Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover readOnly and destructive hints, so the description adds meaningful behavioral context: the verification performs existence/deliverability checks and categories like disposable/role-based/risky. The 'Spends 1 credit' note adds cost behavior not present in annotations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.