Chapter 3: Tools, Prompts, Resources, and Schema Discipline

April 13, 2026 ยท View on GitHub

Welcome to Chapter 3: Tools, Prompts, Resources, and Schema Discipline. In this part of MCP Ruby SDK Tutorial: Building MCP Servers and Clients in Ruby, you will build an intuitive mental model first, then move into concrete implementation details and practical production tradeoffs.

This chapter focuses on modeling MCP primitives with predictable behavior and schema quality.

Learning Goals

  • design tool schemas that validate arguments and output shapes reliably
  • structure prompt and resource handlers for maintainable growth
  • use annotations and metadata consistently across primitives
  • avoid schema drift that breaks client interoperability

Primitive Implementation Checklist

  1. define explicit tool argument schemas and output contracts
  2. model prompt arguments with stable names and titles
  3. expose resource identifiers and templates with clear naming patterns
  4. test edge cases for empty, invalid, and over-broad input payloads

Source References

Summary

You now have a schema-first primitive strategy for Ruby MCP servers.

Next: Chapter 4: Notifications, Logging, and Observability

Source Code Walkthrough

.rubocop.yml

The .rubocop module in .rubocop.yml handles a key part of this chapter's functionality:

inherit_gem:
  rubocop-shopify: rubocop.yml

plugins:
  - rubocop-minitest
  - rubocop-rake

AllCops:
  TargetRubyVersion: 2.7

Gemspec/DevelopmentDependencies:
  Enabled: true

Lint/IncompatibleIoSelectWithFiberScheduler:
  Enabled: true

Minitest/LiteralAsActualArgument:
  Enabled: true

This module is important because it defines how MCP Ruby SDK Tutorial: Building MCP Servers and Clients in Ruby implements the patterns covered in this chapter.

conformance/server.rb

The server module in conformance/server.rb handles a key part of this chapter's functionality:

# frozen_string_literal: true

require "rackup"
require "json"
require "uri"
require_relative "../lib/mcp"

module Conformance
  # 1x1 red PNG pixel (matches TypeScript SDK and Python SDK)
  BASE64_1X1_PNG = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8DwHwAFBQIAX8jx0gAAAABJRU5ErkJggg=="

  # Minimal WAV file (matches TypeScript SDK and Python SDK)
  BASE64_MINIMAL_WAV = "UklGRiYAAABXQVZFZm10IBAAAAABAAEAQB8AAAB9AAACABAAZGF0YQIAAAA="

  module Tools
    class TestSimpleText < MCP::Tool
      tool_name "test_simple_text"
      description "A tool that returns simple text content"

      class << self
        def call(**_args)
          MCP::Tool::Response.new([MCP::Content::Text.new("This is a simple text response for testing.").to_h])
        end
      end
    end

    class TestImageContent < MCP::Tool
      tool_name "test_image_content"
      description "A tool that returns image content"

      class << self
        def call(**_args)
          MCP::Tool::Response.new([MCP::Content::Image.new(BASE64_1X1_PNG, "image/png").to_h])
        end
      end

This module is important because it defines how MCP Ruby SDK Tutorial: Building MCP Servers and Clients in Ruby implements the patterns covered in this chapter.

lib/json_rpc_handler.rb

The json_rpc_handler module in lib/json_rpc_handler.rb handles a key part of this chapter's functionality:

# frozen_string_literal: true

require "json"

module JsonRpcHandler
  class Version
    V1_0 = "1.0"
    V2_0 = "2.0"
  end

  class ErrorCode
    INVALID_REQUEST = -32600
    METHOD_NOT_FOUND = -32601
    INVALID_PARAMS = -32602
    INTERNAL_ERROR = -32603
    PARSE_ERROR = -32700
  end

  DEFAULT_ALLOWED_ID_CHARACTERS = /\A[a-zA-Z0-9_-]+\z/

  extend self

  def handle(request, id_validation_pattern: DEFAULT_ALLOWED_ID_CHARACTERS, &method_finder)
    if request.is_a?(Array)
      return error_response(id: :unknown_id, id_validation_pattern: id_validation_pattern, error: {
        code: ErrorCode::INVALID_REQUEST,
        message: "Invalid Request",
        data: "Request is an empty array",
      }) if request.empty?

      # Handle batch requests
      responses = request.map { |req| process_request(req, id_validation_pattern: id_validation_pattern, &method_finder) }.compact

      # A single item is hoisted out of the array
      return responses.first if responses.one?

This module is important because it defines how MCP Ruby SDK Tutorial: Building MCP Servers and Clients in Ruby implements the patterns covered in this chapter.

How These Components Connect

flowchart TD
    A[.rubocop]
    B[server]
    C[json_rpc_handler]
    A --> B
    B --> C