Test Stdio Transport README example

Author: sambostock

README.md

@@ -84,6 +84,7 @@ end
 
 If you want to build a local command-line application, you can use the stdio transport:
 
+<!-- SNIPPET ID: stdio_transport -->
 ```ruby
 #!/usr/bin/env ruby
 require "mcp"

test/integration/readme_code_snippets_test.rb

@@ -25,6 +25,44 @@ class ReadmeCodeSnippetsTest < ActiveSupport::TestCase
     )
   end
 
+  test "Stdio Transport example works exactly as documented in README" do
+    # This snippet is a standalone example server/transport, so we run it directly and send it requests
+    stdout = Tempfile.create(["stdio_transport", ".rb"]) do |file|
+      FileUtils.chmod("+x", file.path) # Make executable
+      File.write(file.path, extract_readme_code_snippet("stdio_transport"))
+
+      # Reuse example input from README, but drop command to start the server (`$ ...` line)
+      stdin_data = extract_readme_code_snippet("running_stdio_server", language: "console").lines(chomp: true).grep_v(/^\$\s/).join("\n")
+
+      stdout, stderr, status = Open3.capture3(file.path, stdin_data:)
+      assert_predicate(status, :success?, "Expected #{file.path} to exit with success, but got exit status #{status.exitstatus}\n\nSTDOUT: #{stdout}\n\nSTDERR: #{stderr}")
+      assert_empty(stderr, "Expected no stderr in: #{stderr}")
+      refute_empty(stdout, "Expected stdout not to be empty")
+
+      stdout
+    end
+
+    assert_json_lines(
+      [
+        { jsonrpc: "2.0", id: "1", result: {} },
+        {
+          jsonrpc: "2.0",
+          id: "2",
+          result: {
+            tools: [
+              {
+                name: "example_tool",
+                description: "A simple example tool that echoes back its arguments",
+                inputSchema: { type: "object", properties: { message: { type: "string" } }, required: ["message"] },
+              },
+            ],
+          },
+        },
+      ],
+      stdout,
+    )
+  end
+
   private
 
   def assert_json_lines(expected, actual, message = "Expected the given JSON lines")