Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint, idempotentHint, non-destructive and openWorldHint=false, so safety is covered. The description adds genuinely non-derived context: the result is deterministic, it costs 1 credit, and it handles movable feasts and King's Day rules — the last being the main edge-case an agent would worry about.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.