Serve Sim

serve-sim

Drive an Apple Simulator (iOS, iPad, Apple Watch) from an agent using the serve-sim CLI. serve-sim captures simulator IOSurfaces through its in-process native addon, exposes HTTP or WebRTC video plus WebSocket input, and serves a React preview UI. This skill teaches an agent the exact CLI surface, the gesture JSON shape, the gotchas, and the recommended workflows.

When to use

• The user wants an agent to tap, swipe, drag, pinch, or send hardware buttons to a running Apple Simulator.
• The user wants to stream a simulator to a browser (local, LAN, or tunneled) for review or remote control.
• The user wants to inject a synthetic camera feed (file, webcam, or animated placeholder) into a specific app on the simulator.
• The user wants to toggle CoreAnimation debug overlays (off-screen rendering, blended layers, slow animations) for performance work.
• The user wants to simulate a memory warning or rotate the device programmatically.
• The user wants to read the simulator's accessibility tree to find UI elements without pixel hunting.
• The user wants to grant, revoke, or reset an app's privacy permissions — camera, photos, location, contacts, or push notifications.

When NOT to use

• Android emulators → use adb shell tooling.
• Building or installing an iOS app → use xcodebuild or xcrun simctl install.
• React Native in-app runtime debugging (Redux state, network inspection, component tree) → use rn-debugger tooling.
• Real iOS hardware devices → use xcrun devicectl or Xcode.

Prerequisites

Before any other action, verify the host satisfies these. If something is missing, tell the user exactly what to install — do not proceed.

A bundled helper script is available: scripts/check-prereqs.sh. Run it; if it exits non-zero, surface the message to the user.

A booted simulator is required for most subcommands. Check with xcrun simctl list devices booted. If none are booted, tell the user to open Xcode → Simulator or to run xcrun simctl boot <UDID>.

Mental model

┌───────────────┐  IOSurface  ┌────────────────────┐  HTTP/WebRTC  ┌─────────┐
│ iOS Simulator │ ──────────► │ serve-sim process  │ ────────────► │ Browser │
└───────────────┘              │ + native N-API     │   WebSocket   └─────────┘
                               └────────────────────┘
                                         ▲
                                  state files in
                                $TMPDIR/serve-sim/

Key invariants the agent must respect:

• All coordinates are normalized 0..1, with (0, 0) at top-left and (1, 1) at bottom-right of the display. Never pass pixel coordinates.
• One registered server per device. A server process may capture several simulators, and the capture API itself is non-exclusive.
• State lives in $TMPDIR/serve-sim/server-{udid}.json. Use serve-sim --list to query it; do not read the JSON directly unless you know what you are doing.
• The orientation set via rotate is remembered by the helper, and subsequent gestures are rotated client-side. An agent that sends raw coords after a rotation does not need to compensate manually.

Common operations

Most subcommands accept -d <udid|name> to target a specific device when several are booted.

Critical gotcha: prefer tap over gesture for taps

Each serve-sim gesture call opens its own WebSocket. If you issue two back-to-back gesture calls — one with {"type":"begin",...} and one with {"type":"end",...} — the simulator receives them with enough latency between them that the touch is interpreted as a long-press, not a tap. This is a deliberate constraint of the protocol, not a bug to work around.

Rule: for any single-shot tap, use serve-sim tap <x> <y>. Only use gesture for drags, swipes, or multi-step interactions where you must thread the same socket across begin → move × N → end.

Targeting a specific device

When multiple simulators are booted, every subcommand accepts -d <udid|name>. The name match is case-insensitive against the device name returned by xcrun simctl list devices booted. Examples:

npx @expo/serve-sim tap 0.5 0.5 -d "iPhone 16 Pro"
npx @expo/serve-sim button home -d ABC12345-...
npx @expo/serve-sim --list                                # show all running streams

If the user has only one booted simulator, omit -d entirely. The skill should prefer auto-detection over hard-coding device names.

Output modes

By default, serve-sim prints human-readable status to stdout. For agent loops, prefer JSON output:

npx @expo/serve-sim --list -q          # JSON object; one stream is top-level, many are under .streams
npx @expo/serve-sim --detach -q        # JSON with pid/port/url after spawn
npx @expo/serve-sim camera status -q   # JSON with {alive, source, mirror, ...}

Parse -q output programmatically. Never parse the non--q human output — it can change between versions.

Showing the stream in your agent's preview

When the user asks to "see the simulator here", "view it in preview", "open it in this tool", or similar, the goal is to stream the simulator into the same surface the user is chatting with. serve-sim returns a regular HTTP URL — the agent's job is to surface that URL and, if the host exposes a preview tool, hand it off.

Steps:

1. Start serve-sim and capture the URL:

   npx @expo/serve-sim --detach -q
   

   This returns JSON like {"pid":..., "port":3100, "url":"http://localhost:3100", "streamUrl":"http://localhost:3100/helper/<udid>/stream.mjpeg", ...}. The url field is the human-facing preview UI; streamUrl is the raw MJPEG endpoint.

2. Always surface the URL plainly in your response so the user can fallback to opening it manually in any browser.

3. Probe your host's preview tool and hand off the URL if one exists. Examples of tool names you may see in your toolset:

   • preview_start (Claude Code) — call it with { url: "<url>" }.
   • mcp__Claude_Preview__preview_start (some MCP setups).
   • A browser_open, open_url, or similar URL-opening tool — pass the URL.
   • Cursor / Codex CLI / others may not expose a preview tool to the agent. In that case, just print the URL and tell the user how to open it (their browser, their IDE's built-in browser pane, etc.).

4. Do not assume any specific preview tool exists. Inspect the tools available to you in the current session. If one matches the description above, use it. If none does, fall back to step 2 (print the URL prominently).

The stream stays alive until npx @expo/serve-sim --kill. Multiple clients (the host's preview + the user's browser + a tunnel) can read the same URL simultaneously.

See references/workflows.md workflow "Show the simulator stream in the host's preview" for the full recipe.

Workflows

For complete end-to-end recipes (UI automation, camera testing, accessibility-driven taps, deep-link flows, preview handoff), see references/workflows.md. The reference covers the patterns documented in serve-sim's own AGENTS.md.

Cleanup

Always stop helpers when finished, unless the user explicitly wants them to keep running:

npx @expo/serve-sim --kill            # stop all
npx @expo/serve-sim --kill "iPhone 16 Pro"  # stop one

Orphan servers occupy their recorded ports and prevent fresh starts.

Anti-patterns

• Do not pass pixel coordinates. All coords are normalized 0..1. If the user gives pixel values, divide by the screen dimensions reported by GET /config.
• Do not use gesture for plain taps. Use tap. See "Critical gotcha" above.
• Do not assume npx @expo/serve-sim is already running. Verify with --list or by checking $TMPDIR/serve-sim/server-{udid}.json. If absent, start it explicitly.
• Do not skip the prerequisites check on the first invocation in a session. Wrong macOS version, missing Xcode CLI tools, or Node <20 produce confusing errors downstream.
• Do not invent button names. Only these six are valid: home, swipe_home, app_switcher, lock, siri, side_button. See references/buttons-rotation.md for the source-of-truth list.
• Do not parse the non-quiet human output. Use -q for JSON.
• Do not leave camera helpers running across unrelated tasks. Stop them with npx @expo/serve-sim camera --stop-webcam when done.
• Do not guess coordinates when an accessibility lookup returns no match. If you fetched the AX tree (e.g. GET /ax) to find a target element and the query returned no result, fail loudly — tapping a guessed spot is almost always worse than reporting "target not found" back to the user. See references/workflows.md workflow 1 for the guard pattern.

Reference index

• references/gestures.md — exact gesture JSON shapes, edge values, multi-touch, drag/swipe recipes.
• references/buttons-rotation.md — the six valid buttons and the four orientations, with behavioral notes.
• references/camera.md — synthetic camera injection: placeholder, file, webcam, mirror modes, hot-swap.
• references/permissions.md — granting/revoking app privacy permissions, including push notifications.
• references/ca-debug.md — the five CoreAnimation debug flags and when each one helps.
• references/endpoints.md — HTTP and WebSocket endpoints for agents that bypass the CLI.
• references/workflows.md — end-to-end recipes for UI automation, camera testing, deep-link flows.