Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so safety is covered. The description adds value the annotations cannot: a cost signal ('costs 1 unit') and deterministic ordering ('newest first'), both of which matter for planning and budgeting calls.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.