Introduction

The official Ruby SDK for the Model Context Protocol (MCP), implementing both server and client functionality for JSON-RPC 2.0 based communication between LLM applications and context providers.

Key features:

JSON-RPC 2.0 message handling with protocol initialization and capability negotiation
Tool, prompt, and resource registration and invocation
Stdio and Streamable HTTP (including SSE) transports
Client support for communicating with MCP servers
Notifications, sampling, progress tracking, and completions

Quick Start

Here is a minimal MCP server using the stdio transport:

require "mcp"

class ExampleTool < MCP::Tool
  description "A simple example tool that echoes back its arguments"
  input_schema(
    properties: {
      message: { type: "string" },
    },
    required: ["message"]
  )

  class << self
    def call(message:, server_context:)
      MCP::Tool::Response.new([{
        type: "text",
        text: "Hello from example tool! Message: #{message}",
      }])
    end
  end
end

server = MCP::Server.new(
  name: "example_server",
  tools: [ExampleTool],
)

transport = MCP::Server::Transports::StdioTransport.new(server)
transport.open

Run the script and send JSON-RPC requests via stdin:

$ ruby server.rb
{"jsonrpc":"2.0","id":"1","method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{},"clientInfo":{"name":"example","version":"0.1.0"}}}
{"jsonrpc":"2.0","id":"2","method":"tools/list"}
{"jsonrpc":"2.0","id":"3","method":"tools/call","params":{"name":"example_tool","arguments":{"message":"Hello"}}}

For comprehensive documentation, see the full README.

API Documentation

Full API reference is hosted on RubyDoc.info. Select a version to view:

License

This project is transitioning from the MIT License to the Apache License 2.0. See LICENSE for details.