Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=true, and the description adds useful semantic context beyond that: the meaning of well-known keys, the session-start convention for notes, and the canonical funnel steps. This makes expectations about the returned content clear without conflicting with annotations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.