Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish read-only, idempotent, non-destructive behavior, so the description's added value is the countable behavioral nuance: it shows disagreement rather than picking one, and the counts are 'reported next to' each other rather than merged. It also discloses that filters only affect rows, not the two full counts, which is useful beyond the schema.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.