implements the notifications/initialized method

- According to
  https://modelcontextprotocol.io/specification/2025-06-18/basic/lifecycle,
  "After successful initialization, the client MUST send an initialized
  notification to indicate it is ready to begin normal operations".
- This implementation just replies with an empty response (same as ping).

Author: jkraemer

lib/mcp/server.rb

@@ -66,6 +66,7 @@ def initialize(
         Methods::PROMPTS_GET => method(:get_prompt),
         Methods::INITIALIZE => method(:init),
         Methods::PING => ->(_) { {} },
+        Methods::NOTIFICATIONS_INITIALIZED => ->(_) { {} },
 
         # No op handlers for currently unsupported methods
         Methods::RESOURCES_SUBSCRIBE => ->(_) {},

test/mcp/server_test.rb

@@ -141,6 +141,25 @@ class ServerTest < ActiveSupport::TestCase
       assert_instrumentation_data({ method: "unsupported_method" })
     end
 
+    test "#handle notifications/initialized returns empty response" do
+      request = {
+        jsonrpc: "2.0",
+        method: "notifications/initialized",
+        id: "123",
+      }
+
+      response = @server.handle(request)
+      assert_equal(
+        {
+          jsonrpc: "2.0",
+          id: "123",
+          result: {},
+        },
+        response,
+      )
+      assert_instrumentation_data({ method: "notifications/initialized" })
+    end
+
     test "#handle tools/list returns available tools" do
       request = {
         jsonrpc: "2.0",