Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the description need not re-state safety. Beyond that, it adds useful behavioral details: pagination semantics for count and has_more, a prominent deprecation warning for total, and the authentication requirement. This is substantive context an agent could not get from annotations alone.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.