Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare mutating, open-world, non-idempotent, non-destructive behavior. The description adds valuable behavioral context beyond that: account-type limits (free = 3 songs, Pro = 50), that anyone can open the link, and that output is a shareable ordered link. It doesn't cover error responses or what happens at limit overflow, but it adds substantive non-annotation context.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.