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).

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 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

Options:
  -p, --port <port>   Starting port (preview default: 3200, stream 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
      --list [device] List running streams
      --kill [device] Kill running stream(s)

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

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

Connectors

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

Claude Code Desktop

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

{
  "version": "0.0.1",
  "configurations": [
    {
      "name": "ios",
      "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
// → SSE logs    at /.sim/logs

The middleware reads the helper's state from $TMPDIR/serve-sim/ and forwards the user's browser to the live MJPEG + WebSocket endpoints. CORS is wide-open on the helper, so the page renders without a proxy.

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