Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover readOnlyHint and idempotentHint, so the safety profile is known. The description adds context about the data source (FRED), the exact return fields (series ID, title, frequency, units, seasonal adjustment, date range), and the API key requirement, providing value beyond annotations without contradiction.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.