Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish a safe read-only, idempotent, closed-world operation, so the bar is lower. The description still adds valuable edge-case behavior: uncovered cities return the covered list, and groups under 30 permits are suppressed, which an agent could not learn from annotations or schema.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.