Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the annotations (readOnly, non-destructive, idempotent, closed-world), the description adds billing behavior (valid members read on demand, no trial deduction), membership gating, and the precise semantics of the public_record_matched result field. It also conditions the cross-app jump on the user having opened original_page, all of which an agent could not infer from structured data.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.