Test README per server configuration example

This test revealed that the `define_` helper methods were failing to
ensure the server supports the type of capability they were defining.

Author: sambostock

README.md

@@ -155,6 +155,7 @@ 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.
 
+<!-- SNIPPET ID: per_server_configuration -->
 ```ruby
 configuration = MCP::Configuration.new
 configuration.exception_reporter = ->(exception, server_context) {

lib/mcp/server.rb

@@ -87,11 +87,13 @@ def handle_json(request)
     end
 
     def define_tool(name: nil, description: nil, input_schema: nil, annotations: nil, &block)
+      @capabilities.support_tools
       tool = Tool.define(name:, description:, input_schema:, annotations:, &block)
       @tools[tool.name_value] = tool
     end
 
     def define_prompt(name: nil, description: nil, arguments: [], &block)
+      @capabilities.support_prompts
       prompt = Prompt.define(name:, description:, arguments:, &block)
       @prompts[prompt.name_value] = prompt
     end
@@ -102,6 +104,7 @@ def resources_list_handler(&block)
     end
 
     def resources_read_handler(&block)
+      @capabilities.support_resources
       @handlers[Methods::RESOURCES_READ] = block
     end
 
@@ -116,6 +119,7 @@ def tools_list_handler(&block)
     end
 
     def tools_call_handler(&block)
+      @capabilities.support_tools
       @handlers[Methods::TOOLS_CALL] = block
     end
 
@@ -125,6 +129,7 @@ def prompts_list_handler(&block)
     end
 
     def prompts_get_handler(&block)
+      @capabilities.support_prompts
       @handlers[Methods::PROMPTS_GET] = block
     end
 

test/fixtures/files/code_snippet_wrappers/readme/per_server_configuration.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require "mcp"
+
+# Minimally mock Bugsnag for the test
+module Bugsnag
+  class Report
+    attr_reader :metadata
+
+    def initialize
+      @metadata = {}
+    end
+
+    def add_metadata(key, value)
+      @metadata[key] = value
+    end
+  end
+
+  class << self
+    def notify(exception)
+      report = Report.new
+      yield report
+      puts "Bugsnag notified of #{exception.inspect} with metadata #{report.metadata.inspect}"
+    end
+  end
+end
+
+b = binding
+eval(File.read("code_snippet.rb"), b) # rubocop:disable Security/Eval -- We need to run the snippet in this context
+server = b.local_variable_get(:server)
+
+server.define_tool(name: "error_tool") { raise "boom" }
+
+puts server.handle_json({
+  jsonrpc: "2.0",
+  id: "1",
+  method: "tools/call",
+  params: { name: "error_tool", arguments: {} },
+}.to_json)

test/integration/readme_code_snippets_test.rb

@@ -89,6 +89,34 @@ class ReadmeCodeSnippetsTest < ActiveSupport::TestCase
     STDOUT
   end
 
+  test "Per-Server Configuration example works exactly as documented in README" do
+    stdout = run_code_snippet("per_server_configuration")
+
+    request = {
+      jsonrpc: "2.0",
+      id: "1",
+      method: "tools/call",
+      params: { name: "error_tool", arguments: {} },
+    }.to_json
+
+    error = MCP::Server::RequestHandlerError.new("Internal error calling tool error_tool", request)
+    metadata = { model_context_protocol: { request: } }
+
+    response = {
+      jsonrpc: "2.0",
+      id: "1",
+      error: { code: -32603, message: "Internal error", data: "Internal error calling tool error_tool" },
+    }.to_json
+
+    expected_output = <<~OUTPUT
+      Bugsnag notified of #{error.inspect} with metadata #{metadata.inspect}
+      Got instrumentation data {method: "tools/call", tool_name: "error_tool", error: :internal_error, duration: 1.23}
+      #{response}
+    OUTPUT
+
+    assert_equal(expected_output, stdout)
+  end
+
   private
 
   def assert_json_lines(expected, actual, message = "Expected the given JSON lines")