Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare non-destructive, idempotent, non-read-only, so the bar is lower; the description still adds real context by warning that running work may incur provider cost and that cancellation doesn't guarantee refund or rollback. That is exactly the side-effect information the annotations cannot express.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.