diff --git a/src/server/api/__tests__/previewFs.test.ts b/src/server/api/__tests__/previewFs.test.ts
index 38b2df37..f8d47814 100644
--- a/src/server/api/__tests__/previewFs.test.ts
+++ b/src/server/api/__tests__/previewFs.test.ts
@@ -1,5 +1,8 @@
 import { describe, expect, it } from 'bun:test'
-import { contentTypeForPath } from '../previewFs'
+import { mkdtempSync, writeFileSync, mkdirSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import * as path from 'node:path'
+import { contentTypeForPath, handlePreviewFs } from '../previewFs'
 
 describe('contentTypeForPath', () => {
   it('maps common web asset extensions', () => {
@@ -14,3 +17,43 @@ describe('contentTypeForPath', () => {
     expect(contentTypeForPath('/x/file.bin')).toBe('application/octet-stream')
   })
 })
+
+function setupWorkspace() {
+  const root = mkdtempSync(path.join(tmpdir(), 'pfs-'))
+  writeFileSync(path.join(root, 'index.html'), '<h1>ok</h1>')
+  mkdirSync(path.join(root, 'assets'))
+  writeFileSync(path.join(root, 'assets', 'a.css'), 'body{}')
+  return root
+}
+
+describe('handlePreviewFs', () => {
+  it('serves an in-workspace file with content-type', async () => {
+    const root = setupWorkspace()
+    const resolve = async (id: string) => (id === 's1' ? root : null)
+    const res = await handlePreviewFs(new URL('http://127.0.0.1/preview-fs/s1/index.html'), resolve)
+    expect(res.status).toBe(200)
+    expect(res.headers.get('content-type')).toBe('text/html; charset=utf-8')
+    expect(await res.text()).toBe('<h1>ok</h1>')
+  })
+
+  it('serves nested assets', async () => {
+    const root = setupWorkspace()
+    const resolve = async () => root
+    const res = await handlePreviewFs(new URL('http://127.0.0.1/preview-fs/s1/assets/a.css'), resolve)
+    expect(res.status).toBe(200)
+    expect(res.headers.get('content-type')).toBe('text/css; charset=utf-8')
+  })
+
+  it('blocks path traversal with 403', async () => {
+    const root = setupWorkspace()
+    const resolve = async () => root
+    const res = await handlePreviewFs(new URL('http://127.0.0.1/preview-fs/s1/../../etc/passwd'), resolve)
+    expect(res.status).toBe(403)
+  })
+
+  it('404 when session has no workdir', async () => {
+    const resolve = async () => null
+    const res = await handlePreviewFs(new URL('http://127.0.0.1/preview-fs/sX/index.html'), resolve)
+    expect(res.status).toBe(404)
+  })
+})
diff --git a/src/server/api/previewFs.ts b/src/server/api/previewFs.ts
index 61b3e676..9d9dfe9a 100644
--- a/src/server/api/previewFs.ts
+++ b/src/server/api/previewFs.ts
@@ -1,4 +1,9 @@
+import * as fs from 'node:fs'
 import * as path from 'node:path'
+import {
+  isSameOrInsidePathForPlatform,
+  normalizeDriveRootPathForPlatform,
+} from '../services/windowsDrivePath.js'
 
 const CONTENT_TYPES: Record<string, string> = {
   html: 'text/html; charset=utf-8',
@@ -24,3 +29,63 @@ export function contentTypeForPath(filePath: string): string {
   const ext = path.extname(filePath).slice(1).toLowerCase()
   return CONTENT_TYPES[ext] ?? 'application/octet-stream'
 }
+
+export type ResolveWorkDir = (sessionId: string) => Promise<string | null>
+
+const PREFIX = '/preview-fs/'
+const MAX_FILE_BYTES = 50 * 1024 * 1024
+
+/**
+ * Serve a single file from a session's sandboxed workspace directory.
+ *
+ * URL shape: `/preview-fs/<sessionId>/<relPath>` where `<relPath>` may itself
+ * contain `/` separators. The WHATWG URL parser collapses `..` segments before
+ * this handler runs, so a traversal attempt such as
+ * `/preview-fs/s1/../../etc/passwd` arrives with its pathname normalized to
+ * `/etc/passwd` — i.e. the `/preview-fs/` prefix is gone. We treat any request
+ * that lost the prefix as a sandbox escape and return 403. Requests that keep
+ * the prefix are additionally re-validated against the resolved work-dir root.
+ */
+export async function handlePreviewFs(
+  url: URL,
+  resolveWorkDir: ResolveWorkDir,
+): Promise<Response> {
+  if (!url.pathname.startsWith(PREFIX)) {
+    return new Response('forbidden', { status: 403 })
+  }
+
+  const rest = url.pathname.slice(PREFIX.length)
+  const slash = rest.indexOf('/')
+  if (slash <= 0) return new Response('bad request', { status: 400 })
+
+  const sessionId = decodeURIComponent(rest.slice(0, slash))
+  const relRaw = decodeURIComponent(rest.slice(slash + 1))
+
+  const workDir = await resolveWorkDir(sessionId)
+  if (!workDir) return new Response('no workdir', { status: 404 })
+
+  const root = path.resolve(normalizeDriveRootPathForPlatform(workDir))
+  const target = path.resolve(root, relRaw)
+  if (!isSameOrInsidePathForPlatform(target, root)) {
+    return new Response('forbidden', { status: 403 })
+  }
+
+  let stat: fs.Stats
+  try {
+    stat = fs.statSync(target)
+  } catch {
+    return new Response('not found', { status: 404 })
+  }
+  if (!stat.isFile()) return new Response('not a file', { status: 404 })
+  if (stat.size > MAX_FILE_BYTES) return new Response('too large', { status: 413 })
+
+  const data = fs.readFileSync(target)
+  return new Response(data, {
+    status: 200,
+    headers: {
+      'Content-Type': contentTypeForPath(target),
+      'Content-Length': String(stat.size),
+      'Cache-Control': 'no-cache',
+    },
+  })
+}
diff --git a/src/server/index.ts b/src/server/index.ts
index 5b924d78..b4d8a5f3 100644
--- a/src/server/index.ts
+++ b/src/server/index.ts
@@ -15,6 +15,9 @@ import { handleProxyRequest } from './proxy/handler.js'
 import { ProviderService } from './services/providerService.js'
 import { handleHahaOAuthCallback } from './api/haha-oauth.js'
 import { handleHahaOpenAIOAuthCallback } from './api/haha-openai-oauth.js'
+import { handlePreviewFs } from './api/previewFs.js'
+import { sessionService } from './services/sessionService.js'
+import { conversationService } from './services/conversationService.js'
 import { OPENAI_CODEX_REDIRECT_PATH } from '../services/openaiAuth/client.js'
 import { ensureDesktopCliLauncherInstalled } from './services/desktopCliLauncherService.js'
 import { enableConfigs } from '../utils/config.js'
@@ -275,6 +278,32 @@ export function startServer(port = PORT, host = HOST) {
           return handleHahaOpenAIOAuthCallback(url)
         }
 
+        // Preview filesystem — serve sandboxed workspace files for a session.
+        if (url.pathname.startsWith('/preview-fs/')) {
+          if (cors.rejected) {
+            return corsRejectedResponse(cors)
+          }
+
+          if (authRequired) {
+            const authError = await requireH5Token(req)
+            if (authError) {
+              return withCors(authError, cors)
+            }
+          } else if (forceAuth) {
+            const authError = await requireAuth(req)
+            if (authError) {
+              return withCors(authError, cors)
+            }
+          }
+
+          const response = await handlePreviewFs(url, async (sessionId) =>
+            conversationService.getSessionWorkDir(sessionId) ||
+            (await sessionService.getSessionWorkDir(sessionId)) ||
+            null,
+          )
+          return withCors(response, cors)
+        }
+
         // REST API
         if (url.pathname.startsWith('/api/')) {
           if (cors.rejected) {
@@ -396,7 +425,6 @@ export function startServer(port = PORT, host = HOST) {
 }
 
 // ─── Graceful shutdown: kill all CLI subprocesses on exit ────────────────────
-import { conversationService } from './services/conversationService.js'
 
 function cleanupAllSessions() {
   const active = conversationService.getActiveSessions()