Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare it is a non-readonly, non-idempotent, non-destructive mutation, but the description adds real depth: only passed fields are written (partial update), values are re-validated server-side under a row lock (concurrency/validation), and it enumerates error modes (not Premium, account not found, bad value, nothing to apply). It stops short of discussing reversibility or auditing, so a 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.