Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already provide readOnlyHint, idempotentHint, and destructiveHint, but the description adds valuable context: it is read-only and 'never pays or requests private bytes,' which is a meaningful behavioral guarantee beyond the schema. It also discloses the scope of the audit (all 107 levels) and what is checked, without contradicting the annotations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.