Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the behavioral burden. It discloses that grouping commas and currency are handled and that non-numeric rows are excluded and counted, which is valuable. However, it doesn't mention whether the operation is read-only, error handling, or performance, but for a stats tool these are less critical. It adds context 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.