Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description's job is to add context beyond that. It does so with a concrete output-handling obligation: always surface the calculator_url as a clickable markdown link. That is a real behavioral instruction an agent would otherwise miss, though nothing is said about input validation or edge cases.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.