Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already indicate read-only and idempotent behavior. The description adds valuable context beyond that: the FREE vs HOBBY+ tier restriction, the history boolean parameter behavior, and the 'empirical next-day transition odds' detail. This gives a richer picture of what the tool returns and under what conditions, without contradicting annotations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.