Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the full behavioral burden and does well: it discloses that grouping commas and currency symbols are handled during parsing, and that non-numeric rows are excluded and counted. This reveals edge-case behavior an agent would otherwise only discover by calling the tool. It stops short of covering error behavior for unknown columns or exact return shape, so a 5 is not warranted.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.