Return HTTP status code 202 for notification initialized method

lib/mcp/server/transports/streamable_http_transport.rb

@@ -108,6 +108,8 @@ module MCP
 
           if body["method"] == "initialize"
             handle_initialization(body_string, body)
+          elsif body["method"] == MCP::Methods::NOTIFICATIONS_INITIALIZED
+            handle_notification_initialized
           else
             handle_regular_request(body_string, session_id)
           end
@@ -185,6 +187,10 @@ module MCP
           [200, headers, [response]]
         end
 
+        def handle_notification_initialized
+          [202, {}, []]
+        end
+
         def handle_regular_request(body_string, session_id)
           # If session ID is provided, but not in the sessions hash, return an error
           if session_id && !@sessions.key?(session_id)