Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover read-only, idempotent, non-destructive behavior, so the description adds useful extras: pagination is alphabetical, results contain stable slugs, and the call is 'Cheap' from a cost/performance standpoint. No contradiction with annotations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.