diff --git a/src/server/api/__tests__/previewFs.test.ts b/src/server/api/__tests__/previewFs.test.ts
index f8d47814..bcb53224 100644
--- a/src/server/api/__tests__/previewFs.test.ts
+++ b/src/server/api/__tests__/previewFs.test.ts
@@ -2,7 +2,7 @@ import { describe, expect, it } from 'bun:test'
 import { mkdtempSync, writeFileSync, mkdirSync } from 'node:fs'
 import { tmpdir } from 'node:os'
 import * as path from 'node:path'
-import { contentTypeForPath, handlePreviewFs } from '../previewFs'
+import { contentTypeForPath, handlePreviewFs, parseRange } from '../previewFs'
 
 describe('contentTypeForPath', () => {
   it('maps common web asset extensions', () => {
@@ -13,19 +13,62 @@ describe('contentTypeForPath', () => {
     expect(contentTypeForPath('/x/logo.svg')).toBe('image/svg+xml')
     expect(contentTypeForPath('/x/p.png')).toBe('image/png')
   })
+  it('maps video and audio extensions', () => {
+    expect(contentTypeForPath('/x/clip.mp4')).toBe('video/mp4')
+    expect(contentTypeForPath('/x/clip.webm')).toBe('video/webm')
+    expect(contentTypeForPath('/x/clip.mov')).toBe('video/quicktime')
+    expect(contentTypeForPath('/x/clip.m4v')).toBe('video/x-m4v')
+    expect(contentTypeForPath('/x/song.mp3')).toBe('audio/mpeg')
+    expect(contentTypeForPath('/x/song.wav')).toBe('audio/wav')
+    expect(contentTypeForPath('/x/song.ogg')).toBe('audio/ogg')
+  })
   it('falls back to octet-stream for unknown', () => {
     expect(contentTypeForPath('/x/file.bin')).toBe('application/octet-stream')
   })
 })
 
+// Deterministic 256-byte payload (bytes 0..255) so range slices are checkable.
+const VIDEO_BYTES = Uint8Array.from({ length: 256 }, (_, i) => i)
+
 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{}')
+  writeFileSync(path.join(root, 'clip.mp4'), VIDEO_BYTES)
   return root
 }
 
+describe('parseRange', () => {
+  it('returns null when no header', () => {
+    expect(parseRange(null, 100)).toBeNull()
+    expect(parseRange(undefined, 100)).toBeNull()
+  })
+  it('parses explicit closed range', () => {
+    expect(parseRange('bytes=0-99', 256)).toEqual({ start: 0, end: 99 })
+  })
+  it('parses open-ended range to EOF', () => {
+    expect(parseRange('bytes=100-', 256)).toEqual({ start: 100, end: 255 })
+  })
+  it('parses suffix range (last N bytes)', () => {
+    expect(parseRange('bytes=-10', 256)).toEqual({ start: 246, end: 255 })
+  })
+  it('clamps end past EOF', () => {
+    expect(parseRange('bytes=0-9999', 256)).toEqual({ start: 0, end: 255 })
+  })
+  it('reports unsatisfiable when start >= size', () => {
+    expect(parseRange('bytes=999999-', 256)).toBe('unsatisfiable')
+  })
+  it('reports unsatisfiable when start > end', () => {
+    expect(parseRange('bytes=50-10', 256)).toBe('unsatisfiable')
+  })
+  it('ignores malformed headers (falls back to full response)', () => {
+    expect(parseRange('bytes=abc', 256)).toBeNull()
+    expect(parseRange('items=0-9', 256)).toBeNull()
+    expect(parseRange('bytes=-', 256)).toBeNull()
+  })
+})
+
 describe('handlePreviewFs', () => {
   it('serves an in-workspace file with content-type', async () => {
     const root = setupWorkspace()
@@ -56,4 +99,73 @@ describe('handlePreviewFs', () => {
     const res = await handlePreviewFs(new URL('http://127.0.0.1/preview-fs/sX/index.html'), resolve)
     expect(res.status).toBe(404)
   })
+
+  it('serves .mp4 with video content-type and Accept-Ranges on a 200', async () => {
+    const root = setupWorkspace()
+    const resolve = async () => root
+    const res = await handlePreviewFs(new URL('http://127.0.0.1/preview-fs/s1/clip.mp4'), resolve)
+    expect(res.status).toBe(200)
+    expect(res.headers.get('content-type')).toBe('video/mp4')
+    expect(res.headers.get('accept-ranges')).toBe('bytes')
+    expect(res.headers.get('content-length')).toBe('256')
+    expect(new Uint8Array(await res.arrayBuffer())).toEqual(VIDEO_BYTES)
+  })
+
+  it('serves a closed byte-range as 206 with the requested bytes', async () => {
+    const root = setupWorkspace()
+    const resolve = async () => root
+    const res = await handlePreviewFs(
+      new URL('http://127.0.0.1/preview-fs/s1/clip.mp4'),
+      resolve,
+      new Headers({ Range: 'bytes=0-99' }),
+    )
+    expect(res.status).toBe(206)
+    expect(res.headers.get('content-range')).toBe('bytes 0-99/256')
+    expect(res.headers.get('content-length')).toBe('100')
+    expect(res.headers.get('accept-ranges')).toBe('bytes')
+    const body = new Uint8Array(await res.arrayBuffer())
+    expect(body.length).toBe(100)
+    expect(body).toEqual(VIDEO_BYTES.slice(0, 100))
+  })
+
+  it('serves an open-ended byte-range to EOF as 206', async () => {
+    const root = setupWorkspace()
+    const resolve = async () => root
+    const res = await handlePreviewFs(
+      new URL('http://127.0.0.1/preview-fs/s1/clip.mp4'),
+      resolve,
+      new Headers({ Range: 'bytes=100-' }),
+    )
+    expect(res.status).toBe(206)
+    expect(res.headers.get('content-range')).toBe('bytes 100-255/256')
+    expect(res.headers.get('content-length')).toBe('156')
+    const body = new Uint8Array(await res.arrayBuffer())
+    expect(body.length).toBe(156)
+    expect(body).toEqual(VIDEO_BYTES.slice(100))
+  })
+
+  it('returns 416 for an unsatisfiable range (start >= size)', async () => {
+    const root = setupWorkspace()
+    const resolve = async () => root
+    const res = await handlePreviewFs(
+      new URL('http://127.0.0.1/preview-fs/s1/clip.mp4'),
+      resolve,
+      new Headers({ Range: 'bytes=999999-' }),
+    )
+    expect(res.status).toBe(416)
+    expect(res.headers.get('content-range')).toBe('bytes */256')
+  })
+
+  it('returns the whole file (200) when no Range header is sent', async () => {
+    const root = setupWorkspace()
+    const resolve = async () => root
+    const res = await handlePreviewFs(
+      new URL('http://127.0.0.1/preview-fs/s1/clip.mp4'),
+      resolve,
+      new Headers(),
+    )
+    expect(res.status).toBe(200)
+    expect(res.headers.get('content-length')).toBe('256')
+    expect(new Uint8Array(await res.arrayBuffer())).toEqual(VIDEO_BYTES)
+  })
 })
diff --git a/src/server/api/previewFs.ts b/src/server/api/previewFs.ts
index 9d9dfe9a..b00ee60d 100644
--- a/src/server/api/previewFs.ts
+++ b/src/server/api/previewFs.ts
@@ -23,6 +23,15 @@ const CONTENT_TYPES: Record<string, string> = {
   woff2: 'font/woff2',
   txt: 'text/plain; charset=utf-8',
   md: 'text/plain; charset=utf-8',
+  // Video — served inline via <video> with HTTP byte-range streaming.
+  mp4: 'video/mp4',
+  webm: 'video/webm',
+  mov: 'video/quicktime',
+  m4v: 'video/x-m4v',
+  // Audio.
+  mp3: 'audio/mpeg',
+  wav: 'audio/wav',
+  ogg: 'audio/ogg',
 }
 
 export function contentTypeForPath(filePath: string): string {
@@ -33,7 +42,76 @@ export function contentTypeForPath(filePath: string): string {
 export type ResolveWorkDir = (sessionId: string) => Promise<string | null>
 
 const PREFIX = '/preview-fs/'
-const MAX_FILE_BYTES = 50 * 1024 * 1024
+
+/**
+ * Upper bound on what we'll serve. The old 50 MB cap existed because the file
+ * was buffered into memory via `readFileSync`; that would 413 real dubbed
+ * videos. We now STREAM every response through `Bun.file(...)` (including
+ * byte-ranges), so the in-memory pressure is gone and we can raise this a lot.
+ * We keep a generous-but-finite ceiling (2 GiB) purely as a sanity guard
+ * against pathological / runaway files — not as a memory limit.
+ */
+const MAX_FILE_BYTES = 2 * 1024 * 1024 * 1024
+
+export interface ParsedRange {
+  start: number
+  end: number
+}
+
+/**
+ * Parse a single HTTP `Range` header against a known file `size`.
+ *
+ * Supports the common single-range forms:
+ *   - `bytes=start-end`  (explicit closed range)
+ *   - `bytes=start-`     (open-ended → to EOF)
+ *   - `bytes=-N`         (suffix → last N bytes)
+ *
+ * Returns inclusive `{ start, end }` byte offsets clamped to `[0, size-1]`,
+ * `null` when the header is absent/unparseable (caller should fall back to a
+ * full 200 response), or `'unsatisfiable'` when the range cannot be satisfied
+ * (caller should reply 416). Multi-range requests (comma-separated) are not
+ * supported and fall back to a full response.
+ */
+export function parseRange(
+  rangeHeader: string | null | undefined,
+  size: number,
+): ParsedRange | null | 'unsatisfiable' {
+  if (!rangeHeader) return null
+
+  const match = /^bytes=(\d*)-(\d*)$/.exec(rangeHeader.trim())
+  if (!match) return null
+
+  const startRaw = match[1]
+  const endRaw = match[2]
+
+  // `bytes=-N` suffix form: the last N bytes.
+  if (startRaw === '') {
+    if (endRaw === '') return null // `bytes=-` is malformed → ignore.
+    const suffixLen = Number(endRaw)
+    if (!Number.isFinite(suffixLen) || suffixLen <= 0) return 'unsatisfiable'
+    if (size === 0) return 'unsatisfiable'
+    const start = Math.max(0, size - suffixLen)
+    return { start, end: size - 1 }
+  }
+
+  const start = Number(startRaw)
+  if (!Number.isFinite(start)) return null
+
+  // start beyond EOF is unsatisfiable.
+  if (start >= size) return 'unsatisfiable'
+
+  let end: number
+  if (endRaw === '') {
+    end = size - 1 // open-ended → EOF
+  } else {
+    end = Number(endRaw)
+    if (!Number.isFinite(end)) return null
+    if (end < start) return 'unsatisfiable'
+    end = Math.min(end, size - 1) // clamp to EOF
+  }
+
+  return { start, end }
+}
 
 /**
  * Serve a single file from a session's sandboxed workspace directory.
@@ -49,6 +127,7 @@ const MAX_FILE_BYTES = 50 * 1024 * 1024
 export async function handlePreviewFs(
   url: URL,
   resolveWorkDir: ResolveWorkDir,
+  reqHeaders?: Headers,
 ): Promise<Response> {
   if (!url.pathname.startsWith(PREFIX)) {
     return new Response('forbidden', { status: 403 })
@@ -79,12 +158,48 @@ export async function handlePreviewFs(
   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, {
+  const size = stat.size
+  const contentType = contentTypeForPath(target)
+  // Stream straight from disk via Bun.file — never buffer whole media into
+  // memory. Bun's file blob is an acceptable Response body.
+  const file = Bun.file(target)
+
+  const range = parseRange(reqHeaders?.get('range'), size)
+
+  if (range === 'unsatisfiable') {
+    return new Response('range not satisfiable', {
+      status: 416,
+      headers: {
+        'Content-Range': `bytes */${size}`,
+        'Accept-Ranges': 'bytes',
+        'Cache-Control': 'no-cache',
+      },
+    })
+  }
+
+  if (range) {
+    const { start, end } = range
+    // `slice(start, end + 1)` — Bun's slice end is exclusive, range end is
+    // inclusive — streams just the requested window.
+    return new Response(file.slice(start, end + 1), {
+      status: 206,
+      headers: {
+        'Content-Type': contentType,
+        'Content-Range': `bytes ${start}-${end}/${size}`,
+        'Accept-Ranges': 'bytes',
+        'Content-Length': String(end - start + 1),
+        'Cache-Control': 'no-cache',
+      },
+    })
+  }
+
+  // No (or unparseable) Range header → stream the whole file as 200.
+  return new Response(file, {
     status: 200,
     headers: {
-      'Content-Type': contentTypeForPath(target),
-      'Content-Length': String(stat.size),
+      'Content-Type': contentType,
+      'Content-Length': String(size),
+      'Accept-Ranges': 'bytes',
       'Cache-Control': 'no-cache',
     },
   })
diff --git a/src/server/index.ts b/src/server/index.ts
index b4d8a5f3..90d6866e 100644
--- a/src/server/index.ts
+++ b/src/server/index.ts
@@ -296,10 +296,13 @@ export function startServer(port = PORT, host = HOST) {
             }
           }
 
-          const response = await handlePreviewFs(url, async (sessionId) =>
-            conversationService.getSessionWorkDir(sessionId) ||
-            (await sessionService.getSessionWorkDir(sessionId)) ||
-            null,
+          const response = await handlePreviewFs(
+            url,
+            async (sessionId) =>
+              conversationService.getSessionWorkDir(sessionId) ||
+              (await sessionService.getSessionWorkDir(sessionId)) ||
+              null,
+            req.headers,
           )
           return withCors(response, cors)
         }