Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint, idempotentHint, and openWorldHint, so the description need not re-state safety. It adds useful behavioral details beyond the schema: newest-first ordering, mention markup format, plain_content flattening, and pagination fields ('pagination.total' and 'has_more').
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.