Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover readOnly, idempotent, and non-destructive behavior. The description adds value by disclosing the content of the response (definition, context, related terms) and the cost profile ('Cheap — usually under 1k tokens'), which helps an agent weigh this against heavier alternatives.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.