Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations only cover the safety profile (mutating, non-idempotent, not destructive); the description adds genuinely non-derivable behavior: the note is invisible to the user, is attributed in the inbox as written by this agent, and renders Markdown. It omits duplicate behavior (annotations say non-idempotent, so re-posting creates another note) and any auth/permission expectations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.