Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already provide readOnlyHint=true, and the description adds meaningful behavioral context: it is measured from real answers, not estimated, and includes movement over the tracked window. This goes beyond the schema and helps the agent set expectations about data source and output shape, even though return format details are not spelled out.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.