MCP-Rails

Enhance Rails routing and parameter handling for LLM agents with MCP (Model Context Protocol) integration.

mcp-rails is a Ruby on Rails gem that builds on top of the mcp-rb library to seamlessly integrate MCP (Model Context Protocol) servers into your Rails application. It enhances Rails routes by allowing you to tag them with MCP-specific metadata and generates a valid Ruby MCP server (in tmp/mcp/server.rb) that LLM agents, such as Goose, can connect to. Additionally, it provides a powerful way to define and manage strong parameters in your controllers, which doubles as both MCP server configuration and Rails strong parameter enforcement.

This was inspired during the creation of Gaggle.

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Features

• Tagged Routes: Tag Rails routes with mcp: true or specific actions (e.g., mcp: [:index, :create]) to expose them to an MCP server.
• Automatic MCP Server Generation: Generates a Ruby MCP server in tmp/mcp/server.rb for LLM agents to interact with your application.
• Parameter Definition: Define permitted parameters in controllers with rich metadata (e.g., types, examples, required flags) that are used for both MCP server generation and Rails strong parameters.
• HTTP Bridge for LLM Agents: Generates a ruby based MCP server to interact with your application through HTTP requests, ensuring LLM agents follow the same paths as human users.
• Environment Variable Integration: Automatically includes specified environment variables in MCP tool calls.

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Installation

Add this line to your application’s Gemfile:

gem 'mcp-rails'

Then run:

bundle install

Ensure you also have the mcp-rb gem installed, as mcp-rails depends on it:

gem 'mcp-rb'

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Configuration

MCP-Rails can be configured in an initializer file. Create config/initializers/mcp_rails.rb:

MCP::Rails.configure do |config|
  # Server Configuration
  config.server_name = "my-app-server"      # Default: 'mcp-server'
  config.server_version = "2.0.0"           # Default: '1.0.0'
  
  # Output Configuration
  config.output_directory = Rails.root.join("tmp/mcp")  # Default: Rails.root.join('tmp', 'mcp')
  config.bypass_key_path = Rails.root.join("tmp/mcp/bypass_key.txt")  # Default: Rails.root.join('tmp', 'mcp', 'bypass_key.txt')
  
  # Environment Variables
  config.env_vars = ["API_KEY", "ORGANIZATION_ID"]  # Default: ['API_KEY', 'ORGANIZATION_ID']
  
  # Base URL Configuration
  config.base_url = "http://localhost:3000"  # Default: Uses action_mailer.default_url_options
end

If you are an engine developer, you can register your engine’s configuration with MCP-Rails:

MCP::Rails.configure do |config|
  config.register_engine(YourEngine, env_vars: ["YOUR_ENGINE_KEY"])
end

Environment Variables