Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare read-only, idempotent, non-destructive behavior, but the description adds genuinely new operational context: it is read-only and 'costs 1 unit', a billing detail no structured field provides. It doesn't discuss pagination behavior beyond the limit param, keeping it from a 5.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.