Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=false and destructiveHint=false, so the agent knows this is a write operation that is not destructive. The description adds the concept of a 'custom KPI from a formula over other metrics,' which clarifies the semantic behavior. However, it doesn't disclose details like whether the metric is immediately available, whether it can reference other calculated metrics, or what the response contains. With annotations covering the basic safety profile, a 3 is appropriate.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.