Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover the safety profile (readOnly, idempotent, non-destructive), so the bar is lower, yet the description adds substantive context: a free sample is returned here with no charge, and the full result requires $0.01 Base USDC via x402. That payment/authorization behavior is not in any structured field. It stops short of disclosing rate limits, caching, or when the paid URL must be called, so a 4 rather than 5.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.