Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish readOnly, idempotent, non-destructive, closed-world behavior, so the safety profile is covered. The description adds genuinely new behavioral context: the result exposes splits, merges, changed meanings and unmapped codes, and that one-to-one equivalence must not be assumed. That cardinality warning is exactly the kind of nuance annotations cannot express.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.