Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare the mutation profile (readOnly=false, non-idempotent, non-destructive), so the bar is lower. The description adds value the annotations cannot: non-compiling or failing rules abort the entire create (atomic failure), diagnostics arrive in details, and '1 request' signals cost. It doesn't explain what happens to a partially-created list beyond the refusal, but the failure semantics are notably well disclosed.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.