Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations cover the safety profile (readOnly, idempotent, non-destructive, openWorld), and the description adds non-obvious behavioral detail beyond them: required fields are only marked when every row is non-null, nested structures are rejected rather than guessed, and — critically — the free MCP call does NOT execute the capability but returns the x402 endpoint, price, and payment guide. That payment/execution split is exactly the kind of context annotations cannot express.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.