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 demonstrate a curious mental shortcut that people fall into all the time. But before getting into the tools, let’s try the experiment on you. Pick any random number from 1 to 100. Write it down or just keep it in mind. Now answer this: how many African countries are members of the United Nations? That setup leads us to the interesting part. Even though the first number was random and had nothing to do with the second question, it often still influences the estimate. If your first number was high, say 78, your guess about the number of African UN