Fix an incorrect result: null response to an id-bearing notification message#435
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…n message ## 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 modelcontextprotocol#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`.
atesgoral
approved these changes
Jul 1, 2026
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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/cancelledwith an id, the server replied with a successfulresult: 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, andnotifications/progress, a batch that mixes an id-bearing notification with other messages, customnotifications/*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.Types of changes
Checklist