How to Run Your ChatGPT App Locally

[Updated 2026-02-05] If you’ve heard about ChatGPT Apps and want to try building one, you might think you need a paid ChatGPT account to get started. You don’t. With sunpeak, you can develop and test ChatGPT Apps entirely on your local machine using a built-in ChatGPT simulator.

TL;DR: Install sunpeak globally, run sunpeak new and sunpeak dev, then open localhost:6767 to use the ChatGPT simulator. No paid ChatGPT account needed.

What is Sunpeak?

Sunpeak is an open-source ChatGPT App Framework built on the MCP Apps protocol. It gives you:

• A local ChatGPT simulator - Test your apps without connecting to ChatGPT
• Project scaffolding - Start with working code, not a blank file
• Hot reloading - See changes instantly as you code
• MCP server - The backend your app needs, already configured

Think of it as “create-react-app” for ChatGPT Apps.

What are ChatGPT Apps?

ChatGPT Apps are interactive UIs that run inside ChatGPT conversations. When a user asks ChatGPT to do something, your app can render a rich interface directly in the chat - forms, charts, carousels, games, anything you can build with React.

Under the hood, ChatGPT Apps are built on the Model Context Protocol (MCP), which means your app is structured around Resources (pages/views) and Tools (actions the AI can invoke).

Getting Started

Prerequisites

You’ll need Node.js 20 or higher installed on your machine.

Step 1: Install sunpeak and create a project

pnpm add -g sunpeak && sunpeak new

This installs sunpeak globally and walks you through creating a new project. Pick a name and choose a template to start with.

Step 2: Start the development server

cd my-app
sunpeak dev

That’s it. Open http://localhost:6767 in your browser.

Using the Local Simulator

The sunpeak simulator mimics the ChatGPT interface so you can test your ChatGPT App without deploying anything. You’ll see a chat interface where you can invoke your app’s tools.

The simulator shows you exactly how your ChatGPT App will look and behave inside ChatGPT, including:

• Different display modes (inline, modal, full-screen)
• Light and dark themes
• Tool invocations and responses

As you edit your code, the simulator hot-reloads automatically. No manual refresh needed.

Project Structure

When you create a sunpeak project, you get:

my-app/
├── src/
│   └── resources/     # Your app's pages/views
├── tests/
│   └── simulations/   # Your app's common test states
└── package.json

Start by editing the files in src/resources/ to customize your UI.

When You’re Ready to Deploy

Once your app works locally, you can connect it to the real ChatGPT:

1. Expose your local server with ngrok:

   ngrok http 6766

2. In ChatGPT, go to User > Settings > Apps & Connectors > Create (requires a paid account and developer mode)

3. Add your ngrok URL with the /mcp path

But there’s no rush. The local simulator is fully functional for development and learning.

Next Steps

• Try the interactive simulator demo in your browser
• Explore the sunpeak documentation
• Please star sunpeak on GitHub!