Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish read-only, idempotent, open-world, non-destructive behavior, so the bar is lower. The description adds two things beyond them: the auth requirement ('no account needed') and the shape of the result set (ids, names, descriptions, URLs).
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.