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

> Source: <https://stemjson.com/>
> Published: 2026-09-14 11:26:45+00:00

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.