Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so safety and repetition semantics are fully covered. The description contributes one extra behavioral fact — that results include message counts — but says nothing about ordering, empty-feed behavior, or pagination. Adequate, not rich.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.