Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Goes well beyond the annotations: it discloses that re-marking does not reset the first-read timestamp, that there is no unmark operation, that the response includes the new unread_count (avoiding a follow-up call), and the exact undo-token contract including when undo is refused. This is unusually rich behavioral detail for a low-risk mutation.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.