First install the following browser extensions:

• Tampermonkey
• Tampermonkey Editors for Firefox or Chrome

Then install tampermonkey-mcp globally:

npm install -g tampermonkey-mcp@latest

Finally configure your AI assistant to use the tampermonkey tool:

{
  "mcpServers": {
    "tampermonkey": {
      "command": "npx",
      "args": [
        "-y",
        "tampermonkey-mcp@latest"
      ]
    }
  }
}

Your assistant will call tampermonkey_get_connection_code and return a connection code that you can enter in the Tampermonkey Editors extension.

Claude Code configuration example

npm install -g claude

// either use your local checkout
npm run watch
claude mcp add --transport http --scope project tampermonkey http://localhost:4001/mcp

// or the npm package
npm install -g tampermonkey-mcp@latest
claude mcp add --transport stdio --scope project tampermonkey -- npx -y tampermonkey-mcp

// more browser control needed?
// Open chrome://inspect/#remote-debugging
// [x] Allow remote debugging for this browser instance
// or
chromium --user-data-dir=/tmp/chrome-devtools-mcp-test-profile --remote-debugging-port=9222

npm install -g chrome-devtools-mcp@latest
// install Tampermonkey and Tampermonkey Editors
claude mcp add --transport stdio --scope project chrome-devtools-mcp -- npx -y chrome-devtools-mcp@latest --slim --no-usage-statistics --auto-connect --browser-url http://127.0.0.1:9222

Build

npm install
npm run all

Tampermonkey MCP Server Architecture

Overview

This project implements an MCP (Model Context Protocol) server that allows AI assistants to read and modify Tampermonkey userscripts via the Tampermonkey Editors browser extension.

Architecture Components

┌─────────────────────────────────────────────────────────────────────────────┐
│                              AI Assistant                                   │
└─────────────────────────────────────────────────────────────────────────────┘
                                   │
                                   │ MCP Protocol (stdio transport)
                                   ▼
┌─────────────────────────────────────────────────────────────────────────────┐
│                         tampermonkey_mcp (MCP Server)                       │
│                                                                             │
│  ┌───────────────────────────────────────────────────────────────────────┐  │
│  │  server.ts                    │  Main entry point                     │  │
│  │                               │  - HTTP or stdio transport            │  │
│  │                               │  - MCP middleware                     │  │
│  └───────────────────────────────────────────────────────────────────────┘  │
│                                  │                                          │
│                                  ▼                                          │
│  ┌───────────────────────────────────────────────────────────────────────┐  │
│  │  tampermonkey.ts              │  MCP Tool Definitions                 │  │
│  │                               │  - tampermonkey_get_connection_code   │  │
│  │                               │  - tampermonkey_list                  │  │
│  │                               │  - tampermonkey_get                   │  │
│  │                               │  - tampermonkey_patch                 │  │
│  └───────────────────────────────────────────────────────────────────────┘  │
│                                  │                                          │
│                                  ▼                                          │
│  ┌───────────────────────────────────────────────────────────────────────┐  │
│  │  tampermonkey-ws-client.ts    │  WebSocket Server                     │  │
│  │                               │  - Connection code generation         │  │
│  │                               │  - Authentication handshake           │  │
│  │                               │  - Message relay to extension         │  │
│  └───────────────────────────────────────────────────────────────────────┘  │
└─────────────────────────────────────────────────────────────────────────────┘
                                   │
                                   │ WebSocket (local, port derived from code)
                                   ▼
┌─────────────────────────────────────────────────────────────────────────────┐
│                   Tampermonkey Editors Extension                            │
│                                                                             │
│  ┌───────────────────────────────────────────────────────────────────────┐  │
│  │  Service Worker               │  Receives connection code,            │  │
│  │                               │  establishes WebSocket connection     │  │
│  └───────────────────────────────────────────────────────────────────────┘  │
│                                  │                                          │
│                                  │ Chrome Extension Messaging               │
│                                  ▼                                          │
│  ┌───────────────────────────────────────────────────────────────────────┐  │
│  │  Content Script               │  Bridges extension ↔ Tampermonkey     │  │
│  └───────────────────────────────────────────────────────────────────────┘  │
│                                  │                                          │
│                                  ▼                                          │
│  ┌───────────────────────────────────────────────────────────────────────┐  │
│  │  Tampermonkey                 │  Userscript storage & execution       │  │
│  └───────────────────────────────────────────────────────────────────────┘  │
└─────────────────────────────────────────────────────────────────────────────┘

Connection Flow

1. MCP Server starts - Creates WebSocket server on a random available port
2. Connection code generated - Format: <base32(port - 1024)><auth_token><echo_token>
3. User calls tampermonkey_get_connection_code - Gets the connection code
4. User enters code in Tampermonkey Editors - Extension parses code to get port and auth
5. Extension connects to MCP server - WebSocket handshake with authentication
6. MCP tools become available - list, get, patch can now be called

Authentication Protocol

Extension                              MCP Server
    │                                      │
    │───── auth (token) ──────────────────>│
    │                                      │
    │<──── auth (echo_token) ──────────────│
    │                                      │
    │───── auth (echo_token) ─────────────>│
    │                                      │
    │<──── authOK ─────────────────────────│
    │                                      │
    │<───── pings (every 15s) ─────────────│  (keep-alive)
    │───── pongs ─────────────────────────>│
    │                                      │
    │<───── commands (list/get/patch) ─────│
    │───── responses ─────────────────────>│

MCP Tools

tampermonkey_get_connection_code

• Purpose: Initialize connection by getting the WebSocket connection code.
• Required: Must be called first to start the WebSocket server.
• Output: Connection code to enter in Tampermonkey Editors.

tampermonkey_list

• Purpose: List all available userscripts.
• Input: Optional filter by name pattern or include URL pattern.
• Output: Array of userscript metadata (name, namespace, path, requires).

tampermonkey_get

• Purpose: Get content of a specific userscript.
• Input: path (from list), optional ifNotModifiedSince timestamp.
• Output: Script content, lastModified timestamp, or error.

tampermonkey_patch

• Purpose: Update userscript content.
• Input: path, value (new content), optional lastModified for optimistic locking.
• Output: Success confirmation or error.

tampermonkey_put

• Purpose: Create a new userscript with the given name and namespace.
• Input: value (new script), optional lastModified.
• Output: Success confirmation, path of created script, or error.
• Requires: Tampermonkey Editors 1.0.6+ and Tampermonkey 5.6+

tampermonkey_delete

• Purpose: Delete an existing userscript.
• Input: path (from list).
• Output: Success confirmation or error.
• Requires: Tampermonkey Editors 1.0.6+ and Tampermonkey 5.6+

Basic testing

MCP Server capabilities

curl -X POST http://localhost:4001/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
      "protocolVersion": "2024-11-05",
      "clientInfo": {
        "name": "curl-client",
        "version": "1.0.0"
      },
      "capabilities": {}
    }
  }'

test stdio

echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","clientInfo":{"name":"test-client","version":"1.0.0"},"capabilities":{}}}' | npx -y tampermonkey-mcp
echo '{"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}' | npx -y tampermonkey-mcp
echo '{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"tampermonkey_list","arguments":{"pattern":""}}}' | npx -y tampermonkey-mcp