Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish read-only, idempotent, non-destructive behavior, so safety is covered. The description adds genuinely new behavior: facets are computed over the entire filtered bibliography, not the current page, and results are ranked. That is exactly the trait that distinguishes facet aggregation from paginated search.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.