Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly/idempotent/openWorld, but the description adds genuinely new context: keyless access, no pagination, no sorting, entire array returned in one shot. That tells the agent it cannot page or narrow results, which annotations alone would not convey.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.