Fast MCP 🚀

🌟 Interface your Servers with LLMs in minutes

AI models are powerful, but they need to interact with your applications to be truly useful. Traditional approaches mean wrestling with:

• 🔄 Complex communication protocols and custom JSON formats
• 🔌 Integration challenges with different model providers
• 🧩 Compatibility issues between your app and AI tools
• 🧠 Managing the state between AI interactions and your data

Fast MCP solves all these problems by providing a clean, Ruby-focused implementation of the Model Context Protocol, making AI integration a joy, not a chore.

✨ Features

• 🛠️ Tools API - Let AI models call your Ruby functions securely, with in-depth argument validation through Dry-Schema.
• 📚 Resources API - Share data between your app and AI models
• 🔄 Multiple Transports - Choose from STDIO, HTTP, or SSE based on your needs
• 🧩 Framework Integration - Works seamlessly with Rails, Sinatra or any Rack app.
• 🔒 Authentication Support - Secure your AI-powered endpoints with ease
• 🚀 Real-time Updates - Subscribe to changes for interactive applications
• 🎯 Dynamic Filtering - Control tool/resource access based on request context (permissions, API versions, etc.)

💎 What Makes FastMCP Great

# Define tools for AI models to use
server = FastMcp::Server.new(name: 'popular-users', version: '1.0.0')

# Define a tool by inheriting from FastMcp::Tool
class CreateUserTool < FastMcp::Tool
  description "Create a user"
    # These arguments will generate the needed JSON to be presented to the MCP Client
    # And they will be validated at run time.
    # The validation is based off Dry-Schema, with the addition of the description.
  arguments do
    required(:first_name).filled(:string).description("First name of the user")
    optional(:age).filled(:integer).description("Age of the user")
    required(:address).description("The shipping address").hash do
      required(:street).filled(:string).description("Street address")
      optional(:city).filled(:string).description("City name")
      optional(:zipcode).maybe(:string).description("Postal code")
    end
  end

  def call(first_name:, age: nil, address: {})
    User.create!(first_name:, age:, address:)
  end
end

# Register the tool with the server
server.register_tool(CreateUserTool)

# Share data resources with AI models by inheriting from FastMcp::Resource
class PopularUsers < FastMcp::Resource
  uri "myapp:///users/popular"
  resource_name "Popular Users"
  mime_type "application/json"

  def content
    JSON.generate(User.popular.limit(5).as_json)
  end
end

class User < FastMcp::Resource
  uri "myapp:///users/{id}" # This is a resource template
  resource_name "user"
  mime_type "application/json"

  def content
    id = params[:id] # params are computed from the uri pattern

    JSON.generate(User.find(id).as_json)
  end
end

# Register the resource with the server
server.register_resources(PopularUsers, User)

# Accessing the resource through the server
server.read_resource(PopularUsers.uri)

# Notify the resource content has been updated to clients
server.notify_resource_updated(PopularUsers.variabilized_uri)

# Notifiy the content of a resource from a template has been updated to clients
server.notify_resource_updated(User.variabilized_uri(id: 1))

🎯 Dynamic Tool Filtering

Control which tools and resources are available based on request context:

# Tag your tools for easy filtering
class AdminTool < FastMcp::Tool
  tags :admin, :dangerous
  description "Perform admin operations"

  def call
    # Admin only functionality
  end
end

# Filter tools based on user permissions
server.filter_tools do |request, tools|
  user_role = request.params['role']

  case user_role
  when 'admin'
    tools # Admins see all tools
  when 'user'
    tools.reject { |t| t.tags.include?(:admin) }
  else
    tools.select { |t| t.tags.include?(:public) }
  end
end

🚂 Fast Ruby on Rails implementation

bundle add fast-mcp
bin/rails generate fast_mcp:install

This will add a configurable fast_mcp.rb initializer

require 'fast_mcp'

FastMcp.mount_in_rails(
  Rails.application,
  name: Rails.application.class.module_parent_name.underscore.dasherize,
  version: '1.0.0',
  path_prefix: '/mcp', # This is the default path prefix
  messages_route: 'messages', # This is the default route for the messages endpoint
  sse_route: 'sse', # This is the default route for the SSE endpoint
  # Add allowed origins below, it defaults to Rails.application.config.hosts
  # allowed_origins: ['localhost', '127.0.0.1', 'example.com', /.*\.example\.com/],
  # localhost_only: true, # Set to false to allow connections from other hosts
  # whitelist specific ips to if you want to run on localhost and allow connections from other IPs
  # allowed_ips: ['127.0.0.1', '::1']
  # authenticate: true,       # Uncomment to enable authentication
  # auth_token: 'your-token' # Required if authenticate: true
) do |server|
  Rails.application.config.after_initialize do
    # FastMcp will automatically discover and register:
    # - All classes that inherit from ApplicationTool (which uses ActionTool::Base)
    # - All classes that inherit from ApplicationResource (which uses ActionResource::Base)
    server.register_tools(*ApplicationTool.descendants)
    server.register_resources(*ApplicationResource.descendants)
    # alternatively, you can register tools and resources manually:
    # server.register_tool(MyTool)
    # server.register_resource(MyResource)
  end
end

The install script will also:

• add app/resources folder
• add app/tools folder
• add app/tools/sample_tool.rb
• add app/resources/sample_resource.rb
• add ApplicationTool to inherit from
• add ApplicationResource to inherit from as well

Rails-friendly class naming conventions

For Rails applications, FastMCP provides Rails-style class names to better fit with Rails conventions:

• ActionTool::Base - An alias for FastMcp::Tool
• ActionResource::Base - An alias for FastMcp::Resource

These are automatically set up in Rails applications. You can use either naming convention in your code:

# Using Rails-style naming:
class MyTool < ActionTool::Base
  description "My awesome tool"

  arguments do
    required(:input).filled(:string)
  end

  def call(input:)
    # Your implementation
  end
end

# Using standard FastMcp naming:
class AnotherTool < FastMcp::Tool
  # Both styles work interchangeably in Rails apps
end

When creating new tools or resources, the generators will use the Rails naming convention by default:

# app/tools/application_tool.rb
class ApplicationTool < ActionTool::Base
  # Base methods for all tools
end

# app/resources/application_resource.rb
class ApplicationResource < ActionResource::Base
  # Base methods for all resources
end

Easy Sinatra setup

I'll let you check out the dedicated sinatra integration docs.

🚀 Quick Start

Create a Server with Tools and Resources and STDIO transport

require 'fast_mcp'

# Create an MCP server
server = FastMcp::Server.new(name: 'my-ai-server', version: '1.0.0')

# Define a tool by inheriting from FastMcp::Tool
class SummarizeTool < FastMcp::Tool
  description "Summarize a given text"

  arguments do
    required(:text).filled(:string).description("Text to summarize")
    optional(:max_length).filled(:integer).description("Maximum length of summary")
  end

  def call(text:, max_length: 100)
    # Your summarization logic here
    text.split('.').first(3).join('.') + '...'
  end
end

# Register the tool with the server
server.register_tool(SummarizeTool)

# Create a resource by inheriting from FastMcp::Resource
class StatisticsResource < FastMcp::Resource
  uri "data/statistics"
  resource_name "Usage Statistics"
  description "Current system statistics"
  mime_type "application/json"

  def content
    JSON.generate({
      users_online: 120,
      queries_per_minute: 250,
      popular_topics: ["Ruby", "AI", "WebDev"]
    })
  end
end

# Register the resource with the server
server.register_resource(StatisticsResource)

# Start the server
server.start

🧪 Testing with the inspector

MCP has developed a very useful inspector. You can use it to validate your implementation. I suggest you use the examples I provided with this project as an easy boilerplate. Clone this project, then give it a go !

npx @modelcontextprotocol/inspector examples/server_with_stdio_transport.rb

Or to test with an SSE transport using a rack middleware:

npx @modelcontextprotocol/inspector examples/rack_middleware.rb

Or to test over SSE with an authenticated rack middleware:

npx @modelcontextprotocol/inspector examples/authenticated_rack_middleware.rb

You can test your custom implementation with the official MCP inspector by using:

# Test with a stdio transport:
npx @modelcontextprotocol/inspector path/to/your_ruby_file.rb

# Test with an HTTP / SSE server. In the UI select SSE and input your address.
npx @modelcontextprotocol/inspector

Sinatra

# app.rb
require 'sinatra'
require 'fast_mcp'

use FastMcp::RackMiddleware.new(name: 'my-ai-server', version: '1.0.0') do |server|
  # Register tools and resources here
  server.register_tool(SummarizeTool)
end

get '/' do
  'Hello World!'
end

Integrating with Claude Desktop

Add your server to your Claude Desktop configuration at:

• macOS: ~/Library/Application Support/Claude/claude_desktop_config.json
• Windows: %APPDATA%\Claude\claude_desktop_config.json

{
  "mcpServers": {
    "my-great-server": {
      "command": "ruby",
      "args": ["/Users/path/to/your/awesome/fast-mcp/server.rb"]
    }
  }
}

How to add a MCP server to Claude, Cursor, or other MCP clients?

Please refer to configuring_mcp_clients

📊 Supported Specifications

Feature	Status
✅ JSON-RPC 2.0	Full implementation for communication
✅ Tool Definition & Calling	Define and call tools with rich argument types
✅ Resource & Resource Templates Management	Create, read, update, and subscribe to resources
✅ Transport Options	STDIO, HTTP, and SSE for flexible integration
✅ Framework Integration	Rails, Sinatra, Hanami, and any Rack-compatible framework
✅ Authentication	Secure your AI endpoints with token authentication
✅ Schema Support	Full JSON Schema for tool arguments with validation

🗺️ Use Cases

• 🤖 AI-powered Applications: Connect LLMs to your Ruby app's functionality
• 📊 Real-time Dashboards: Build dashboards with live AI-generated insights
• 🔗 Microservice Communication: Use MCP as a clean protocol between services
• 📚 Interactive Documentation: Create AI-enhanced API documentation
• 💬 Chatbots and Assistants: Build AI assistants with access to your app's data

🔒 Security Features

Fast MCP includes built-in security features to protect your applications:

DNS Rebinding Protection

The HTTP/SSE transport validates the Origin header on all incoming connections to prevent DNS rebinding attacks, which could allow malicious websites to interact with local MCP servers.

# Configure allowed origins (defaults to ['localhost', '127.0.0.1'])
FastMcp.rack_middleware(app,
  allowed_origins: ['localhost', '127.0.0.1', 'your-domain.com', /.*\.your-domain\.com/],
  localhost_only: false,
  allowed_ips: ['192.168.1.1', '10.0.0.1'],
  # other options...
)

Authentication

Fast MCP supports token-based authentication for all connections:

# Enable authentication
FastMcp.authenticated_rack_middleware(app,
  auth_token: 'your-secret-token',
  # other options...
)

📖 Documentation

• 🚀 Getting Started Guide
• 🧩 Integration Guide
• 🛤️ Rails Integration
• 🌐 Sinatra Integration
• 📚 Resources
• 🛠️ Tools
• 🔒 Security
• 🎯 Dynamic Filtering

💻 Examples

Check out the examples directory for more detailed examples:

• 🔨 Basic Examples:

  • Simple Server
  • Tool Examples

• 🌐 Web Integration:

  • Rack Middleware
  • Authenticated Endpoints

🧪 Requirements

• Ruby 3.2+

👥 Contributing

We welcome contributions to Fast MCP! Here's how you can help:

1. Fork the repository
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

Please read our Contributing Guide for more details.

📄 License

This project is available as open source under the terms of the MIT License.

🙏 Acknowledgments

• The Model Context Protocol team for creating the specification
• The Dry-Schema team for the argument validation.
• All contributors to this project