Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=false and openWorldHint=true, but the description adds crucial behavioral context: the operation is billable at ~$0.005 per email, every submitted address is billed regardless of verdict, and cost.amount_charged in responses shows actual spend. This exceeds annotation coverage and informs the agent of cost implications.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.