Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already provide readOnly, idempotent, and non-destructive hints, so the behavior bar is lower. The description adds valuable normalization behavior: spacing, punctuation, and capitalization don't affect matching. It also discloses that results are ranked by field, which is beyond what the annotations or schema state.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.