Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations provide clear behavioral hints (read-only, non-destructive, idempotent, open-world), which cover safety and idempotency. The description adds no additional behavioral context, but since annotations are comprehensive, the bar is lower. There is no contradiction with annotations (e.g., it doesn't imply mutation), so the description doesn't detract, but it also doesn't supplement with details like rate limits or authentication needs. With good annotations, a 4 reflects that the description doesn't add value but doesn't mislead.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.