Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover readOnly/idempotent/non-destructive, but the description adds real behavioral context: color/color_size are null when there is no palette color, and on a text page the style is fixed and the response reports it as not editable. This disclosed edge-case behavior is genuinely useful beyond the annotations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.