Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the annotations (readOnlyHint=false, openWorldHint=true), the description discloses the side effect that the host is emailed and that replies surface via get_messages, plus the lifespan of the channel. That is genuinely useful behavioral context an agent cannot get from the structured fields, though it says nothing about delivery failure or rate limiting.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.