Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations cover the safety profile (read-only, idempotent, non-destructive, openWorld=false), and the description goes further in the ways that matter: it discloses the x402 paywall at 0.02 USDC per call, that no-data results are free, and the provenance limitation that price history only exists from our first observation of a listing. That last point materially affects interpretation of the days window and is not derivable from the schema.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.