Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish read-only, idempotent, and non-destructive behavior, so the bar is lower. The description adds useful context by stating that presence includes both people and agents and that it reflects current state, which goes beyond the annotations. There is no contradiction.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.