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Fix an incorrect result: null response to an id-bearing notification message

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## Motivation and Context

A JSON-RPC notification carries no id and must not receive any response.
When a client erroneously sent a notification-only method such as `notifications/cancelled` with an id,
the server replied with a successful `result: null`, which is neither a valid MCP result nor
a legal reply to a notification.

Such a message is now treated as a request for a method that has no request handler and rejected with
Method not found, matching the behavior of the TypeScript and Python SDKs. A well-formed notification
that omits the id continues to receive no response.

Fixes #434.

## How Has This Been Tested?

Added regression tests covering id-bearing `notifications/cancelled`, `notifications/initialized`,
and `notifications/progress`, a batch that mixes an id-bearing notification with other messages,
custom `notifications/*` methods, and the same case over the HTTP transport.

## Breaking Changes

None for spec-conformant clients, which never put an id on a notification.
The only behavioral change is that a malformed id-bearing notification now receives a Method not found error
instead of a spurious `result: null`.
This commit is contained in:
Koichi ITO 2026-07-01 01:30:56 +09:00
commit 6d42cf7826
1 changed files with 133 additions and 0 deletions
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32
test/mcp/server/transports/streamable_http_transport_test.rb
View file

@ -80,6 +80,38 @@ module MCP
assert_equal({}, body["result"])
end
test "id-bearing notification message is rejected with Method not found" 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: "notifications/cancelled", id: "123" }.to_json,
)
response = @transport.handle_request(request)
assert_equal 200, response[0]
io = StringIO.new
response[2].call(io)
body = JSON.parse(io.string.match(/^data: (.+)$/)[1])
assert_equal "123", body["id"]
refute body.key?("result")
assert_equal JsonRpcHandler::ErrorCode::METHOD_NOT_FOUND, body["error"]["code"]
end
test "handles POST request with invalid JSON" do
request = create_rack_request(
"POST",