Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint and idempotentHint, so the safety profile is covered. The description adds valuable behavioral details: it returns earned achievements, progress toward locked ones, and shows '???' for secret achievements until earned. This goes beyond the annotations with meaningful context.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.