Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover read-only, idempotent, non-destructive, closed-world behavior, so the bar is lower. The description adds useful context beyond annotations: it reads only the local catalog, has no upstream row counts or latest dates, and returns an error when the dataset_id does not exist.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.