Thesys just open-sourced their generative UI rendering engine. Interesting timing given where Google a2ui and Vercel's json-render are headed. The difference worth noting: a2ui and json-render both treat JSONL as the contract between the LLM and the renderer. Thesys is betting that's the wrong primitive. Their engine uses a code-like syntax (OpenUI Lang) instead — LLM writes it, renderer executes it. The argument is that LLMs are fundamentally better at generating code than generating structured data, so you get cleaner output and ~67% fewer tokens. The broader vision seems to be a model-agnostic, design-system-agnostic layer that sits between any LLM and your actual UI components. You bring your own components and design tokens, the engine handles translating LLM output into rendered interfaces — charts, forms, tables, cards. Generative UI as a category is still figuring out what the right abstraction is. This is a concrete stake in the ground against JSON-as-spec.