Skills
skills/aradotso/trending-skills/serve-sim-apple-simulator

serve-sim-apple-simulator

Installation
SKILL.md

serve-sim — Apple Simulator Hosting Tool

Skill by ara.so — Daily 2026 Skills collection.

serve-sim is the npx serve of Apple Simulators. It spawns a Swift helper that captures a booted iOS Simulator's framebuffer via simctl io, exposes it as an MJPEG stream + WebSocket control channel, and serves a React preview UI in your browser. Works with any booted iOS Simulator — no Xcode plugin, no app instrumentation required.

Requirements

  • macOS with Xcode command line tools installed (xcrun simctl must be available)
  • At least one booted iOS Simulator
  • Node.js / npm (for npx)

Quick Start

# Start preview server (auto-detects booted simulator)
npx serve-sim
# → Preview at http://localhost:3200

CLI Reference

serve-sim [device...]                 # Start preview server (default: localhost:3200)
serve-sim --no-preview [device...]    # Stream only, no web UI
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]
serve-sim ca-debug <option> <on|off> [-d udid]
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)

Common CLI Examples

# Target a specific device by name
serve-sim "iPhone 16 Pro"

# Start on a custom port
serve-sim -p 4000

# Start as background daemon, returns JSON with stream info
serve-sim --detach

# List all running streams
serve-sim --list

# Kill all running helpers
serve-sim --kill

# Send home button press
serve-sim button home -d <udid>

# Rotate simulator
serve-sim rotate landscape_left -d <udid>
# orientations: portrait | portrait_upside_down | landscape_left | landscape_right

# Toggle CoreAnimation debug flags
serve-sim ca-debug slow-animations on -d <udid>
# options: blended | copies | misaligned | offscreen | slow-animations

# Simulate memory warning
serve-sim memory-warning -d <udid>

# Multiple simulators at once
serve-sim "iPhone 16 Pro" "iPad Pro"

# Send a touch gesture (JSON format)
serve-sim gesture '{"type":"tap","x":200,"y":400}' -d <udid>

Features

  • 60 FPS MJPEG video stream in browser
  • Touch, swipe, pinch (hold Option key) gestures
  • Keyboard input and hotkeys forwarded to simulator (CMD+SHIFT+H = home)
  • Simulator logs forwarded to browser
  • Drag and drop images/videos onto simulator
  • Apple Watch, iPad, and iOS support
  • Connect-style middleware for embedding in existing dev servers

Architecture

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

State files are written to $TMPDIR/serve-sim/. The Swift binary is bundled in the npm package — no separate Xcode build needed at runtime.

Integration Patterns

Claude Code Desktop

Create .claude/launch.json in your project root:

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

Claude will automatically start the simulator preview when you open the project.

Expo / Metro Dev Server

Customize metro.config.js to embed serve-sim at http://localhost:8081/.sim:

// metro.config.js
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;

Then run npx expo start — simulator preview available at http://localhost:8081/.sim.

Express / Connect Dev Server

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

const app = express();

// First start the helper in daemon mode
// $ npx serve-sim --detach

app.use(simMiddleware({ basePath: "/.sim" }));

app.listen(3000, () => {
  console.log("Dev server at http://localhost:3000");
  console.log("Simulator preview at http://localhost:3000/.sim");
});

Vite Dev Server

// vite.config.ts
import { defineConfig } from "vite";
import { simMiddleware } from "serve-sim/middleware";

export default defineConfig({
  server: {
    middlewareMode: false,
  },
  plugins: [
    {
      name: "serve-sim",
      configureServer(server) {
        // Start helper first: npx serve-sim --detach
        server.middlewares.use("/.sim", simMiddleware({ basePath: "/.sim" }));
      },
    },
  ],
});

Next.js Custom Server

// server.ts
import { createServer } from "http";
import { parse } from "url";
import next from "next";
import { simMiddleware } from "serve-sim/middleware";

const dev = process.env.NODE_ENV !== "production";
const app = next({ dev });
const handle = app.getRequestHandler();
const simHandler = simMiddleware({ basePath: "/.sim" });

app.prepare().then(() => {
  createServer((req, res) => {
    const parsedUrl = parse(req.url!, true);
    if (parsedUrl.pathname?.startsWith("/.sim")) {
      return simHandler(req, res, () => handle(req, res, parsedUrl));
    }
    handle(req, res, parsedUrl);
  }).listen(3000);
});

Middleware API

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

// Mount options
simMiddleware({
  basePath: "/.sim",  // URL prefix for all serve-sim routes
});

// Middleware exposes:
// GET  /.sim        → Preview HTML UI
// GET  /.sim/api    → State JSON (connected devices, stream URLs)
// GET  /.sim/logs   → SSE log stream from simulator

The middleware reads state from $TMPDIR/serve-sim/ and proxies the browser to the live MJPEG + WebSocket endpoints. CORS is open on the helper, so no additional proxy config is needed.

Daemon / Detach Mode

Use --detach to start the helper as a background process and get JSON output for scripting:

# Start daemon and capture JSON output
STREAM_INFO=$(npx serve-sim --detach --quiet)
echo $STREAM_INFO
# {"udid":"...","mjpeg":"http://localhost:3100/stream","ws":"ws://localhost:3100/control"}

# List running streams as JSON
npx serve-sim --list --quiet

# Kill specific device stream
npx serve-sim --kill "iPhone 16 Pro"

# Kill all streams
npx serve-sim --kill

Programmatic Usage (TypeScript)

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

const app = connect();

// Attach middleware — requires serve-sim helper already running (--detach)
app.use(
  simMiddleware({
    basePath: "/.sim",
  })
);

export default app;

Gesture JSON Format

When using serve-sim gesture '<json>':

# Tap at coordinates
serve-sim gesture '{"type":"tap","x":200,"y":400}' -d <udid>

# Swipe gesture
serve-sim gesture '{"type":"swipe","startX":200,"startY":800,"endX":200,"endY":200,"duration":0.3}' -d <udid>

Development Setup

git clone https://github.com/EvanBacon/serve-sim
cd serve-sim
bun install

# Build JS bundles
bun run --filter serve-sim build

# Rebuild Swift helper binary
bun run --filter serve-sim build:swift

# Watch mode for development
bun run --filter serve-sim dev

Troubleshooting

No simulator detected

# Check booted simulators
xcrun simctl list devices booted

# Boot a simulator if none running
xcrun simctl boot "iPhone 16 Pro"

Port already in use

# Use a different port
serve-sim -p 4200

# Kill existing helpers first
serve-sim --kill

Stream not appearing in middleware

# Ensure helper is running first
serve-sim --detach

# Verify state files exist
ls $TMPDIR/serve-sim/

# Check running streams
serve-sim --list

Swift binary not found / won't execute

# Ensure Xcode CLT are installed
xcode-select --install

# Verify simctl works
xcrun simctl list

Multiple simulators — wrong device targeted

# List all booted simulators with UDIDs
xcrun simctl list devices booted

# Target by UDID explicitly
serve-sim -d <udid>

Rebuild Swift helper if binary is stale

bun run --filter serve-sim build:swift
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