StemJSON
StemJSON MCP Server

Generate mobile features in your AI chat

Describe a feature to Claude or ChatGPT; this server helps it write a valid StemJSON module and hands back a link. Open it on your iPhone or iPad to run the module in StemStudio AI.

How to connect

Works with Claude · Claude Code · ChatGPT · any MCP client

From prompt to phone

your AI chat

you A checkers game I can play against an AI - build it with StemJSON.

scaffold_module ✓ validate_module ✓ submit_module ✓

AI Validated and accepted. Open it on your phone - or download checkers.stem there: stemjson.com/t/…

A playable game - AI opponent, kings, multi-jump chains - in one StemJSON file.

Checkers module rendered natively on iPhone - full board with red and black pieces, 'Red to move' status card, and New game controls
Rendered natively on iPhone

Connect

server URL 
https://stem-cloud-api-183076946186.europe-west1.run.app/mcp
  1. Settings → Connectors on claude.ai or in the desktop app.
  2. Add custom connector, paste the server URL.
  3. Sign in with GitHub when prompted.

Free with your existing AI subscription. GitHub sign-in, rate limits apply. Questions or higher limits: email us.

What you get back

A share link

It opens a page for your module. On iPhone or iPad, download the .stem file there and open it in StemStudio AI to run it.

The .stem file

A module is a single JSON file - download it from the share link. Use it in the example apps, or in any app with the StemRuntime SDK.

Validation

The server checks structure, references, and design rules; failures go back to the AI to fix, not to you.

Where your modules run

On your phone

StemStudio AI

Import a module and run it natively on iPhone and iPad, then refine it with AI or in the JSON editor.

About StemStudio AI →
Runnable today

Public examples

Complete example apps with the runtime integrated. Clone one, add your .stem file, and run it.

Swift  ·  Kotlin
In your own app

StemRuntime SDK

Add the SDK to your iOS or Android app and render StemJSON inside it: ship new screens from your backend without an app update, or add a Stem AI Area where your users generate features on the fly.

About the runtime →

Under the hood

Nine tools cover the whole authoring loop - spec lookups while writing, validation before anything reaches you, submission at the end. Also useful on their own when writing StemJSON by hand or reviewing generated modules.

  • begin_stemjson_module Returns the authoring workflow the AI follows.
  • scaffold_module Starter skeleton for a new module.
  • get_example Complete, working example modules.
  • lookup_component Contract for any UI component.
  • lookup_action Contract for any action.
  • search_spec Full-text search over the spec.
  • get_spec_section Read a spec section in place.
  • validate_module The same validator the runtimes use.
  • submit_module Accepts the module, returns a share link.
Specification StemRuntime SDK

Paste the URL. Build something.

Setup steps