Abstract : The development of cross-platform UI at scale requires not only style guides, design patterns, and design language systems but also a design system architecture that enforces principled contracts between visual decisions, behavioral expectations, and accessibility semantics. Design tokens encode visual intention in a portable, platform-agnostic format, enabling design decisions to be semantically mapped to platform representations without re-encoding at each surface. Component contracts abstract stable interaction semantics, accessibility expectations, state transitions, and error handling behaviors that must remain consistent across device classes and rendering backends. Together, these architectural mechanisms address the fundamental sources of interface drift—behavioral divergence, semantic discontinuity, and undetected accessibility regressions—all of which compound across platform-driven release cycles. Interaction correctness and cross-platform consistency are treated as first-class reliability properties, not post-hoc quality concerns. Token standardization and tool-agnostic interchange formats motivate treating tokens as versioned public interfaces subject to the same governance disciplines applied to APIs. Breaking change detection, managed escape hatches, and continuous drift detection pipelines maintain interface integrity as teams scale and surface proliferation increases. Quality is measured through metrics including token coverage, component reuse rates, contract coverage, and assertion parity across platform targets.