serve-sim

The npx serve of Apple Simulators.

Host your simulator for use with Agent tools like Codex, Cursor, or Claude Desktop — locally, over your LAN, or host on a remote mac and tunnel anywhere.

npx serve-sim
# → Preview at http://localhost:3200

https://github.com/user-attachments/assets/fbf890f4-c8c7-4684-82be-d677b8a188f8

serve-sim spawns a small Swift helper that captures the simulator's framebuffer via simctl io, exposes it as an MJPEG stream + WebSocket control channel, and serves a React preview UI on top. It works with any booted iOS Simulator — no Xcode plugin, no instrumentation in your app.

Features

• Full 60 FPS video stream in the browser.
• Swipe from the bottom to go home.
• gestures like pinch to zoom by holding the option key.
• Simulator logs are forwarded to the browser for browser-use MCP tools to read from.
• Drag and drop videos and images to add them to the simulator device.
• Keyboard commands and hot keys are forwarded to the simulator, including CMD+SHIFT+H to go home.
• Apple Watch, iPad, and iOS support.

Why?

Hosted simulators can be hard to test, serve-sim enables you to test the hosted infra locally first for faster iteration. When you're ready to host a simulator remotely, simply tunnel the served URL and users can interact with the simulator as if it were running locally on their device.

I develop the Expo framework, but this tool is completely agnostic to React Native and can be used for any iOS interaction you need.

Install

Requires macOS with Xcode command line tools (xcrun simctl) and a maintained Node.js LTS release (currently Node 20+). Older or end-of-life Node versions are not supported. bun is not required to run the CLI. Camera injection uses a host-side helper built for macOS 14+.

Note: Apple Silicon (arm64) only. The bundled serve-sim-bin helper ships as an arm64 binary and does not run on Intel (x86_64) Macs.

CLI

serve-sim [device...]                 Start preview server (default: localhost:3200)
serve-sim --no-preview [device...]    Stream in foreground without a preview server
serve-sim gesture '<json>' [-d udid]  Send a touch gesture
serve-sim button [name] [-d udid]     Send a button press (default: home)
serve-sim type <text> [-d udid]       Type text via the simulator keyboard
                                      (US keyboard only; also --stdin / --file <path>)
serve-sim rotate <orientation> [-d udid]
                                      portrait | portrait_upside_down |
                                      landscape_left | landscape_right
serve-sim ca-debug <option> <on|off> [-d udid]
                                      Toggle a CoreAnimation debug flag
                                      (blended|copies|misaligned|offscreen|slow-animations)
serve-sim memory-warning [-d udid]    Simulate a memory warning

serve-sim camera <bundle-id> [-d udid] [source-options]
                                      Inject a synthetic camera feed and (re)launch the app
serve-sim camera switch <placeholder|webcam|file> [arg] [-d udid]
                                      Hot-swap the running helper's source (no relaunch)
serve-sim camera mirror <auto|on|off> [-d udid]
                                      Hot-swap preview-layer mirror mode
serve-sim camera status [-d udid]     Print helper state as JSON ({alive, source, ...})
serve-sim camera --list-webcams       List host camera devices
serve-sim camera --stop-webcam [-d udid]
                                      Stop the camera helper for a device

Options:
  -p, --port <port>   Starting port (preview default: 3200; helper default: 3100)
  -d, --detach        Spawn helper and exit (daemon mode)
  -q, --quiet         JSON-only output
      --no-preview    Skip the web UI; stream in foreground only
      --codec <codec> Stream codec for the preview UI: 'auto' (H.264 when the
                      browser can decode it) or 'mjpeg' (force software JPEG —
                      e.g. on VMs without H.264 encode)
      --transport <http|webrtc>
                      Stream transport (default: http)
      --webrtc-codec <vp8|vp9|h264>
                      WebRTC video codec (default: h264)
      --list [device] List running streams
      --kill [device] Kill running stream(s)

Camera options (used with `serve-sim camera <bundle-id>`):
  -f, --file <path>          Image or video file (kind auto-detected from
                             extension/magic bytes; videos loop at native FPS)
      --webcam [name]        Live host webcam (defaults to the built-in
                             front camera when [name] is omitted)
      --mirror [on|off|auto] Override preview-layer mirroring (default: auto =
                             front mirrored, back not). Data-output buffers
                             are never auto-mirrored, matching AVF defaults.
      --no-mirror            Shortcut for --mirror off
      --build                Rebuild the dylib + helper from source

Examples

serve-sim                              # auto-detect booted sim, open preview
serve-sim "iPhone 16 Pro"              # target a specific device
serve-sim --detach                     # start a background helper, return JSON
serve-sim --list                       # show running streams
serve-sim --kill                       # stop all helpers

# Type text into the focused field
serve-sim type "Hello, world!"
echo "from stdin" | serve-sim type --stdin
serve-sim type --file ./snippet.txt

# Camera injection
serve-sim camera com.acme.MyApp                            # animated placeholder
serve-sim camera com.acme.MyApp --webcam                   # default webcam
serve-sim camera com.acme.MyApp --webcam "MacBook Pro Camera"
serve-sim camera com.acme.MyApp --file ~/Pictures/face.png # static image
serve-sim camera com.acme.MyApp --file ~/Movies/loop.mp4   # looping video

# Hot-swap source on a running helper (no app relaunch)
serve-sim camera switch placeholder
serve-sim camera switch webcam
serve-sim camera switch ~/Movies/loop.mp4                  # auto-detects file kind

# Other helpers
serve-sim camera mirror on
serve-sim camera status                                    # JSON: alive, source, mirror
serve-sim camera --list-webcams
serve-sim camera --stop-webcam

Multiple booted simulators are supported — pass several device names, or leave it empty to attach to all of them.

Camera

serve-sim camera <bundle-id> replaces the simulator's camera feed for a single app. A small host-side helper writes BGRA frames into a POSIX shared-memory region; an injected dylib (DYLD_INSERT_LIBRARIES) swizzles AVFoundation inside the simulator process so the app reads from that region instead of the simulator's stub camera.

The helper is one-per-device and outlives any single app launch, so multiple apps on the same simulator can share the feed — just run serve-sim camera <other-bundle-id> again to relaunch the next app with the dylib attached. Source changes (camera switch) and mirror changes (camera mirror) flow through the helper's control socket and don't relaunch the app.

Sources:

• placeholder — animated programmatic frames (default).
• file — image (PNG/JPEG/HEIC/…) or video (mp4/mov/m4v/webm/…). The CLI sniffs the kind from the extension and falls back to magic bytes for files without an extension.
• webcam — live AVCaptureDevice (built-in, Continuity, external).

Connectors

serve-sim can be used with dev servers, browser, and AI editors for more seamless integration.

Agent Skill

An Agent Skill ships in skills/serve-sim — it teaches AI coding agents (Claude Code, Cursor, Codex CLI, Gemini CLI, and any host implementing the open Agent Skills standard) how to drive a simulator through the CLI: taps, gestures, hardware buttons, rotation, camera injection, and handing the stream off to the host's preview pane.

bunx add-skill EvanBacon/serve-sim
# in Claude Code:
/plugin marketplace add EvanBacon/serve-sim

See skills/serve-sim/README.md for the full capability list.

Claude Code Desktop

Create a .claude/launch.json and define a server:

{
  "version": "0.0.1",
  "configurations": [
    {
      "name": "Apple",
      "runtimeExecutable": "npx",
      "runtimeArgs": ["serve-sim"],
      "port": 3200
    }
  ]
}

Expo

Automatically start the serve-sim process with npx expo start and access the URL at http://localhost:8081/.sim.

First, customize the metro.config.js file (bunx expo customize):

// Learn more https://docs.expo.io/guides/customizing-metro
const { getDefaultConfig } = require("expo/metro-config");
const connect = require("connect");
const { simMiddleware } = require("serve-sim/middleware");

/** @type {import('expo/metro-config').MetroConfig} */
const config = getDefaultConfig(__dirname);

config.server = config.server || {};
const originalEnhanceMiddleware = config.server.enhanceMiddleware;
config.server.enhanceMiddleware = (metroMiddleware, server) => {
  const middleware = originalEnhanceMiddleware
    ? originalEnhanceMiddleware(metroMiddleware, server)
    : metroMiddleware;
  const app = connect();
  app.use(simMiddleware({ basePath: "/.sim" }));
  app.use(middleware);
  return app;
};

module.exports = config;

Embed in your dev server

serve-sim/middleware is a Connect-style middleware that mounts the same preview UI inside your existing dev server (Metro, Vite, Next, plain Express, etc.). Run serve-sim --detach once to start the streaming helper, then add the middleware:

import { simMiddleware } from "serve-sim/middleware";

app.use(simMiddleware({ basePath: "/.sim" }));
// → preview HTML at /.sim
// → state JSON  at /.sim/api

The middleware reads the helper's state from $TMPDIR/serve-sim/ and points the browser at the helper's stream, interaction WebSocket, and WebKit DevTools endpoints. By default those URLs target the helper's own port directly (CORS is wide-open on the helper), so a plain app.use(...) mount works without touching your server's WebSocket handling.

Single-port / remote proxying

To expose the preview to remote viewers behind a single port (the way standalone serve-sim does), pass proxyHelpers: true. The browser then reaches the stream, control socket, and DevTools through same-origin /.sim/helper/<device> and /.sim/devtools URLs, so the per-device helper port and inspect-webkit bridge can stay local to the host. This routes WebSockets through the middleware, so you must forward your server's upgrade events to handleUpgrade:

const middleware = simMiddleware({ basePath: "/.sim", proxyHelpers: true });
app.use(middleware);

const server = app.listen(3000);
server.on("upgrade", (req, socket, head) => middleware.handleUpgrade(req, socket, head));

If you enable proxyHelpers but don't wire upgrade, the page still loads video over HTTP but loses simulator input and DevTools (their sockets never reach the proxy). When terminating TLS at a reverse proxy, forward X-Forwarded-Proto so the helper URLs use https/wss and avoid mixed-content blocks.

How it works

┌──────────────┐   simctl io   ┌─────────────────┐  MJPEG / WS  ┌─────────┐
│ iOS Simulator│ ────────────► │ serve-sim-bin   │ ───────────► │ Browser │
└──────────────┘   (Swift)     │ (per-device)    │              └─────────┘
                               └─────────────────┘
                                       ▲
                                  state file in
                                $TMPDIR/serve-sim/
                                       ▲
                               ┌──────────────────┐
                               │ serve-sim CLI /  │
                               │ middleware       │
                               └──────────────────┘

The Swift helper (bin/serve-sim-bin) is a tiny standalone binary — no Xcode dependency at runtime. The CLI embeds it via bun build --compile, so installing the npm package is enough.

Development

bun install
bun run --filter serve-sim build         # build the JS bundles
bun run --filter serve-sim build:swift   # rebuild the Swift helper
bun run --filter serve-sim dev           # watch mode

License

Apache-2.0