ruby-sdk/README.md

Model Context Protocol

A Ruby gem implementing Model Context Protocol + JSON RPC 2.0 spec

Overview

Model Context Protocol provides a standardized way to interact with language models through a JSON RPC 2.0 interface.

It handles request/response formatting, error handling, and tool integrations in a type-safe manner using Sorbet.

See https://spec.modelcontextprotocol.io/specification/

This implementation breaks from the transport examples in the spec as it implements a POST-based JSON RPC call rather than a stateful SSE transport.

MCP Server

The ModelContextProtocol::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

• Implements JSON-RPC 2.0 message handling
• Supports protocol initialization and capability negotiation
• Manages tool registration and invocation
• Provides type-safe request/response handling using Sorbet

Supported Methods

• initialize - Initializes the protocol and returns server capabilities
• ping - Simple health check that returns "pong"
• tools/list - Lists all registered tools and their schemas
• tools/call - Invokes a specific tool with provided arguments
• prompts/list - Lists all registered prompts and their schemas
• prompts/get - Retrieves a specific prompt by name

Usage

Rails Controller

Implement an ApplicationController which calls the Server#handle method, eg

module ModelContextProtocol
  class ApplicationController < ActionController::Base

    sig { void }
    def index
      server = ModelContextProtocol::Server.new(
        name: "my_server",
        tools: [SomeTool, AnotherTool],
        prompts: [MyPrompt],
        server_context: { user_id: current_user.id },
      )
      render(json: server.handle_json(request.body.read).to_h)
    end
  end
end

or, if you want/need to parse the json yourself you can do the following

module ModelContextProtocol
  class ApplicationController < ActionController::Base

    sig { void }
    def index
      server = ModelContextProtocol::Server.new(
        name: "my_server",
        tools: [SomeTool, AnotherTool],
        prompts: [MyPrompt],
        server_context: { user_id: current_user.id },
      )
      render(json: server.handle(JSON.parse(request.body.read)).to_h)
    end
  end
end

Stdio Transport

If you want to build a simple command-line application, you can use the stdio transport:

#!/usr/bin/env ruby
require "model_context_protocol"
require "model_context_protocol/transports/stdio"

# Create a simple tool
class ExampleTool < ModelContextProtocol::Tool
  description "A simple example tool that echoes back its arguments"
  input_schema type: "object",
    properties: {
      message: { type: "string" },
    },
    required: ["message"]

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

# Set up the server
server = ModelContextProtocol::Server.new(
  name: "example_server",
  tools: [ExampleTool],
)

# Create and start the transport
transport = ModelContextProtocol::Transports::StdioTransport.new(server)
transport.open

You can run this script and then type in requests to the server at the command line.

$ ./stdio_server.rb
{"jsonrpc":"2.0","id":"1","result":"pong"}
{"jsonrpc":"2.0","id":"2","result":["ExampleTool"]}
{"jsonrpc":"2.0","id":"3","result":["ExampleTool"]}

Testing without a transport

To see sample responses without setting up a client to hit the server, you can simply run

server = ModelContextProtocol::Server.new(
  name: "my_server",
  tools: [SomeTool, AnotherTool],
  prompts: [MyPrompt],
  server_context: { user_id: current_user.id },
)
request = {
  jsonrpc: "2.0",
  method: "tools/list",
  params: {},
  id: "1"
}
server.handle(request)

Configuration

The gem can be configured using the ModelContextProtocol.configure block:

ModelContextProtocol.configure do |config|
  config.exception_reporter = ->(exception, server_context) do
    # Your exception reporting logic here
    # For example with Bugsnag:
    Bugsnag.notify(exception) do |report|
      report.add_metadata(:model_context_protocol, server_context)
    end
  end

  config.instrumentation_callback = -> (data) { puts "Got instrumentation data #{data.inspect}" }
end

or by creating an explicit configuration and passing it into the server.
This is useful for systems where an application hosts more than one MCP server but
they might require different instrumentation callbacks.

configuration = ModelContextProtocol::Configuration.new
configuration.exception_reporter = ->(exception, server_context) do
  # Your exception reporting logic here
  # For example with Bugsnag:
  Bugsnag.notify(exception) do |report|
    report.add_metadata(:model_context_protocol, server_context)
  end
end

configuration.instrumentation_callback = -> (data) { puts "Got instrumentation data #{data.inspect}" }

server = ModelContextProtocol::Server.new(
  # ... all other options
  configuration:,
)

Server Protocol Version

The server's protocol version can be overridden using the protocol_version class method:

ModelContextProtocol::Server.protocol_version = "2024-11-05"

This will make all new server instances use the specified protocol version instead of the default version. The protocol version can be reset to the default by setting it to nil:

ModelContextProtocol::Server.protocol_version = nil

Be sure to check the MCP spec for the protocol version to understand the supported features for the version being set.

Exception Reporting

The exception reporter receives two arguments:

• exception: The Ruby exception object that was raised
• server_context: A hash containing contextual information about where the error occurred

The server_context hash includes:

• For tool calls: { tool_name: "name", arguments: { ... } }
• For general request handling: { request: { ... } }

When an exception occurs:

1. The exception is reported via the configured reporter
2. For tool calls, a generic error response is returned to the client: { error: "Internal error occurred", isError: true }
3. For other requests, the exception is re-raised after reporting

If no exception reporter is configured, a default no-op reporter is used that silently ignores exceptions.

Tools

MCP spec includes Tools which provide functionality to LLM apps.

This gem provides a ModelContextProtocol::Tool class that can be used to create tools in two ways:

1. As a class definition:

class MyTool < ModelContextProtocol::Tool
  description "This tool performs specific functionality..."
  input_schema [{ type: "text", text: "message" }]
  annotations(
    title: "My Tool",
    read_only_hint: true,
    destructive_hint: false,
    idempotent_hint: true,
    open_world_hint: false
  )

  input_schema type: 'object',
    properties: {
      message: { type: 'string' },
    },
    required: ['message']

  def self.call(message:, server_context:)
    Tool::Response.new([{ type: "text", text: "OK" }])
  end
end

tool = MyTool

2. By using the ModelContextProtocol::Tool.define method with a block:

tool = ModelContextProtocol::Tool.define(
  name: "my_tool",
  description: "This tool performs specific functionality...",
  annotations: {
    title: "My Tool",
    read_only_hint: true
  }
) do |args, server_context|
  Tool::Response.new([{ type: "text", text: "OK" }])
end

The server_context parameter is the server_context passed into the server and can be used to pass per request information, e.g. around authentication state.

Tool Annotations

Tools can include annotations that provide additional metadata about their behavior. The following annotations are supported:

• title: A human-readable title for the tool
• read_only_hint: Indicates if the tool only reads data (doesn't modify state)
• destructive_hint: Indicates if the tool performs destructive operations
• idempotent_hint: Indicates if the tool's operations are idempotent
• open_world_hint: Indicates if the tool operates in an open world context

Annotations can be set either through the class definition using the annotations class method or when defining a tool using the define method.

Prompts

MCP spec includes Prompts, which enable servers to define reusable prompt templates and workflows that clients can easily surface to users and LLMs.

The ModelContextProtocol::Prompt class provides two ways to create prompts:

1. As a class definition with metadata:

class MyPrompt < ModelContextProtocol::Prompt
  prompt_name "my_prompt"  # Optional - defaults to underscored class name
  description "This prompt performs specific functionality..."
  arguments [
    Prompt::Argument.new(
      name: "message",
      description: "Input message",
      required: true
    )
  ]

  class << self
    def template(args, server_context:)
      Prompt::Result.new(
        description: "Response description",
        messages: [
          Prompt::Message.new(
            role: "user",
            content: Content::Text.new("User message")
          ),
          Prompt::Message.new(
            role: "assistant",
            content: Content::Text.new(args["message"])
          )
        ]
      )
    end
  end
end

prompt = MyPrompt

2. Using the ModelContextProtocol::Prompt.define method:

prompt = ModelContextProtocol::Prompt.define(
  name: "my_prompt",
  description: "This prompt performs specific functionality...",
  arguments: [
    Prompt::Argument.new(
      name: "message",
      description: "Input message",
      required: true
    )
  ]
) do |args, server_context:|
  Prompt::Result.new(
    description: "Response description",
    messages: [
      Prompt::Message.new(
        role: "user",
        content: Content::Text.new("User message")
      ),
      Prompt::Message.new(
        role: "assistant",
        content: Content::Text.new(args["message"])
      )
    ]
  )
end

The server_context parameter is the server_context passed into the server and can be used to pass per request information, e.g. around authentication state or user preferences.

Key Components

• Prompt::Argument - Defines input parameters for the prompt template
• Prompt::Message - Represents a message in the conversation with a role and content
• Prompt::Result - The output of a prompt template containing description and messages
• Content::Text - Text content for messages

Usage

Register prompts with the MCP server:

server = ModelContextProtocol::Server.new(
  name: "my_server",
  prompts: [MyPrompt],
  server_context: { user_id: current_user.id },
)

The server will handle prompt listing and execution through the MCP protocol methods:

• prompts/list - Lists all registered prompts and their schemas
• prompts/get - Retrieves and executes a specific prompt with arguments

Instrumentation

The server allows registering a callback to receive information about instrumentation. To register a handler pass a proc/lambda to as instrumentation_callback into the server constructor.

ModelContextProtocol.configure do |config|
  config.instrumentation_callback = -> (data) { puts "Got instrumentation data #{data.inspect}" }
end

The data contains the following keys: method: the metod called, e.g. ping, tools/list, tools/call etc tool_name: the name of the tool called prompt_name: the name of the prompt called resource_uri: the uri of the resource called error: if looking up tools/prompts etc failed, e.g. tool_not_found duration: the duration of the call in seconds

tool_name, prompt_name and resource_uri are only populated if a matching handler is registered. This is to avoid potential issues with metric cardinality

Releases

TODO