A captured MCP message contains jsonrpc, method and params members but no id member. How must the receiver treat it?
Select an answer to reveal the explanation.
Short Explanation
No id means no return address, so there's nobody to reply to. That's a notification: fire and forget. If you're tempted to send an error back, remember you'd have nothing to put in the id.
Full Explanation
MCP follows JSON-RPC 2.0 message types. A request carries an id and expects a response; a notification is a one-way message that MUST NOT include an id, and the receiver MUST NOT send a response. A message with jsonrpc, method and params but no id therefore matches the notification shape exactly. Examples include notifications/cancelled on stdio, notifications/progress scoped to a request, and notifications/tools/list_changed delivered on a subscriptions/listen stream. Treating it as a malformed request is wrong, because a missing id is precisely what distinguishes a notification and is not an error. Answering it with a result carrying a null id contradicts the rule that notifications get no response, and MCP also forbids null request ids. Interpreting it as a response is impossible, because responses carry result or error members rather than a method, and correlating messages by guessing at the oldest pending request would corrupt request tracking. Exam caveat: notifications flow both ways, so a server can send them, but servers in this revision do not initiate JSON-RPC requests. Operational check: when debugging traffic, classify each message by its members — method with id is a request, method without id is a notification, and result or error with id is a response.