Resources

Resources

The ModelContextProtocol::Server::Resource base class allows subclasses to define a resource that the MCP client can use.

Define the resource properties and optionally annotations, then implement the call method to build your resource. Use the respond_with instance method to ensure your resource responds with appropriately formatted response data.

You can also send MCP log messages to clients from within your resource by calling a valid logger level method on the client_logger and passing a string message. For server-side debugging and monitoring, use the server_logger to write logs that are not sent to clients.

Resource Definition

Use the define block to set resource properties and configure annotations.

Property	Description
name	The name of the resource
title	Human-readable display name
description	Short description of what the resource contains
mime_type	MIME type of the resource content
uri	URI identifier for the resource
annotations	Block for defining resource annotations

Annotation Definition

Define any resource annotations using an annotations block nested within the define block.

Property	Description
audience	Target audience for the resource (array of symbols like [:user, :assistant])
priority	Priority level (numeric value, e.g., 0.9)
last_modified	Last modified timestamp (ISO 8601 string)

Resource Methods

Define your resource properties and annotations, implement the call method to build resource content and respond_with to serialize the response. You can wrap long running operations in a cancellable block to allow clients to cancel the request. Also, you can automatically send progress notifications to clients by wrapping long-running operations in a progressable block.

Method	Context	Description
define	Class definition	Block for defining resource metadata and annotations
call	Instance method	Main method to implement resource logic and build response
cancellable	Within call	Wrap long-running operations to allow client cancellation (e.g., cancellable { slow_operation })
progressable	Within call	Wrap long-running operations to send clients progress notifications (e.g., progressable { slow_operation })
respond_with	Within call	Return properly formatted response data (e.g., respond_with text: or respond_with binary:)

Available Instance Variables

Resources have access to their configured properties and server context.

Variable	Context	Description
mime_type	Within call	The configured MIME type for this resource
uri	Within call	The configured URI identifier for this resource
client_logger	Within call	Client logger instance for sending MCP log messages (e.g., client_logger.info("message"))
server_logger	Within call	Server logger instance for debugging and monitoring (e.g., server_logger.debug("message"))
context	Within call	Hash containing server configuration context values

Examples

Text Resource

This is an example resource that returns a text response:

class TestResource < ModelContextProtocol::Server::Resource
  define do
    name "top-secret-plans.txt"
    title "Top Secret Plans"
    description "Top secret plans to do top secret things"
    mime_type "text/plain"
    uri "file:///top-secret-plans.txt"
  end

  def call
    respond_with text: "I'm finna eat all my wife's leftovers."
  end
end

Annotated Resource

This is an example resource with annotations:

class TestAnnotatedResource < ModelContextProtocol::Server::Resource
  define do
    name "annotated-document.md"
    description "A document with annotations showing priority and audience"
    mime_type "text/markdown"
    uri "file:///docs/annotated-document.md"
    annotations do
      audience [:user, :assistant]
      priority 0.9
      last_modified "2025-01-12T15:00:58Z"
    end
  end

  def call
    respond_with text: "# Annotated Document\n\nThis document has annotations."
  end
end

Binary Resource

This is an example resource that returns binary data:

class TestBinaryResource < ModelContextProtocol::Server::Resource
  define do
    name "project-logo.png"
    description "The logo for the project"
    mime_type "image/png"
    uri "file:///project-logo.png"
  end

  def call
    # In a real implementation, we would retrieve the binary resource
    # This is a small valid base64 encoded string (represents "test")
    data = "dGVzdA=="
    respond_with binary: data
  end
end