Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly/idempotent/non-destructive, so the safety profile is covered; the description adds real behavioral detail beyond that — the significance test used (Welch t-test at 95%), the rule that `winner` is only named when significant, and the fallback states "no_significant_difference" and "insufficient_samples". This is meaningful outcome-condition disclosure that the annotations cannot express.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.