Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and a closed world, so the safety profile is covered. The description adds genuinely new behavioral context on top of that: the response content varies by subscription tier, with certain metrics and a wider top-N list appearing only on Pro. That plan-dependent variability is exactly the kind of thing annotations cannot express. It stops short of 5 because it says nothing about rate limits, caching, or freshness of the 24h figures.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.