Fix to return a response code 202 if the server accepts JSON-RPC notifications and responses

If the server accepts JSON-RPC notifications and responses, it must return an HTTP status code of 202 without a body.

refs
- https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#sending-messages-to-the-server
- https://www.jsonrpc.org/specification

Follow up: #111

Author: dak2

lib/mcp/server/transports/streamable_http_transport.rb

@@ -108,8 +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
+          elsif notification_or_response?(body)
+            handle_notification_or_response
           else
             handle_regular_request(body_string, session_id)
           end
@@ -168,6 +168,11 @@ def parse_request_body(body_string)
           [400, { "Content-Type" => "application/json" }, [{ error: "Invalid JSON" }.to_json]]
         end
 
+        def notification_or_response?(body)
+          # Check if the body is a notification or a response
+          !(body["id"] && body["method"])
+        end
+
         def handle_initialization(body_string, body)
           session_id = SecureRandom.uuid
 
@@ -187,7 +192,7 @@ def handle_initialization(body_string, body)
           [200, headers, [response]]
         end
 
-        def handle_notification_initialized
+        def handle_notification_or_response
           [202, {}, []]
         end
 

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -622,6 +622,63 @@ class StreamableHTTPTransportTest < ActiveSupport::TestCase
           assert_empty(response[2])
         end
 
+        test "handles POST request when json-rpc notification object is received" do
+          notification_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "some_notification" }.to_json,
+          )
+
+          response = @transport.handle_request(notification_request)
+          assert_equal 202, response[0]
+          assert_empty(response[1])
+          assert_empty(response[2])
+        end
+
+        test "handles POST request when json-rpc response object with result is received" do
+          # Test response (has id but no method)
+          response_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", id: "123", result: { content: { type: "text" } } }.to_json,
+          )
+
+          response = @transport.handle_request(response_request)
+          assert_equal 202, response[0]
+          assert_empty(response[1])
+          assert_empty(response[2])
+        end
+
+        test "handles POST request when json-rpc response object with error is received" do
+          response_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", id: "123", error: { content: { type: "text" } } }.to_json,
+          )
+
+          response = @transport.handle_request(response_request)
+          assert_equal 202, response[0]
+          assert_empty(response[1])
+          assert_empty(response[2])
+        end
+
+        test "handles POST request when json-rpc error object is received" do
+          response_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", id: "123", error: { content: { type: "text" } } }.to_json,
+          )
+
+          response = @transport.handle_request(response_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)