Codex

I don't write code.

Instead I tinker with a file called AGENTS.md. Instructions for an agent. When the agent makes a mistake, I add a line so it never makes that mistake again.

That file is what Codex reads.

The word codex comes from Latin caudex. A block of wood. Romans coated wooden tablets in wax and scratched words into them. Bound several together as a record. They called it a codex. When wood gave way to parchment, the name stayed.

In 529, Emperor Justinian compiled every decree scattered across the Roman Empire into one system. Not individual laws, but a law to govern laws. Every edict in one volume, so you only had to look in one place. They called it Codex Justinianus, a legal code.

In 2026, I'm building my own Codex Justinianus. Rules, lessons from failures, memory, all in one file so the agent only has to look in one place.

OpenAI shared a number on their blog this February. Three engineers. Five months. A million lines of production code. Lines written by humans: zero. What they wrote wasn't code. It was codex. Markdown wrapped around the agent. They call it harness engineering.

To use codex (the app), you first write your own codex (legal code). Not on parchment. On markdown. This codex is what codex (the agent) follows. I get a little nervous every time I open the IDE. Code I never wrote is following my codex. It runs fine. I didn't write a single line.

Legal code, agent, app. One word holds this tool's entire workflow. Whether whoever named it intended all of this, I have no idea.

Codex. Well named.