Quick Start: Learn five mental models that cover every HTTP response, then read live codes with curl . Total time: 20 minutes. What You’ll Learn What an HTTP status code is, and what problem it solves. Five mental models that make any code readable, not just the ones you’ve memorized. The mnemonic for all five classes, and four more models for the hard cases. The handful of codes you’ll meet 95% of the time. When the code lies to you (200 with an error inside). How to read status codes yourself with curl . Where to go to learn the long tail. The Basics Every HTTP response carries a three-digit status code: the server’s verdict on your request, delivered before any body content. The first digit classifies responses into five categories; knowing those five is enough to read any code you encounter.