MCP Ruby SDK 
The official Ruby SDK for Model Context Protocol servers and clients.
Detailed guides are available at https://ruby.sdk.modelcontextprotocol.io.
Features
- Build MCP servers that expose tools, prompts, and resources to any MCP host
- Build MCP clients that connect to any MCP server, with automatic lifecycle negotiation and OAuth 2.1 authorization
- Speak every standard transport: stdio and Streamable HTTP (including SSE), with a Rails integration
- Cover the full protocol surface: server-to-client requests, multi round-trip requests, notifications, progress, logging, cancellation, completions, and pagination
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.
Quick Start
The following minimal programs show both sides of the protocol: a server that exposes a single tool, and a client that spawns such a server and drives it over stdio.
MCP Server
A minimal server defines a tool and serves it over the stdio transport:
require "mcp"
# Create a simple tool
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
# Set up the server
server = MCP::Server.new(
name: "example_server",
tools: [ExampleTool],
)
# Create and start the transport
transport = MCP::Server::Transports::StdioTransport.new(server)
transport.open
Save the script as server.rb, run it, and send JSON-RPC requests via stdin:
$ ruby server.rb
{"jsonrpc":"2.0","id":"1","method":"ping"}
{"jsonrpc":"2.0","id":"2","method":"tools/list"}
{"jsonrpc":"2.0","id":"3","method":"tools/call","params":{"name":"example_tool","arguments":{"message":"Hello"}}}
The same server can also run over Streamable HTTP, including mounted inside a Rails application; see Server Transports.
MCP Client
A minimal client spawns a stdio server as a subprocess, connects, and lists and calls its tools:
stdio_transport = MCP::Client::Stdio.new(
command: "bundle",
args: ["exec", "ruby", "path/to/server.rb"],
env: { "API_KEY" => "my_secret_key" },
read_timeout: 30
)
client = MCP::Client.new(transport: stdio_transport)
# Perform the MCP initialization handshake before sending any requests.
client.connect
# List available tools.
tools = client.tools
tools.each do |tool|
puts "Tool: #{tool.name} - #{tool.description}"
end
# Call a specific tool.
response = client.call_tool(
tool: tools.first,
arguments: { message: "Hello, world!" }
)
# Close the transport when done.
stdio_transport.close
The same client can connect to Streamable HTTP servers with MCP::Client::HTTP;
see Client Transports.
Examples
Runnable examples are available in examples/,
including a complete Rails application in examples/rails.
Documentation
License
This project is licensed under the Apache License 2.0 for new contributions, with existing code under MIT. See the LICENSE file for details.