Reverse Engineering ChatGPT Web: How OpenAI Built for a Billion Users
Dennis BrotzkyJuly 2, 2026 Open a new tab, type chatgpt.com, and ask it something. No account, no sign in, no spinner. The page is interactive immediately, and the answer starts streaming a moment after you hit enter. That experience is served to roughly 1 billion people, making chatgpt.com a top website on the entire internet and one of the most used web apps in the world. On the surface it seems like a simple interface, but after digging in it's anything but. I spent days reverse engineering their web app by digging through the page source, bundled code, and network requests to understand how it was built. What I'll dig into Start with the constraints The migration from Next.js to React Router Fastest path to paint CSS at billion-user scale Don't reinvent the wheel with components Every answer is a render problem Can you type yet? Feature flagging everything The fastest path to the first token Letting a billion strangers in The difference between ChatGPT and Claude Before we start: I don't work at OpenAI and I've never seen their source code. What makes this piece different from my Linear and Conductor breakdowns is that OpenAI hasn't really published anything about ChatGPT web. This piece is a mixture of history, investigation, and technical insights. The whole story has been assembled from chatgpt.com itself (the HTML, JavaScript, CSS, and network requests) plus random talks, tweets, and their career page. Video 6 chatgpt.com on a fresh load followed by a prompt.
A universal experience for anyone Start with the constraints The two most important things I consider when architecting a web app are the goal and the constraints. These two factors shape every downstream decision. Who is the customer? How important is the initial load? Do you need an account to use it? Does it need to show up in search engines? What's the budget? How important is the developer experience? Is there existing architecture? And so on... From the outside, the goal of ChatGPT feels clear to me: be the site the entire world uses to interact with AI. It's incredibly important that the site is accessible to anyone around the world on any device. It should cater to everyone from an Android user on a spotty connection who just discovered ChatGPT, to a power user with a Pro subscription on a MacBook Pro. Having those constraints in place eliminates entire architectures for you.