
fast-mcp
A Ruby Implementation of the Model Context Protocol
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Fast MCP is a Ruby gem that simplifies the integration of AI models with your Ruby applications. It provides a clean implementation of the Model Context Protocol, eliminating complex communication protocols, integration challenges, and compatibility issues. With Fast MCP, you can easily connect AI models to your servers, share data resources, choose from multiple transports, integrate with frameworks like Rails and Sinatra, and secure your AI-powered endpoints. The gem also offers real-time updates and authentication support, making AI integration a seamless experience for developers.
README:
No complex protocols, no integration headaches, no compatibility issues β just beautiful, expressive Ruby code.
AI models are powerful, but they need to interact with your applications to be truly useful. Traditional approaches mean wrestling with:
- π Complex communication protocols and custom JSON formats
- π Integration challenges with different model providers
- π§© Compatibility issues between your app and AI tools
- π§ 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.
- π οΈ Tools API - Let AI models call your Ruby functions securely, with in-depth argument validation through Dry-Schema.
- π Resources API - Share data between your app and AI models
- π Multiple Transports - Choose from STDIO, HTTP, or SSE based on your needs
- π§© Framework Integration - Works seamlessly with Rails, Sinatra or any Rack app.
- π Authentication Support - Secure your AI-powered endpoints with ease
- π Real-time Updates - Subscribe to changes for interactive applications
- π― Dynamic Filtering - Control tool/resource access based on request context (permissions, API versions, etc.)
# 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))
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
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
For Rails applications, FastMCP provides Rails-style class names to better fit with Rails conventions:
-
ActionTool::Base
- An alias forFastMcp::Tool
-
ActionResource::Base
- An alias forFastMcp::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
I'll let you check out the dedicated sinatra integration docs.
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
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
# 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
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"]
}
}
}
Please refer to configuring_mcp_clients
Feature | Status |
---|---|
β JSON-RPC 2.0 | Full implementation for communication |
β Tool Definition & Calling | Define and call tools with rich argument types |
β Resource & Resource Templates Management | Create, read, update, and subscribe to resources |
β Transport Options | STDIO, HTTP, and SSE for flexible integration |
β Framework Integration | Rails, Sinatra, Hanami, and any Rack-compatible framework |
β Authentication | Secure your AI endpoints with token authentication |
β Schema Support | Full JSON Schema for tool arguments with validation |
- π€ AI-powered Applications: Connect LLMs to your Ruby app's functionality
- π Real-time Dashboards: Build dashboards with live AI-generated insights
- π Microservice Communication: Use MCP as a clean protocol between services
- π Interactive Documentation: Create AI-enhanced API documentation
- π¬ Chatbots and Assistants: Build AI assistants with access to your app's data
Fast MCP includes built-in security features to protect your applications:
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...
)
Fast MCP supports token-based authentication for all connections:
# Enable authentication
FastMcp.authenticated_rack_middleware(app,
auth_token: 'your-secret-token',
# other options...
)
- π Getting Started Guide
- π§© Integration Guide
- π€οΈ Rails Integration
- π Sinatra Integration
- π Resources
- π οΈ Tools
- π Security
- π― Dynamic Filtering
Check out the examples directory for more detailed examples:
-
π¨ Basic Examples:
-
π Web Integration:
- Ruby 3.2+
We welcome contributions to Fast MCP! Here's how you can help:
- Fork the repository
- Create your feature branch (
git checkout -b my-new-feature
) - Commit your changes (
git commit -am 'Add some feature'
) - Push to the branch (
git push origin my-new-feature
) - Create a new Pull Request
Please read our Contributing Guide for more details.
This project is available as open source under the terms of the MIT License.
- The Model Context Protocol team for creating the specification
- The Dry-Schema team for the argument validation.
- All contributors to this project
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