Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations cover the safety profile (readOnly, idempotent, non-destructive), and the description adds materially new behavior: it returns a free sample plus paid URL and pay steps, with no charge on this call, while the full result costs $0.03 Base USDC via x402. Payment gating is exactly the non-obvious operational context an agent needs before invoking.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.