Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations cover the safety profile (readOnly, idempotent, non-destructive), so the bar is lower, yet the description adds meaningful non-obvious context: a paid x402 call at 0.1 USDC on Solana or Base. That cost/auth detail is exactly the kind of behavior annotations cannot convey. It stops short of noting rate limits or what happens on unsupported symbols.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.