MCP Ruby SDK is the official Ruby framework for building Model Context Protocol servers and clients. It provides a complete implementation of the MCP specification for handling JSON-RPC 2.0 communications between AI assistants and external services. Key capabilities: - Core `MCP::Server` class that manages protocol initialization, capability negotiation, and message handling - Full support for stdio and Streamable HTTP transports (including Server-Sent Events) - Tool registration and invocation with schema validation - Prompt management for templated interactions - Resource registration and retrieval with URI template support - Notifications for real-time updates when tools, prompts, or resources change - Custom method definitions beyond standard MCP protocol methods - Built-in instrumentation callbacks for monitoring and observability Install via RubyGems (`gem 'mcp'`) to add MCP server capabilities to Ruby applications.

MCP Ruby SDK [](https://rubygems.org/gems/mcp) [](https://github.com/modelcontextprotocol/ruby-sdk/blob/main/LICENSE) [](https://github.com/modelcontextprotocol/ruby-sdk/actions/workflows/ci.yml)

The official Ruby SDK for Model Context Protocol servers and clients.

Installation

Add this line to your application's Gemfile:

gem 'mcp'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install mcp

You may need to add additional dependencies depending on which features you wish to access.

Building an MCP Server

The MCP::Server class is the core component that handles JSON-RPC requests and responses. It implements the Model Context Protocol specification, handling model context requests and responses.

Key Features

Supported Methods

Custom Methods

The server allows you to define custom JSON-RPC methods beyond the standard MCP protocol methods using the define_custom_method method:

server = MCP::Server.new(name: "my_server")
Define a custom method that returns a result
server.define_custom_method(method_name: "add") do |params|
  params[:a] + params[:b]
end
Define a custom notification method (returns nil)
server.define_custom_method(method_name: "notify") do |params|
  # Process notification
  nil
end

Key Features:

Usage Example:

# Client request
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "add",
  "params": { "a": 5, "b": 3 }
}
Server response
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": 8
}

Error Handling:

Notifications

The server supports sending notifications to clients when lists of tools, prompts, or resources change. This enables real-time updates without polling.

#### Notification Methods

The server provides the following notification methods:

#### Notification Format

Notifications follow the JSON-RPC 2.0 specification and use these method names:

Progress

The MCP Ruby SDK supports progress tracking for long-running tool operations, following the MCP Progress specification.

#### How Progress Works

1. Client Request: The client sends a progressToken in the _meta field when calling a tool 2. Server Notification: The server sends notifications/progress messages back to the client during tool execution 3. Tool Integration: Tools call server_context.report_progress to report incremental progress

#### Server-Side: Tool with Progress

Tools that accept a server_context: parameter can call report_progress on it. The server automatically wraps the context in an MCP::ServerContext instance that provides this method:

```ruby class LongRunningTool < MCP::Tool description "A tool that reports progress during execution"