GPT-5.3-Codex vs. Claude Code: a comparison

This post is a quick overview of an Abto Software blog article about GPT‑5.3-Codex vs. Claude Code: a comparison. Anthropic released Claude Opus 4.6, and almost at the same moment, OpenAI rolled out GPT-5.3-Codex. That timing made the comparison too tempting to ignore. So we decided to test both models in a practical way: by asking them to build a small website around a well-known psychological bias. The idea was simple. The website would help nstrate a curious mental shortcut that people fall into all the time. But before getting into the tools, let’s try the experiment on you.