Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare read-only, idempotent and open-world, but the description adds information the structured fields cannot: listings can include used, refurbished, rental and bundle offers, so titles and sellers must be inspected before trusting a price, plus a per-call cost model ($0.25 per successful call, failed calls free).
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.