Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare this is a non-read-only, non-destructive, non-idempotent creation. The description contributes the 'empty' initial-state detail, which is real added value. It says nothing about auth/permission needs, duplicate-title behavior, or whether the new run is returned with an ID — gaps that matter for a create tool.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.