Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations cover the safety profile (readOnly, idempotent, non-destructive), so the bar is lower. The description adds valuable context beyond annotations: pagination size and ordering, that it returns titles not text, and the past-the-end empty-page behavior. It does not need to explain return values given the output schema exists.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.