Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish readOnlyHint=false, so the mutation is expected; the description adds concrete behavioral details: author-only enforcement, content length bounds (5-10000 chars), and possible error conditions (not found, not author, validation). This goes beyond what annotations convey.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.