Chapter 4: Notifications, Logging, and Observability

Welcome to Chapter 4: Notifications, Logging, and Observability. 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 explains runtime observability patterns for Ruby MCP servers.

Learning Goals

• send list-change notifications correctly for tools/prompts/resources
• configure and emit structured log messages across severity levels
• align server logging behavior with client logging/setLevel controls
• avoid noisy or sensitive telemetry in production environments

Notification and Logging Surfaces

Surface	Method
Tool list changes	notify_tools_list_changed
Prompt list changes	notify_prompts_list_changed
Resource list changes	notify_resources_list_changed
Structured logs	notify_log_message

Operational Notes

• notifications require an active transport/session context.
• logging emission is filtered by client-selected log level.
• redact secrets before serialization into log message payloads.

Source References

• Ruby SDK README - Notifications
• Ruby SDK README - Logging
• MCP Logging Spec

Summary

You now have a practical observability model for Ruby MCP services.

Next: Chapter 5: Transports: stdio, Streamable HTTP, and Session Modes

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