Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already carry readOnlyHint=true, idempotentHint=true, and non-destructive status, so the bar is lower. The description adds genuine value beyond annotations by disclosing both output paths: success yields formatted JSON, and failure yields 'the exact parse error' — which tells the agent invalid input returns a diagnostic rather than crashing or silently passing through.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.