Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover read-only, non-destructive, and open-world behavior, so the description does not need to restate those traits. It adds useful behavioral context beyond annotations: quota is scoped to a property and returns notConfigured: true when no Bing Webmaster API key is configured.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.