Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already provide readOnlyHint=true and destructiveHint=false, so the safety profile is known. The description adds value by disclosing that only names and provenance are returned, no passages, and by specifying the returned fields (title, author, library_id, guide URL). This shapes expectations beyond the read-only hint and is helpful for downstream processing.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.