... used by 10,000+ developers, and the biggest lesson was that JSON-based approaches break at scale. LLMs keep producing invalid output, rendering is slow, and custom design systems are a pain to wire up. So we built OpenUI Lang , a rendering spec inspired by code rather than data formats. LLMs generate it reliably the way they generate well-formed functions. The benchmarks: up to 67% fewer tokens , 3x faster rendering , streaming-native, and safe by default (no arbitrary code ...