Show HN: StemJSON – a language for LLMs to extend native mobile apps on the fly

StemJSON, a new declarative JSON-based language designed for LLMs to generate native mobile app UI and logic, was posted to Hacker News as a Show HN project. The format lets an LLM emit StemJSON from a user prompt, which a runtime validates and renders as a fully native module inside a developer-defined sandbox, with no new binary and no backend round-trip for the UI. The project is patent pending and is positioned for server-driven UI, prototyping tools, and alongside hand-written code.

StemJSON is a declarative language based on JSON, well-suited for AI to generate comprehensive, performant native app UI and logic - whether building new apps or extending existing ones. The format is open. Drop it into AI tools, prototyping environments, IDE extensions, or alongside hand-written code to simplify and accelerate native app design, development, and delivery. A sandboxed area inside your app where end users describe a feature in their own words - and the runtime renders a fully native module on the spot. Apps that adapt to each user, on demand. An LLM emits StemJSON from the user's prompt; the runtime validates it, renders it natively, and keeps it inside the sandbox you defined. No new binary, no backend round-trip for the UI. Patent pending. User prompt LLM StemJSON Native module Other ways to use it Server-driven UI Push complete functional modules from your backend - state, actions, expressions, navigation, validation - straight to the device. your server → StemJSON → device Prototyping & design tools From a prompt or a Figma export to a runnable native prototype. Tools emit StemJSON; the runtime renders it on-device - no glue code in between. Figma / AI tool → StemJSON → runnable app Alongside hand-written code Drop StemJSON modules into a native app for the parts that change often - settings, onboarding, content feeds - and keep the rest hand-written.