Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Goes well beyond the annotations (which only cover read-only/idempotent/open-world) by disclosing the pricing model ($0.005 per call, 10 free/day, x402 payment-required result) and the error contract (isError with message for invalid input or upstream failure, and that failures are not charged). These are exactly the behavioral facts an agent needs before calling a paid, open-world tool.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.