Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint and idempotentHint. The description adds valuable behavioral detail about the server-side recomputation and the returned validation flags, without contradicting the annotations. It doesn't however discuss failure modes or rate limits, but the bar is lower due to annotations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.