Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish mutation-without-destruction (readOnly=false, destructive=false, idempotent=false), and the description adds real behavioral depth beyond them: horizon pins override ordered_ids position, unknown or de-queued ids are skipped, duplicates are rejected, and the returned shape is disclosed. It stops short of noting the non-idempotency implication or any permission/rate constraints, so 4 rather than 5.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.