Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish read-only, idempotent, non-destructive behavior, but the description adds substantial context beyond them: pricing ($0.01/call, 10 free/day, x402 payment-required response), error behavior (isError, not charged), refresh cadence (every 6 hours), and methodology (empirical rates, shrinkage toward coarser buckets). It also discloses what is bucketed from the first 60 seconds, which helps an agent interpret results.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.