hammerspoon

hammerspoon-mcp

Give AI agents deep control over macOS — windows, audio, Bluetooth, Spaces, Focus mode, and 200+ OS APIs — through one MCP server.

What is this?

An MCP server that bridges AI agents to Hammerspoon — the most powerful macOS automation tool available. Unlike screenshot-and-click tools, this gives agents programmatic OS-level access: manage windows by ID, switch audio devices by name, move apps between Spaces, toggle Focus mode, and subscribe to real-time OS events.

75 tools. 12 subscribable resources. Zero native dependencies.

Quick Start

# 1. Install Hammerspoon (if you haven't)
brew install --cask hammerspoon

# 2. Enable the IPC module (one-time setup)
#    Add this line to ~/.hammerspoon/init.lua:
#    require("hs.ipc")
#    Then reload your Hammerspoon config (Cmd+Alt+Ctrl+R or restart the app)

# 3. (Optional) Install the hs CLI symlink for convenience:
#    Run in Hammerspoon console: hs.ipc.cliInstall()
#    Or manually: ln -sf /Applications/Hammerspoon.app/Contents/Frameworks/hs/hs ~/.local/bin/hs

# 4. Run the MCP server
npx hammerspoon-mcp

Note: The server auto-discovers the hs binary — it checks ~/.local/bin/hs, /usr/local/bin/hs, /opt/homebrew/bin/hs, and the app bundle path. You don't need hs on your PATH if Hammerspoon is installed to /Applications.

Architecture

┌─────────────────────────────────────────────────────────┐
│  MCP Client (Claude, Cursor, MeshClaw, etc.)            │
└──────────────────────────┬──────────────────────────────┘
                           │ stdio (JSON-RPC)
┌──────────────────────────▼──────────────────────────────┐
│  hammerspoon-mcp (Node.js)                              │
│  ├─ tools → execSync("hs -c '...'")  (request/response)│
│  └─ watchers → named pipe reader     (push events)     │
└──────────────────────────┬──────────────────────────────┘
                           │ hs CLI / named pipe
┌──────────────────────────▼──────────────────────────────┐
│  Hammerspoon.app (Lua runtime + ObjC bridge)            │
│  200+ modules: window, screen, spaces, audio, wifi,     │
│  bluetooth, battery, USB, filesystem, accessibility...  │
└──────────────────────────┬──────────────────────────────┘
                           │
              macOS APIs (CoreGraphics, IOKit, etc.)

MCP Client Configuration

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

claude mcp add hammerspoon -- npx hammerspoon-mcp

{
  "servers": {
    "hammerspoon": {
      "command": "npx",
      "args": ["hammerspoon-mcp"]
    }
  }
}

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

Tools

🪟 Window Management (7)

🖥️ Spaces / Virtual Desktops (9)

📱 Application Control (3)

⌨️ Input (3)

🖼️ Screen (3)

🔊 Audio Devices (6)

🔵 Bluetooth (5)

Requires blueutil: brew install blueutil

🌙 Focus / Do Not Disturb (6)

🔋 System State (6)

🔆 Display (1)

⏱️ Timers (3)

📁 Finder / Files (4)

🛠️ Utilities (5)

👁️ Watchers — Live OS Event Streams (13)

🧪 Escape Hatch (1)

This gives access to all 200+ Hammerspoon modules not covered by dedicated tools — USB details, serial ports, network interfaces, Canvas drawing, menubar items, etc.

Resources (Subscriptions)

MCP clients can subscribe to resource URIs and receive push notifications when OS events fire:

How event streaming works

Agent calls: hs_watch_wifi
  → Server injects Lua watcher into Hammerspoon runtime
  → macOS WiFi change fires
  → Watcher writes JSON to named pipe (/tmp/hs-mcp-events.pipe)
  → Node reader emits event
  → Server calls sendResourceUpdated("watcher://wifi")
  → Client reads resource → gets { ssid: "NewNetwork", ... }

Examples

"Tile my editor and terminal side by side"

hs_window_layout({ preset: "side-by-side" })

"Switch my audio to AirPods"

hs_audio_default({ output: "airpods" })
→ "set default output: AirPods Pro"

"Move Slack to Space 3"

hs_list_windows({ app: "Slack" })  → { id: 4523, ... }
hs_spaces_move_window({ windowId: 4523, spaceIndex: 3 })

"Start a 25-minute focus session"

hs_focus_schedule({ minutes: 25 })
hs_timer_start({ name: "pomodoro", minutes: 25, message: "Break time!" })

"Notify me when I switch away from my IDE"

hs_watch_apps()
→ Events stream to watcher://apps whenever focus changes

"Connect my headphones and set volume to 40%"

hs_bluetooth_connect({ device: "WH-1000XM5" })
hs_audio_volume({ device: "WH-1000XM5", volume: 40 })

Compared to Alternatives

Prerequisites

The hs CLI symlink (hs.ipc.cliInstall()) is optional — the server auto-discovers the binary inside the Hammerspoon app bundle.

Verify setup

# If hs is on your PATH:
hs -c 'return "ok"'

# Or directly via the app bundle:
/Applications/Hammerspoon.app/Contents/Frameworks/hs/hs -c 'return "ok"'

# Should print: ok

If this works, npx hammerspoon-mcp will connect successfully.

Permissions

Project Structure

hammerspoon-mcp/
├── src/
│   ├── index.ts              MCP server entry point + resource registration
│   ├── bridge.ts             hs CLI bridge (auto-discovers binary, eval, evalJson)
│   ├── tools.ts              Core tools: window, app, input, screen, system, eval (23)
│   ├── watcher-tools.ts      Watcher management tools (13)
│   ├── watchers.ts           Named pipe event bridge
│   ├── spaces-tools.ts       Spaces / virtual desktop tools (9)
│   ├── audio-tools.ts        Audio device management (6)
│   ├── focus-tools.ts        DND / Focus mode (6)
│   ├── bluetooth-tools.ts    Bluetooth via blueutil (5)
│   └── extra-tools.ts        Brightness, timers, system info, Finder, utils (13)
├── package.json
├── tsconfig.json
├── LICENSE                   MIT
└── README.md

Development

git clone <repo>
cd hammerspoon-mcp
npm install
npm run dev     # Run with tsx (hot reload)
npm run build   # Compile to dist/
npm start       # Run compiled version

How It Works

1. Startup — the bridge auto-discovers the hs binary (checks ~/.local/bin, /usr/local/bin, /opt/homebrew/bin, app bundle) and validates connectivity
2. Tools execute synchronously via hs -c '<lua>' — typically 30–80ms per call. Stdout noise (-- Loading extension: ...) is stripped automatically
3. Watchers inject Lua code that writes JSON lines to a FIFO pipe; a Node reader process emits them as MCP resource updates
4. Resources buffer the last 50 events per watcher; clients read on-demand after receiving update notifications
5. Graceful shutdown stops all watchers and cleans up the pipe on SIGINT/SIGTERM

Known Limitations

• Node.js 22+ is specified in engines but the server builds and runs fine on Node 20 (tested)
• hs_spaces_goto, addSpaceToScreen, removeSpace briefly flash Mission Control (macOS limitation — enable "Reduce motion" to minimize)
• Bluetooth tools require blueutil (brew install blueutil)
• Focus/DND reading uses heuristics (no public Apple API exists)
• hs_screenshot returns base64 — large for full-screen captures
• Saved layouts persist only in Hammerspoon's runtime (lost on HS reload)
• Named pipe path is /tmp/hs-mcp-events.pipe — only one server instance at a time

License

MIT © 2026 Mohammed Babur