Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly, idempotent, non-destructive, open-world behavior, so the bar is lower. The description adds genuinely useful behavior beyond them: US-only coverage via NWS, the shape of the response (named periods with temperature, wind, short + detailed forecast), and the existence of a different mode. It does not mention rate limits or data staleness, keeping it out of 5 territory.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.