Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered. The description adds valuable behavioral detail by enumerating specific render-time warning categories (non-ASCII text, clipping past composition end, missing duration) that the runtime would silently drop or clip. No contradiction 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.