SyncUiState } from '@/app/storage/sync/types'
+
+/** Global subtle sync status for UI chips. */
+export const syncUiState = ref<SyncUiState>('idle')
+export const syncUiDetail = ref<string | null>(null)
+export const pendingSyncCount = ref(0)
+
+export const syncStatusLabel = computed(() => {
+  switch (syncUiState.value) {
+    case 'syncing':
+      return syncUiDetail.value ?? 'Syncing…'
+    case 'offline':
+      return 'Offline · will sync'
+    case 'error':
+      return syncUiDetail.value ?? 'Sync failed'
+    default:
+      return null
+  }
+})
+
+export function setSyncUi(state: SyncUiState, detail: string | null = null) {
+  syncUiState.value = state
+  syncUiDetail.value = detail
+}
+
+export function setPendingSyncCount(count: number) {
+  pendingSyncCount.value = count
+}