Reverse Engineering ChatGPT Web: How OpenAI Built for a Billion Users

TLDR Dev · 1 day ago

OpenAI's ChatGPT web app serves approximately 1 billion users through a carefully optimized architecture that migrated from Next.js to Remix and now uses React Router 7 with server-side rendering. The initial HTML payload is 84 KB compressed with a time to first byte of 50-65ms, delivering a 548-node DOM that prioritizes the chat input as the primary interaction point. The engineering approach uses off-the-shelf libraries rather than custom frameworks, with the server controlling client behavior through prefetch decisions and streaming data to enable rapid interaction without requiring user authentication.