typescript-sdk/.changeset/cancelled-request-id-zero.md

@modelcontextprotocol/core-internal	@modelcontextprotocol/client	@modelcontextprotocol/server
patch	patch	patch

Treat request id 0 as a real id. Two guards tested a RequestId for truthiness, so the legal JSON-RPC ids 0 and '' were read as absent. Id 0 is not a corner case: the outbound request counter is zero-based, so it is the first id every peer assigns, which on the server→client leg is the first sampling/createMessage, elicitation/create, or roots/list a server sends.

• notifications/cancelled carrying id 0 was ignored, and the in-flight handler ran to completion with its AbortSignal never fired.
• A notification sent with relatedRequestId: 0 wrongly passed the debounce gate (for methods opted into debouncedNotificationMethods). Because the pending set is keyed by method alone, a second such notification in the same tick was silently dropped rather than sent.

Absent is now the only value that means "no id".