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 +108,9 @@ 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)
+            # For notifications and responses only, return 202 Accepted
+            handle_notification_or_response
           else
             handle_regular_request(body_string, session_id)
           end
@@ -168,6 +169,10 @@ def parse_request_body(body_string)
           [400, { "Content-Type" => "application/json" }, [{ error: "Invalid JSON" }.to_json]]
         end
 
+        def notification_or_response?(body)
+          !(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

@@ -8,9 +8,15 @@ class Server
     module Transports
       class StreamableHTTPTransportTest < ActiveSupport::TestCase
         setup do
+          tool = Tool.define(
+            name: "test_tool",
+            title: "Test tool",
+            description: "A test tool",
+          )
+
           @server = Server.new(
             name: "test_server",
-            tools: [],
+            tools: [tool],
             prompts: [],
             resources: [],
           )
@@ -620,6 +626,36 @@ class StreamableHTTPTransportTest < ActiveSupport::TestCase
           assert_empty(response[2])
         end
 
+        test "POST tools/list returns 200 with body" do
+          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"]
+
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_MCP_SESSION_ID" => session_id,
+            },
+            { jsonrpc: "2.0", method: MCP::Methods::TOOLS_LIST, id: "123" }.to_json,
+          )
+
+          response = @transport.handle_request(request)
+          assert_equal 200, response[0]
+          assert_equal({ "Content-Type" => "application/json" }, response[1])
+
+          body = JSON.parse(response[2][0])
+          assert_equal "2.0", body["jsonrpc"]
+          assert_equal "123", body["id"]
+          assert_equal "test_tool", body["result"]["tools"][0]["name"]
+        end
+
         private
 
         def create_rack_request(method, path, headers, body = nil)