Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish read-only, idempotent, and non-destructive behavior, so the description does not need to repeat them. It adds useful context by disclosing that descriptions and requirements are bounded to keep responses manageable, a behavioral trait not visible in the schema or annotations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.