Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations only declare destructiveHint=false, so the description carries most of the behavioral burden. It does clearly state the state-change nature ('mark as read'), but it does not mention idempotency, whether there is a response, or any effect on other state (e.g., unread counts). For a simple mutation, this is adequate but not richly transparent.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.