Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare the mutation/safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=false), but the description adds non-obvious behavior: no credit cost and the fact that the frame encloses nothing existing. The OAuth/API-key sign-in requirement with a concrete header format is genuinely useful preconditions context that structured fields do not carry.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.