Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=true and destructiveHint=false, so the non-mutating nature is covered. The description adds valuable behavioral details: it returns whether the comment would be accepted, the exact blocker (code + message), sanitized HTML, resolved mentions, and warnings, plus the caveat that rate-limit/quota are not re-checked. This goes beyond the annotations and gives the agent a clear picture of the tool's behavior.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.