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

[dak2]

Motivation and Context

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

How Has This Been Tested?

• Check existing and added test cases for this case.
• Confirm the server returns 202 when the client sends a notification or response.

Breaking Changes

There is a possibility that, if users depend on status code 200.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Follow up: #111

[atesgoral]

On the tests: I don't see anywhere in either the MCP or the JSON-RPC spec where a result or error property is used in a request with an id. These are response properties. Requests have method and params.

[dak2]

@atesgoral
Thanks for your review.

As you pointed out, it seems I misunderstood. I've deleted the test case I added.

Also, I believe this case is already covered by an existing test in test/mcp/server/transports/streamable_http_transport_test.rb (the case where a server returns 202 when a JSON-RPC notification request object is passed, and 200 when a JSON-RPC regular request object is passed), so I haven't added any specific tests.

Could you please review it again?

[koic]

It seems odd that there is no reproduction test. Without a reproduction case for this issue, wouldn't it be difficult to notice if a regression occurs?

[dak2]

@koic

Thanks for your checking!

I believe the following three points need to be verified for this requirement:

1. When receiving a JSON-RPC notification object, return a 202 status code without a response body.
2. When receiving a JSON-RPC response object, return a 202 status code without a response body.
3. Do not return a 202 when receiving a regular JSON-RPC request object.

Regarding the second point, as atesgoral pointed out, since it's a response object, it doesn't get passed as a request in the first place, so I don't believe it needs to be verified in testing.

For case 1, see lines

ruby-sdk/test/mcp/server/transports/streamable_http_transport_test.rb

Lines 596 to 621 in eb0d9c0

	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

, and case 3 is

ruby-sdk/test/mcp/server/transports/streamable_http_transport_test.rb

Lines 20 to 50 in eb0d9c0

	test "handles POST request with valid JSON-RPC message" do
	# First create a session
	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"]
	# Now make the ping request with the session ID
	request = create_rack_request(
	"POST",
	"/",
	{
	"CONTENT_TYPE" => "application/json",
	"HTTP_MCP_SESSION_ID" => session_id,
	},
	{ jsonrpc: "2.0", method: "ping", 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_empty(body["result"])
	end

I believe these cases is already covered by above test cases, so I don't think we need to write additional tests.
What do you think about this?