Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations only provide false hints, so the description carries the behavioral burden. It discloses a consequential trait: the rule is injected only on every NEW chat, not existing chats. It also provides useful limits (15000 chars, max 20 rules per user), though it does not describe edge-case behavior at those limits.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.