Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations cover the safety profile (readOnly, idempotent, non-destructive, closed world), and the description adds behavioral facts they cannot carry: it is a local fetch, it returns a free sample with the paid URL and pay steps, no charge is incurred on this call, and the full result costs $0.02 Base USDC via x402. The payment/charging model is the kind of context an agent must know before calling.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.