XC-MCP

An MCP server that lets AI agents build, run and drive iOS apps — without drowning in Xcode's output.

XC-MCP wraps xcodebuild, simctl and idb as 77 tools. Its job is not just to expose those commands but to make their output fit in a model's context: summaries first, full detail on demand, and the accessibility tree instead of screenshots wherever the app allows it.

Why it exists

Xcode tooling produces output at a scale no context window survives. Measured against a real app (Grapla, Xcode 27, iPhone 17 Pro simulator):

A single raw build log would overflow a 200k-token context on its own. The full output is never thrown away: it is cached and returned as an MCP resource link, so an agent can drill into the exact errors it needs.

Quick start

Claude Code:

claude mcp add xc-mcp -- npx -y xc-mcp

Codex CLI:

codex mcp add xc-mcp -- npx -y xc-mcp

or add it to ~/.codex/config.toml directly:

[mcp_servers.xc-mcp]
command = "npx"
args = ["-y", "xc-mcp"]

Claude Desktop (~/Library/Application Support/Claude/claude_desktop_config.json):

{
  "mcpServers": {
    "xc-mcp": { "command": "npx", "args": ["-y", "xc-mcp"] }
  }
}

Then ask your agent to build and run your app. rtfm({}) lists the tool categories.

Requirements

xcode-select --install

# idb — required for all UI automation
brew tap facebook/fb
brew install facebook/fb/idb-companion facebook/fb/idb-cli

brew install idb-companion no longer works — it moved from Homebrew core to Meta's facebook/fb tap.

Run idb-doctor whenever UI automation misbehaves. It checks the CLI, the companion version, the Xcode framework layout, and stale companion registrations.

Xcode 27

idb-companion must be 1.5.1 or newer. Xcode 27 moved SimulatorKit.framework, and older companions fail silently: the accessibility tree reads correctly and every tap reports success, while the simulator discards the events. idb-ui-tap, idb-ui-gesture and idb-ui-input detect this and refuse to run rather than pretend.

brew upgrade facebook/fb/idb-companion
brew list --versions idb-companion   # expect >= 1.5.1

• There is no Simulator.app — Xcode 27 replaced it with DeviceHub.app. simctl-boot with openGui opens whichever this Xcode ships.
• If every idb call fails with Connection refused, a dead companion is still registered in /tmp/idb/state. Run idb disconnect <udid>; idb-doctor detects this.

How it works

Summaries first, detail on demand

Tools that produce large output return a summary plus an ID. The full output stays in a 30-minute cache and is exposed two ways: an MCP resource at xcmcp://response/{id}, and the matching *-get-details tool for clients without resource support.

xcodebuild-build({ projectPath: "./MyApp.xcodeproj", scheme: "MyApp" })
// → { buildId, success: false, errorCount: 3, warningCount: 1, durationMs: 18000 }

xcodebuild-get-details({ buildId, detailType: "errors-only" })
// → the three errors, not the 419k-token log

The accessibility tree before screenshots

For UI automation, reading the accessibility tree is cheaper and faster than sending an image, and it gives exact coordinates instead of visual estimates. Measured on the same screen:

The first idb call in a session takes several seconds while the target cache warms.

This only works when the app is accessible — which is the point. An app VoiceOver can navigate is an app an agent can navigate. accessibility-quality-check says which path to take; accessibility-audit says what to fix.

idb-ui-find-element({ query: "Sign In" })
// → { centerX: 201, centerY: 572, visible: true }

idb-ui-tap({ x: 201, y: 572 })

Coordinates are not always tappable. The tree reports frames in scrolled-content space, so an element can sit below the fold. Every match carries visible and, when off-screen, an offscreenReason naming the gesture that brings it into view.

Tools that declare their risk

Every tool carries MCP annotations — readOnlyHint, destructiveHint, idempotentHint — so a client can ask before running simctl-erase, idb-crash-delete or idb-clear-keychain, and run simctl-list freely. Build, test, audit and crash tools also declare an outputSchema and return validated structuredContent.

Tools

77 tools. TOOL_SIGNATURES.md indexes every tool with its annotations; parameters come from the server itself via rtfm({ toolName: "..." }).

Configuration

Flags combine: ["-y", "xc-mcp", "--mini", "--build-only"] — the same args array works in Claude Desktop and Codex's config.toml. Both matter mainly for clients that load every tool description upfront; see the history below.

How context cost shaped this server

XC-MCP has been rebuilt three times, and each rebuild answered the same question — what does a tool cost in context? — differently, because the answer kept changing underneath it. The tool count went 51 → 28 → 77, and each time the right number was set by what clients and the protocol could do.

v1 · Aug 2025 — 51 tools, every description paid upfront

MCP clients of the time loaded every tool's name, description and input schema when they connected, and kept them in context for the whole session. Tool count was a tax on context, paid before the agent did any work.

v1.3.2 cut the tax per tool: descriptions shrank to a few words and the real documentation moved behind rtfm, fetched only when asked for. A separate, smaller xc-mini-mcp package was also published — and reverted shortly after.

v2 · Nov 2025 — 28 tools, consolidated into routers

If the count is the tax, cut the count. v2 folded 21 tools into six operation-enum routers — simctl-device({ operation: "boot" }) instead of simctl-boot — sharing one schema per router.

That bought a debt nobody could see yet. A router is a single tool, so it carries a single description and a single set of properties for every operation behind it. That cost nothing while tools had no per-tool properties worth losing.

v3 · Nov 2025 — betting on deferred loading

v3 set defer_loading: true on every tool, expecting tools to load on demand and baseline cost to fall toward zero. The goal was right; the mechanism could not work.

defer_loading is a field of the Messages API. It belongs on tool definitions sent to the model, alongside the tool-search server tools, and an MCP server has no way to set it. The MCP SDK also drops unrecognised keys from a tool's registration, so the flag never reached a client — a live tools/list later showed 0 of 77 tools carrying it.

Deferral did arrive, on the client side. Clients with tool search, Claude Code among them, began listing only tool names at startup and fetching a tool's schema when it is first used. No server flag was involved.

v4 · Jun 2026 — 70 tools, routers dissolved

Two developments reversed the v2 trade-off.

• Client-side tool search made tool count cheap. With schemas deferred, 77 tools cost roughly what their names cost — about 1,240 tokens at startup, server instructions included.
• The protocol gained per-tool metadata. Revision 2025-03-26 added tool annotations (#185). Revision 2025-06-18 added structured tool output (#371), resource links in tool results (#603), and a human-readable title (#663).

Annotations are precisely what a router cannot express. simctl-device hid both boot, which is harmless, and erase, which destroys a simulator, behind one tool — so no client could tell them apart. The routers were dissolved into discrete tools that each declare their own risk and output shape, and resource links replaced ad-hoc cache IDs as the way to return large output. The server negotiates protocol 2025-06-18.

v4.1 · Sep 2026 — 71 tools, Xcode 27 readiness

Xcode 27 broke idb in a way that failed silently: older companions read the accessibility tree correctly but dropped every tap while reporting success. v4.1 added idb-doctor, a 1.5.1 companion floor, and a preflight that makes HID-writing tools refuse to run rather than pretend.

v4.2 · Sep 2026 — 77 tools, tested against a real app

Driving a real app end-to-end found four bugs that had passed 1,456 tests, all in parsing idb's output. The tests mocked a format idb never emits — newline-delimited objects with label fields — where idb really returns one JSON array using AXLabel. So accessibility-quality-check rated every screen unusable and idb-ui-find-element never matched anything, each while returning well-formed, plausible JSON. Fixtures now use captured idb output, and reintroducing one of those bugs fails ten tests.

The same pass added crash reporting, element visibility and test-isolation tools, and removed the inert defer_loading flag.

The trade-off, version by version

Two lessons carried forward. Context cost is the client's call — a server should describe its tools honestly and let the client decide how to spend context on them. And a structured response can hide a broken tool — test against the real command, not a mock of it.

Release detail is in CHANGELOG.md.

Migrating from v2/v3 routers

Drop the operation field and call the matching tool. Operation-specific parameters are unchanged.

idb-targets keeps its operation enum. rtfm({ toolName: "simctl-device" }) still suggests the replacements.

Development

git clone https://github.com/conorluddy/xc-mcp.git
cd xc-mcp && npm install
npm run build     # compile to dist/
npm test          # jest
npm run lint      # eslint

Point any MCP client at a local build with node /path/to/xc-mcp/dist/index.js. A running server holds dist/ in memory, so restart the client session after rebuilding.

CLAUDE.md covers the architecture, how to add a tool, and the rules learned the hard way.

Contributing

[!WARNING] Contributions are welcome but reviewed slowly — this repo sits well down my priority list. Forking and adapting it is usually the faster route.

Pull requests need a passing build, tests and lint.

License

MIT, as declared in package.json.