MCP Ruby SDK

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

• Implements JSON-RPC 2.0 message handling
• Supports protocol initialization and capability negotiation
• Manages tool registration and invocation
• Supports prompt registration and execution
• Supports resource registration and retrieval
• Supports stdio & Streamable HTTP (including SSE) transports
• Supports notifications for list changes (tools, prompts, resources)

Supported Methods

• initialize - Initializes the protocol and returns server capabilities
• ping - Simple health check
• 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
• resources/list - Lists all registered resources and their schemas
• resources/read - Retrieves a specific resource by name
• resources/templates/list - Lists all registered resource templates and their schemas

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:

• Accepts any method name as a string
• Block receives the request parameters as a hash
• Can handle both regular methods (with responses) and notifications
• Prevents overriding existing MCP protocol methods
• Supports instrumentation callbacks for monitoring

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:

• Raises MCP::Server::MethodAlreadyDefinedError if trying to override an existing method
• Supports the same exception reporting and instrumentation as standard methods

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 three notification methods:

• notify_tools_list_changed - Send a notification when the tools list changes
• notify_prompts_list_changed - Send a notification when the prompts list changes
• notify_resources_list_changed - Send a notification when the resources list changes

Notification Format

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

• notifications/tools/list_changed
• notifications/prompts/list_changed
• notifications/resources/list_changed

Transport Support

• stdio: Notifications are sent as JSON-RPC 2.0 messages to stdout
• Streamable HTTP: Notifications are sent as JSON-RPC 2.0 messages over HTTP with streaming (chunked transfer or SSE)
• Stateless Streamable HTTP: Notifications are not supported and all calls are request/response interactions; allows for easy multi-node deployment.

Usage Example

server = MCP::Server.new(name: "my_server")

# Default Streamable HTTP - session oriented
transport = MCP::Server::Transports::StreamableHTTPTransport.new(server)

# OR Stateless Streamable HTTP - session-less
transport = MCP::Server::Transports::StreamableHTTPTransport.new(server, stateless: true)

server.transport = transport

# When tools change, notify clients
server.define_tool(name: "new_tool") { |**args| { result: "ok" } }
server.notify_tools_list_changed

Unsupported Features ( to be implemented in future versions )

• Log Level
• Resource subscriptions
• Completions

Usage

Rails Controller

When added to a Rails controller on a route that handles POST requests, your server will be compliant with non-streaming

Streamable HTTP transport

requests.

You can use the Server#handle_json method to handle requests.

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

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

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

Server Context and Configuration Block Data

server_context

The server_context is a user-defined hash that is passed into the server instance and made available to tools, prompts, and exception/instrumentation callbacks. It can be used to provide contextual information such as authentication state, user IDs, or request-specific data.

Type:

server_context: { [String, Symbol] => Any }

Example:

server = MCP::Server.new(
  name: "my_server",
  server_context: { user_id: current_user.id, request_id: request.uuid }
)

This hash is then passed as the server_context argument to tool and prompt calls, and is included in exception and instrumentation callbacks.

Configuration Block Data

##### Exception Reporter

The exception reporter receives:

• exception: The Ruby exception object that was raised
• server_context: The context hash provided to the server

Signature:

exception_reporter = ->(exception, server_context) { ... }

##### Instrumentation Callback

The instrumentation callback receives a hash with the following possible keys:

• method: (String) The protocol method called (e.g., "ping", "tools/list")
• tool_name: (String, optional) The name of the tool called
• prompt_name: (String, optional) The name of the prompt called
• resource_uri: (String, optional) The URI of the resource called
• error: (String, optional) Error code if a lookup failed
• duration: (Float) Duration of the call in seconds

Type:

instrumentation_callback = ->(data) { ... }
# where data is a Hash with keys as described above

Example:

config.instrumentation_callback = ->(data) {
  puts "Instrumentation: #{data.inspect}"
}

Server Protocol Version

The server's protocol version can be overridden using the protocol_version keyword argument:

configuration = MCP::Configuration.new(protocol_version: "2024-11-05")
MCP::Server.new(name: "test_server", configuration: configuration)

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:

MCP::Configuration.new(protocol_version: nil)

If an invalid protocol_version value is set, an ArgumentError is raised.

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 MCP::Tool class that can be used to create tools in three ways:

1. As a class definition:

class MyTool  [!NOTE]
> This **Tool Annotations** feature is supported starting from `protocol_version: '2025-03-26'`.

### Tool Output Schemas

Tools can optionally define an `output_schema` to specify the expected structure of their results. This works similarly to how `input_schema` is defined and can be used in three ways:

1. **Class definition with output_schema:**

class WeatherTool (data) {

puts "Got instrumentation data #{data.inspect}"

}

end

The data contains the following keys:

- `method`: the method 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

### Resources

MCP spec includes [Resources](https://modelcontextprotocol.io/specification/2025-06-18/server/resources).

### Reading Resources

The `MCP::Resource` class provides a way to register resources with the server.

resource = MCP::Resource.new(

uri: "https://example.com/my_resource",

name: "my-resource",

title: "My Resource", # WARNING: This is a Draft and is not supported in the Version 2025-06-18 (latest) specification.

description: "Lorem ipsum dolor sit amet",

mime_type: "text/html",

)

server = MCP::Server.new(

name: "my_server",

resources: [resource],

)

The server must register a handler for the `resources/read` method to retrieve a resource dynamically.

server.resources_read_handler do |params|

[{

uri: params[:uri],

mimeType: "text/plain",

text: "Hello from example resource! URI: #{params[:uri]}"

}]

end

otherwise `resources/read` requests will be a no-op.

### Resource Templates

The `MCP::ResourceTemplate` class provides a way to register resource templates with the server.

resource_template = MCP::ResourceTemplate.new(

uri_template: "https://example.com/my_resource_template",

name: "my-resource-template",

title: "My Resource Template", # WARNING: This is a Draft and is not supported in the Version 2025-06-18 (latest) specification.

description: "Lorem ipsum dolor sit amet",

mime_type: "text/html",

)

server = MCP::Server.new(

name: "my_server",

resource_templates: [resource_template],

)

## Building an MCP Client

The `MCP::Client` class provides an interface for interacting with MCP servers.

This class supports:

- Tool listing via the `tools/list` method (`MCP::Client#tools`)
- Tool invocation via the `tools/call` method (`MCP::Client#call_tools`)
- Resource listing via the `resources/list` method (`MCP::Client#resources`)
- Resource reading via the `resources/read` method (`MCP::Client#read_resources`)
- Prompt listing via the `prompts/list` method (`MCP::Client#prompts`)
- Prompt retrieval via the `prompts/get` method (`MCP::Client#get_prompt`)
- Automatic JSON-RPC 2.0 message formatting
- UUID request ID generation

Clients are initialized with a transport layer instance that handles the low-level communication mechanics.
Authorization is handled by the transport layer.

## Transport Layer Interface

If the transport layer you need is not included in the gem, you can build and pass your own instances so long as they conform to the following interface:

class CustomTransport

# Sends a JSON-RPC request to the server and returns the raw response.

#

# @param request [Hash] A complete JSON-RPC request object.

# https://www.jsonrpc.org/specification#request_object

# @return [Hash] A hash modeling a JSON-RPC response object.

# https://www.jsonrpc.org/specification#response_object

def send_request(request:)

# Your transport-specific logic here

# - HTTP: POST to endpoint with JSON body

# - WebSocket: Send message over WebSocket

# - stdio: Write to stdout, read from stdin

# - etc.

end

end

### HTTP Transport Layer

Use the `MCP::Client::HTTP` transport to interact with MCP servers using simple HTTP requests.

You'll need to add `faraday` as a dependency in order to use the HTTP transport layer:

gem 'mcp'

gem 'faraday', '>= 2.0'

Example usage:

http_transport = MCP::Client::HTTP.new(url: "https://api.example.com/mcp")

client = MCP::Client.new(transport: http_transport)

List available tools

tools = client.tools

tools.each do |tool|

puts "Bearer my_token"

}

)

client = MCP::Client.new(transport: http_transport)

client.tools # will make the call using Bearer auth

You can add any custom headers needed for your authentication scheme, or for any other purpose. The client will include these headers on every request.

### Tool Objects

The client provides a wrapper class for tools returned by the server:

- `MCP::Client::Tool` - Represents a single tool with its metadata

This class provides easy access to tool properties like name, description, input schema, and output schema.