Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish the safety profile (readOnlyHint=true, idempotentHint=true, destructiveHint=false), so the description's burden is reduced. The description adds one useful behavioral fact beyond annotations: the result is 'cached,' meaning it may be stale relative to a freshly evaluated aggregation. It does not disclose what happens when no cached result exists or whether the result is returned in the same shape as an evaluation.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.