Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly/idempotent/non-destructive/openWorld, and the description adds genuinely new facts: the result is free, cached (i.e., not freshly computed), and scoped to one fixed domain. It also sketches the return contents. It does not say how stale the cache is or whether the sample domain can be changed, but for a cached read that is minor.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.