Prototypes support mobile responsiveness out of the box.
The primary target is desktop, but mobile matters. A mobile layout isn't a separate initiative — it's basic accessibility. Getting it wrong means the product becomes unusable for a portion of users before any deliberate mobile work begins.
The solution is to get layout and navigation right once and maintain responsiveness at the component level. The intermediate layer is the widget-and-text-block rule: because widgets and text blocks are isolated, each adapts independently without side effects on its neighbors.
Full mobile optimization is a somewhat different problem: interactive charts, controls sized for fingers, correct behavior under slow connections, offline states, and so on. All of that can be built on top of the pre-prepared foundation whenever the time is right.
The distinction matters. "Mobile-ready" means the structure holds. "Mobile-optimized" means the experience is designed for it. The design system provides the former; the latter is deliberate product work on top.