Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations only declare readOnlyHint=true; the description adds useful behavioral context by disclosing what is returned (typed columns, meanings, sort key, coverage). It says nothing about metadata freshness, permissions, or truncation, but is solid beyond the annotation baseline.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.