Every product team has a style guide. Six months in, the web team's components look one way, the iOS team has quietly diverged, the Android team is tracking a third set of design decisions, and the "single source of truth" has become three parallel versions of partial truth. The team knows this and has tried every fix — ran a design-system sprint, nominated a component steward, froze the Figma library. Drift resumes within a quarter. The problem is not discipline. The problem is that style guides built for human upkeep cannot keep pace with the number of screens, variants, and platform-specific implementations a real product ships. This article sets out what a style guide needs to look like in 2026 to stop needing manual upkeep at all — the four structural properties that make a guide durable, the tools that enforce those properties, and why the portable unit of consistency is increasingly the rule (a prompt, a token, a spec), not the document .…