Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description bears the full burden. It clearly discloses the key behavioral trait: without payment, an x402 402 payment challenge is returned, and wallet enables the free trial/paid path. It does not mention error/return formats, but the payment behavior is a material and well-communicated detail.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.