Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly, idempotent, non-destructive, so safety is covered. The description adds one genuinely useful behavioral fact not in annotations: related fields the caller is not authorized for are masked rather than omitted or erroring, which affects how the agent should interpret the response. It says nothing about permissions required or whether an unauthorized id yields 404 vs 403.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.