Building a ChatGPT App Without a Paid Account or Developer Mode (April 2026)

Want to build a ChatGPT App but don’t have a paid ChatGPT account? You don’t need one. You can build, test, and ship a production-ready ChatGPT App without ever logging into ChatGPT.

TL;DR: Run npx sunpeak new sunpeak-app, then cd sunpeak-app && pnpm dev. Open localhost:3000 to build and test your ChatGPT App in a local replica of the ChatGPT runtime. No subscription, no credits, no manual refresh cycle.

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, dashboards, data visualizations, multi-step workflows, anything you’d build as a web app.

ChatGPT Apps are built on the Model Context Protocol (MCP), an open standard for connecting AI applications to external systems. Your App consists of two MCP primitives:

• Resources are HTML pages rendered in sandboxed iframes inside the chat. This is the UI your user sees and interacts with.
• Tools are actions the AI model can invoke, like fetching data or running a calculation. When a tool has an associated resource, ChatGPT renders that resource as an interactive App.

Because ChatGPT Apps use MCP, the same App also works in Claude (as a connector), VS Code Copilot, and other MCP-compatible hosts. You write one codebase and it runs everywhere.

Why you don’t need a paid account to develop

OpenAI requires developer mode (and a paid ChatGPT Plus, Team, or Enterprise subscription) to connect custom Apps to ChatGPT. That makes sense for live testing, but it creates a barrier for developers who just want to build.

The sunpeak Inspector removes that barrier. It replicates the ChatGPT App runtime on your local machine, so you can develop and test your entire App without any ChatGPT account. The Inspector runs at localhost:3000 and shows exactly how your App will look and behave inside ChatGPT.

Here’s what the local development flow looks like:

npx sunpeak new sunpeak-app
cd sunpeak-app
pnpm dev

Open localhost:3000 and your App is running in the Inspector. That’s it.

What you can do locally

The Inspector replicates the full ChatGPT App runtime, so local development covers everything you need during the build phase:

• Build and test your UI components with hot reloading (every code change appears instantly, no manual refresh)
• Switch between all three display modes: inline, fullscreen, and picture-in-picture
• Toggle light and dark themes
• Configure tool invocations with mock responses
• Load simulation files for specific App states
• Test across hosts by switching between ChatGPT and Claude from the sidebar dropdown

That last point matters because your ChatGPT App is an MCP App. It runs in Claude (as a connector) and other MCP hosts too. The Inspector lets you verify cross-host behavior from one interface.

Automated testing without an account

Beyond clicking around in the Inspector, sunpeak includes a testing framework so you can write automated tests that run against the local Inspector using Playwright. No paid account, no API credits.

pnpm test           # unit + e2e tests
pnpm test:visual    # visual regression testing
pnpm test:eval      # multi-model LLM evals

Simulation files in tests/simulations/ provide deterministic mock data, so your tests produce the same result every run. You can add these to a CI/CD pipeline and validate every pull request automatically. The complete guide to testing ChatGPT Apps covers this in depth.

This is where skipping the paid account saves real money. If you run 50 test iterations a day against the real ChatGPT, that’s 50 API calls burning credits and requiring a subscription for every developer on your team. With sunpeak, those same tests run locally for free.

When you actually need a paid account

A paid ChatGPT account is only required for three things:

1. Live testing against ChatGPT’s AI responses. The Inspector mocks tool responses deterministically, which is better for development. But if you need to see how ChatGPT’s actual model triggers your tools and renders your App, you’ll need to connect to the real thing.
2. Deploying your App for end users. When real ChatGPT users interact with your App, it’s served through ChatGPT’s infrastructure, which requires a developer account.
3. Publishing to the ChatGPT marketplace. Listing your App for others to discover requires a verified developer account.

For everything else, local development, automated testing, building production bundles, and even testing in Claude, a paid ChatGPT account isn’t needed.

Connecting to ChatGPT when you’re ready

When you do want to test against the real ChatGPT, expose your local MCP server with a tunneling tool like ngrok (a free ngrok account works):

ngrok http 8000

Copy the forwarding URL and add it in ChatGPT:

1. Go to Settings > Apps > Advanced settings
2. Enable Developer mode (requires Plus, Team, or Enterprise)
3. Click Create app and enter your ngrok URL with the /mcp path, for example: https://abc123.ngrok-free.dev/mcp

Start a new conversation, invoke one of your tools, and ChatGPT will call your local MCP server and render your App UI in the chat. The local development guide walks through this in more detail.

Building for production

When your App is ready to ship, build and start the production server:

pnpm build && pnpm start

pnpm build compiles your resources into optimized bundles. pnpm start launches the MCP server on port 8000 (configurable via the PORT environment variable). Deploy to any Node.js host, or use createMcpHandler for Express setups and createHandler for serverless platforms. The deployment guide covers production configuration.

Your App works in ChatGPT, Claude, and any other MCP-compatible host from the same build. No separate builds per host.

Get started with the quickstart guide, explore the ChatGPT App framework, or try the full tutorial.