Return HTTP status code 202 for notification initialized method

lib/mcp/server/transports/streamable_http_transport.rb

@@ -108,6 +108,8 @@ def handle_post(request)
 
           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 @@ def handle_initialization(body_string, body)
           [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 && [email protected]?(session_id)

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -595,6 +595,33 @@ class StreamableHTTPTransportTest < ActiveSupport::TestCase
           assert_equal "Internal server error", body["error"]
         end
 
+        test "POST notifications/initialized returns 202 with no body" do
+          # Create a session first (optional for notification, but keep consistent with flow)
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          notif_request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_MCP_SESSION_ID" => session_id,
+            },
+            { jsonrpc: "2.0", method: MCP::Methods::NOTIFICATIONS_INITIALIZED }.to_json,
+          )
+
+          response = @transport.handle_request(notif_request)
+          assert_equal 202, response[0]
+          assert_empty(response[1])
+          assert_empty(response[2])
+        end
+
         private
 
         def create_rack_request(method, path, headers, body = nil)