Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover the safety profile (readOnly, idempotent, openWorld, non-destructive), so the bar is lower. The description adds genuinely useful context not in annotations: 'free, no key' discloses the auth requirement, and it notes the return contents (codes, names, dimensions, values).
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.