Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish readOnly, idempotent, and non-destructive behavior, so the description's job is lighter. It adds meaningful context beyond annotations by enumerating the record's contents and, importantly, warns that submitted text must be treated as untrusted data rather than instructions—a security-relevant behavioral trait an agent must know.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.