diff --git a/packages/docs/programmable/sdk/api/composables/index.md b/packages/docs/programmable/sdk/api/composables/index.md
index 925e916d0..ff08b7d8f 100644
--- a/packages/docs/programmable/sdk/api/composables/index.md
+++ b/packages/docs/programmable/sdk/api/composables/index.md
@@ -38,6 +38,10 @@ These are the main composables most `@open-pencil/vue` consumers will use.
 - [useStrokeControls](./use-stroke-controls)
 - [useEffectsControls](./use-effects-controls)
 
+## Document workspaces
+
+- [useDocumentWorkspace](./use-document-workspace)
+
 ## Variables, navigation, and localization
 
 - [useVariablesEditor](./use-variables-editor)
diff --git a/packages/docs/programmable/sdk/api/composables/use-document-workspace.md b/packages/docs/programmable/sdk/api/composables/use-document-workspace.md
new file mode 100644
index 000000000..dfca6ad1b
--- /dev/null
+++ b/packages/docs/programmable/sdk/api/composables/use-document-workspace.md
@@ -0,0 +1,72 @@
+---
+title: useDocumentWorkspace
+description: Manage document lists, refreshes, lazy previews, and workspace events.
+---
+
+# useDocumentWorkspace
+
+`useDocumentWorkspace()` provides headless state for document browsers backed by local or remote storage.
+
+It manages:
+
+- initial, manual, focused-window, reconnect, and interval refreshes
+- deduplicated refresh requests and source invalidation events
+- lazy preview loading with bounded concurrency
+- preview object URL creation and cleanup
+- per-preview errors
+
+## Usage
+
+```ts
+import { useDocumentWorkspace } from '@open-pencil/vue'
+
+const workspace = useDocumentWorkspace({
+  source: {
+    refresh: () => documentService.list(),
+    loadPreview: (id) => documentService.loadPreview(id),
+    subscribe: (listener) => documentService.subscribe(listener),
+  },
+  refreshInterval: 60_000,
+})
+```
+
+The source must return items with `id`, `name`, and `updatedAt`. A changed `updatedAt` value invalidates an existing or in-flight preview.
+
+## Lazy previews
+
+Apply `previewDirective` to the element that should trigger loading, then resolve the current object URL with `previewURL()`:
+
+```vue
+<script setup lang="ts">
+const { documents, previewDirective: vDocumentPreview, previewURL } = workspace
+</script>
+
+<template>
+  <article v-for="document in documents" :key="document.id">
+    <div v-document-preview="document.id">
+      <img v-if="previewURL(document.id)" :src="previewURL(document.id) ?? undefined" alt="" />
+    </div>
+  </article>
+</template>
+```
+
+When `IntersectionObserver` is unavailable, the directive loads the preview immediately.
+
+## Errors
+
+`error` contains the latest document-list refresh failure. Preview failures are available by document ID through `previewErrors`; use `onPreviewError` when errors should also be reported to an application service.
+
+```ts
+const workspace = useDocumentWorkspace({
+  source,
+  onPreviewError(id, error) {
+    reportPreviewError({ id, error })
+  },
+})
+```
+
+Calling `loadPreview(id)` retries a failed preview. Successful loads clear the corresponding preview error.
+
+## Cleanup
+
+The composable revokes generated object URLs and unsubscribes from the source when its component unmounts. Call `clearPreviews()` when switching an external workspace or provider without unmounting the component.
diff --git a/packages/fig/src/thumbnail.ts b/packages/fig/src/thumbnail.ts
index eb495b863..c181d7654 100644
--- a/packages/fig/src/thumbnail.ts
+++ b/packages/fig/src/thumbnail.ts
@@ -14,6 +14,7 @@ export type FigThumbnailLimits = {
 type ThumbnailEntry = {
   method: number
   compressedSize: number
+  outputSize: number
   localOffset: number
 }
 
@@ -26,6 +27,7 @@ const DEFAULT_MAX_TAIL = 4 * 1024 * 1024
 const DEFAULT_MAX_COMPRESSED = 8 * 1024 * 1024
 const DEFAULT_MAX_OUTPUT = 16 * 1024 * 1024
 const THUMBNAIL_NAME = 'thumbnail.png'
+const PNG_SIGNATURE = new Uint8Array([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])
 
 function view(bytes: Uint8Array): DataView {
   return new DataView(bytes.buffer, bytes.byteOffset, bytes.byteLength)
@@ -43,6 +45,10 @@ function boundedLimit(value: number | undefined, fallback: number): number {
   return Number.isFinite(value) && value && value > 0 ? value : fallback
 }
 
+function hasPNGSignature(bytes: Uint8Array): boolean {
+  return PNG_SIGNATURE.every((byte, index) => bytes[index] === byte)
+}
+
 function findThumbnailEntry(
   central: Uint8Array,
   maxCompressed: number,
@@ -66,7 +72,12 @@ function findThumbnailEntry(
     const name = decoder.decode(central.subarray(offset + 46, offset + 46 + nameLength))
     if (name === THUMBNAIL_NAME) {
       if (compressedSize > maxCompressed || outputSize > maxOutput) return null
-      return { method, compressedSize, localOffset: data.getUint32(offset + 42, true) }
+      return {
+        method,
+        compressedSize,
+        outputSize,
+        localOffset: data.getUint32(offset + 42, true)
+      }
     }
     offset = next
   }
@@ -85,10 +96,14 @@ async function readEntryPayload(
     entry.localOffset + 30 + headerView.getUint16(26, true) + headerView.getUint16(28, true)
   if (dataStart + entry.compressedSize > reader.size) return null
   const compressed = await reader.read(dataStart, dataStart + entry.compressedSize)
-  if (entry.method === 0) return compressed.byteLength <= maxOutput ? compressed : null
+  if (entry.method === 0) {
+    return compressed.byteLength === entry.outputSize && hasPNGSignature(compressed)
+      ? compressed
+      : null
+  }
   if (entry.method !== 8) return null
   const output = inflateSync(compressed, { out: new Uint8Array(maxOutput + 1) })
-  return output.byteLength <= maxOutput ? output : null
+  return output.byteLength === entry.outputSize && hasPNGSignature(output) ? output : null
 }
 
 /**
diff --git a/packages/vue/src/document/workspace/use.ts b/packages/vue/src/document/workspace/use.ts
index 9745c078c..100b6f223 100644
--- a/packages/vue/src/document/workspace/use.ts
+++ b/packages/vue/src/document/workspace/use.ts
@@ -31,6 +31,7 @@ export type UseDocumentWorkspaceOptions<Item extends DocumentWorkspaceItem> = {
   refreshOnReconnect?: boolean
   previewConcurrency?: number
   previewMimeType?: string
+  onPreviewError?(id: string, error: unknown): void
 }
 
 export function useDocumentWorkspace<Item extends DocumentWorkspaceItem>(
@@ -41,6 +42,7 @@ export function useDocumentWorkspace<Item extends DocumentWorkspaceItem>(
   const error = shallowRef<unknown>(null)
   const lastRefreshedAt = shallowRef<Date | null>(null)
   const previewUrls = ref<Record<string, string>>({})
+  const previewErrors = shallowRef<Record<string, unknown>>({})
   const previewCleanups = new WeakMap<Element, () => void>()
   const previewGenerations = new Map<string, number>()
   const previewQueue: string[] = []
@@ -64,9 +66,17 @@ export function useDocumentWorkspace<Item extends DocumentWorkspaceItem>(
   function reconcilePreviewUrls(items: readonly Item[]): void {
     const previousItems = new Map(documents.value.map((item) => [item.id, item.updatedAt]))
     const currentItems = new Map(items.map((item) => [item.id, item.updatedAt]))
-    for (const id of Object.keys(previewUrls.value)) {
-      if (previousItems.get(id) !== currentItems.get(id)) removePreviewURL(id)
+    const trackedIds = new Set([...Object.keys(previewUrls.value), ...activePreviews, ...queued])
+    for (const id of trackedIds) {
+      if (previousItems.get(id) === currentItems.get(id)) continue
+      removePreviewURL(id)
+      if (!currentItems.has(id) || activePreviews.has(id)) continue
+      if (!queued.has(id)) {
+        queued.add(id)
+        previewQueue.push(id)
+      }
     }
+    drainPreviewQueue()
   }
 
   function clearPreviews(): void {
@@ -94,6 +104,18 @@ export function useDocumentWorkspace<Item extends DocumentWorkspaceItem>(
     }
   }
 
+  function clearPreviewError(id: string): void {
+    if (!(id in previewErrors.value)) return
+    previewErrors.value = Object.fromEntries(
+      Object.entries(previewErrors.value).filter(([previewId]) => previewId !== id)
+    )
+  }
+
+  function recordPreviewError(id: string, error: unknown): void {
+    previewErrors.value = { ...previewErrors.value, [id]: error }
+    options.onPreviewError?.(id, error)
+  }
+
   function drainPreviewQueue(): void {
     while (activePreviews.size < concurrency) {
       const id = previewQueue.shift()
@@ -106,13 +128,25 @@ export function useDocumentWorkspace<Item extends DocumentWorkspaceItem>(
         .loadPreview(id)
         .then((bytes) => {
           if (bytes?.byteLength && generation === (previewGenerations.get(id) ?? 0)) {
+            clearPreviewError(id)
             replacePreviewURL(id, bytes)
           }
           return undefined
         })
-        .catch(() => null)
+        .catch((error: unknown) => {
+          if (!disposed && generation === (previewGenerations.get(id) ?? 0)) {
+            recordPreviewError(id, error)
+          }
+        })
         .finally(() => {
           activePreviews.delete(id)
+          if (
+            !disposed &&
+            generation !== (previewGenerations.get(id) ?? 0) &&
+            documents.value.some((item) => item.id === id)
+          ) {
+            loadPreview(id)
+          }
           drainPreviewQueue()
         })
     }
@@ -120,6 +154,7 @@ export function useDocumentWorkspace<Item extends DocumentWorkspaceItem>(
 
   function loadPreview(id: string): void {
     if (previewUrls.value[id] || activePreviews.has(id) || queued.has(id)) return
+    clearPreviewError(id)
     queued.add(id)
     previewQueue.push(id)
     drainPreviewQueue()
@@ -236,6 +271,7 @@ export function useDocumentWorkspace<Item extends DocumentWorkspaceItem>(
     error: readonly(error),
     lastRefreshedAt: readonly(lastRefreshedAt),
     previewUrls: readonly(previewUrls),
+    previewErrors: readonly(previewErrors),
     hasDocuments: computed(() => documents.value.length > 0),
     refresh,
     invalidate,
diff --git a/src/app/integrations/storage/s3/client.ts b/src/app/integrations/storage/s3/client.ts
index a13ff39ca..9b8e9f7f3 100644
--- a/src/app/integrations/storage/s3/client.ts
+++ b/src/app/integrations/storage/s3/client.ts
@@ -170,7 +170,9 @@ export async function headObjectSize(
 ): Promise<number | null> {
   const res = await s3Request(config, objectURL(config, key), { method: 'HEAD' })
   if (res.status === 404) return null
-  const size = Number(res.headers.get('content-length'))
+  const sizeHeader = res.headers.get('content-length')
+  if (sizeHeader == null) return null
+  const size = Number(sizeHeader)
   return Number.isSafeInteger(size) && size >= 0 ? size : null
 }
 
@@ -182,6 +184,7 @@ export async function getObjectRange(
 ): Promise<Uint8Array | null> {
   if (
     !Number.isSafeInteger(start) ||
+    start < 0 ||
     !Number.isSafeInteger(endExclusive) ||
     endExclusive <= start
   ) {
diff --git a/src/app/storage/workspace/events.ts b/src/app/storage/workspace/events.ts
index 47589ff98..e6d112999 100644
--- a/src/app/storage/workspace/events.ts
+++ b/src/app/storage/workspace/events.ts
@@ -1,5 +1,7 @@
+import type { StorageProviderID } from '@/app/integrations/storage/types'
+
 export type StorageWorkspaceEvent = {
-  providerId: string
+  providerId: StorageProviderID
   documentId?: string
   kind: 'changed' | 'synced'
 }
diff --git a/src/app/storage/workspace/source.ts b/src/app/storage/workspace/source.ts
index 6548ab36b..d93a50ee0 100644
--- a/src/app/storage/workspace/source.ts
+++ b/src/app/storage/workspace/source.ts
@@ -70,14 +70,15 @@ export function createStorageWorkspaceSource(
     },
 
     async loadPreview(id: string): Promise<Uint8Array | null> {
+      const providerID = activeStorageProviderID.value
       const localStore = getLocalCanvasStore()
       const local = await localStore.readThumb(id)
       if (local?.byteLength) return local
-      const adapter = createActiveStorageAdapter()
+      const adapter = createActiveStorageAdapter(providerID)
       if (!adapter.getThumbnail) return null
       const remote = await adapter.getThumbnail(id)
       if (!remote?.byteLength) return null
-      await localStore.writeThumb(id, remote)
+      if (activeStorageProviderID.value === providerID) await localStore.writeThumb(id, remote)
       return remote
     }
   }
diff --git a/tests/e2e/storage/workspace.spec.ts b/tests/e2e/storage/workspace.spec.ts
index 157515045..232b1ed92 100644
--- a/tests/e2e/storage/workspace.spec.ts
+++ b/tests/e2e/storage/workspace.spec.ts
@@ -34,17 +34,26 @@ test('configured storage lists previews through ranges before opening the docume
       return
     }
     if (url.pathname.endsWith('/remote-1.fig') && route.request().headers().range) {
-      rangeGets++
-      const match = route
-        .request()
-        .headers()
-        .range?.match(/^bytes=(\d+)-(\d+)$/)
-      if (!match) {
+      const range = route.request().headers().range
+      const explicit = range?.match(/^bytes=(\d+)-(\d+)$/)
+      const suffix = range?.match(/^bytes=-(\d+)$/)
+      let start: number
+      let end: number
+      if (explicit) {
+        start = Number(explicit[1])
+        end = Math.min(Number(explicit[2]), fixture.byteLength - 1)
+      } else if (suffix) {
+        const length = Math.min(Number(suffix[1]), fixture.byteLength)
+        start = fixture.byteLength - length
+        end = fixture.byteLength - 1
+      } else {
+        await route.fulfill({ status: 416 })
+        return
+      }
+      if (!Number.isSafeInteger(start) || !Number.isSafeInteger(end) || start < 0 || end < start) {
         await route.fulfill({ status: 416 })
         return
       }
-      const start = Number(match[1])
-      const end = Number(match[2])
       await route.fulfill({
         status: 206,
         headers: {
@@ -53,6 +62,7 @@ test('configured storage lists previews through ranges before opening the docume
         contentType: 'application/octet-stream',
         body: fixture.subarray(start, end + 1)
       })
+      rangeGets++
       return
     }
     if (url.pathname.endsWith('/remote-1.fig') && route.request().method() === 'HEAD') {
@@ -91,7 +101,7 @@ test('configured storage lists previews through ranges before opening the docume
   const preview = page.locator('[data-document-id="remote-1"] img')
   await expect(preview).toBeVisible()
   await expect(preview).toHaveAttribute('src', /^blob:/)
-  expect(rangeGets).toBeGreaterThan(0)
+  expect(rangeGets).toBe(3)
   expect(fullDocumentGets).toBe(0)
 
   await page.locator('[data-document-id="remote-1"]').click()
diff --git a/tests/engine/app/storage/persist.test.ts b/tests/engine/app/storage/persist.test.ts
index d4cc4ff8c..6061844cb 100644
--- a/tests/engine/app/storage/persist.test.ts
+++ b/tests/engine/app/storage/persist.test.ts
@@ -47,6 +47,10 @@ describe('local-first storage persistence', () => {
       { store, enqueueCanvas }
     )
 
-    expect((await store.readThumb('canvas-preview'))?.byteLength).toBeGreaterThan(0)
+    const thumbnail = await store.readThumb('canvas-preview')
+    expect(thumbnail?.byteLength).toBeGreaterThan(0)
+    expect(thumbnail?.subarray(0, 8)).toEqual(
+      new Uint8Array([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])
+    )
   })
 })
diff --git a/tests/engine/io/fig/thumbnail-range.test.ts b/tests/engine/io/fig/thumbnail-range.test.ts
index d652177c2..e40596715 100644
--- a/tests/engine/io/fig/thumbnail-range.test.ts
+++ b/tests/engine/io/fig/thumbnail-range.test.ts
@@ -1,6 +1,8 @@
 import { describe, expect, test } from 'bun:test'
 import { readFileSync } from 'node:fs'
 
+import { zipSync } from 'fflate'
+
 import { extractFigThumbnailFromReader } from '@open-pencil/fig'
 
 function memoryReader(bytes: Uint8Array, ranges: Array<[number, number]>) {
@@ -31,6 +33,14 @@ describe('fig ranged thumbnail extraction', () => {
     )
   })
 
+  test('rejects malformed thumbnail payloads', async () => {
+    const bytes = zipSync({
+      'canvas.fig': new Uint8Array([1]),
+      'thumbnail.png': new TextEncoder().encode('not a png')
+    })
+    expect(await extractFigThumbnailFromReader(memoryReader(bytes, []))).toBeNull()
+  })
+
   test('rejects thumbnails above configured output limits', async () => {
     const bytes = new Uint8Array(readFileSync('tests/fixtures/gold-preview.fig'))
     const thumbnail = await extractFigThumbnailFromReader(memoryReader(bytes, []), {
diff --git a/tests/engine/vue/document-workspace.test.ts b/tests/engine/vue/document-workspace.test.ts
index a21765e63..1eb2523c5 100644
--- a/tests/engine/vue/document-workspace.test.ts
+++ b/tests/engine/vue/document-workspace.test.ts
@@ -158,6 +158,62 @@ describe('useDocumentWorkspace', () => {
     expect(sourceListener).toBeNull()
   })
 
+  test('invalidates an in-flight preview when the document changes', async () => {
+    const previewLoad = deferred<Uint8Array | null>()
+    const freshPreviewLoad = deferred<Uint8Array | null>()
+    const refresh = vi
+      .fn<() => Promise<DocumentWorkspaceItem[]>>()
+      .mockResolvedValueOnce([{ id: 'one', name: 'One', updatedAt: 'first' }])
+      .mockResolvedValueOnce([{ id: 'one', name: 'One', updatedAt: 'second' }])
+    const loadPreview = vi
+      .fn<() => Promise<Uint8Array | null>>()
+      .mockImplementationOnce(() => previewLoad.promise)
+      .mockImplementationOnce(() => freshPreviewLoad.promise)
+    const createObjectURL = vi.spyOn(URL, 'createObjectURL').mockReturnValue('blob:preview')
+    const mounted = mountWorkspace({ refresh, loadPreview })
+    await flushTasks()
+
+    mounted.workspace.loadPreview('one')
+    expect(loadPreview).toHaveBeenCalledTimes(1)
+    await mounted.workspace.refresh()
+    previewLoad.resolve(new Uint8Array([1]))
+    await previewLoad.promise
+    await flushTasks()
+
+    expect(createObjectURL).not.toHaveBeenCalled()
+    expect(loadPreview).toHaveBeenCalledTimes(2)
+    freshPreviewLoad.resolve(new Uint8Array([2]))
+    await freshPreviewLoad.promise
+    await flushTasks()
+    expect(createObjectURL).toHaveBeenCalledTimes(1)
+    mounted.unmount()
+  })
+
+  test('surfaces preview failures to consumers', async () => {
+    const failure = new Error('preview failed')
+    const onPreviewError = vi.fn()
+    const holder: WorkspaceHolder = { current: null }
+    const component = defineComponent({
+      setup() {
+        holder.current = useDocumentWorkspace({
+          source: { refresh: async () => [], loadPreview: async () => Promise.reject(failure) },
+          refreshOnFocus: false,
+          refreshOnReconnect: false,
+          onPreviewError
+        })
+        return () => h('div')
+      }
+    })
+    const app = renderer.createApp(component)
+    app.mount(hostNode())
+    holder.current?.loadPreview('one')
+    await flushTasks()
+
+    expect(holder.current?.previewErrors.value.one).toBe(failure)
+    expect(onPreviewError).toHaveBeenCalledWith('one', failure)
+    app.unmount()
+  })
+
   test('deduplicates previews, limits concurrency, and revokes URLs on unmount', async () => {
     const loads = new Map<string, Deferred<Uint8Array | null>>()
     const loadPreview = vi.fn((id: string) => {