Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations confirm read-only, idempotent, non-destructive. The description adds value by detailing exclusions (own messages, deleted conversations) and the returned fields, but does not need to repeat annotations. Slightly more context could be given about pagination, but that is covered in the schema.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.