Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly/idempotent, so the bar is low, yet the description still adds real behavioral context: a cost of 1 unit, a consent precondition for personal frameworks, and an interaction protocol (one question at a time, verbatim, A/B answers only). None of that is derivable from the structured fields.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.