Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the full burden for behavioral disclosure. It reveals that messages are 'recent' and that channel accepts a name or id, but it does not disclose output format, ordering, pagination, limit semantics, or any access constraints.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.