Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the readOnly/idempotent/non-destructive annotations, the description discloses error semantics an agent needs: missing entities return not_found rather than a tool error, while API failures do raise errors. It also previews what each entity type returns (benchmark links, abstract/authors/PDF/code links, related benchmarks).
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.