Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare the tool read-only and idempotent, so the bar is lower. The description adds meaningful behavioral detail by specifying the structure check and the Luhn mod-10 computation with the A=10…Z=35 conversion, making the validation logic transparent beyond what annotations provide.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.