What is Codex?

You just spent a whole stack of lessons getting cozy with Claude Code. Now let's meet its cousin. Codex is an AI coding agent made by OpenAI, the company behind ChatGPT. Same as Claude Code, it can read your project, change files, run commands and explain what it did.

By the end of this lesson you'll know what Codex is, where you can use it, what it's good at and how it stacks up next to Claude Code.

An AI Coding Agent From OpenAI#

Codex is a coding agent, and that word "agent" is doing real work. A chatbot answers questions. An agent actually DOES stuff.

When you hand Codex a task, here's what it can do.

• Dig through the files in your project to understand how everything fits together
• Make edits to your code
• Run commands, like your tests or your build
• Show you what changed so you can keep it or toss it

You describe what you want in plain English, same as you've been doing with Claude Code, and Codex figures out the steps.

The Places You Can Use Codex#

Codex shows up in a few different places, and they all share the same brain. Pick whichever one feels most comfy.

1. The Codex CLI (Your Terminal)#

The CLI is a program you run in your terminal by typing codex. It works right on the files sitting on your computer. This is the closest match to Claude Code, and it's the one we'll set up next lesson.

Fun fact for later... the Codex CLI is open source. Its code is public on GitHub at github.com/openai/codex.

2. The IDE Extension (Your Code Editor)#

If you like living inside a code editor, Codex has an extension for VS Code and the editors built on it, like Cursor and Windsurf. JetBrains IDEs and Xcode have their own ways to pick Codex as the assistant. The extension sits right beside your code, so you can point at the file you've got open and ask about it.

3. The ChatGPT Desktop App#

The ChatGPT desktop app (for Mac and Windows, with a Linux version in preview) has Codex built right in. You open a folder, pick Codex instead of regular ChatGPT and give it a task. Great for longer jobs, and great for folks who'd rather not stare at a terminal.

4. Codex Cloud (On the Web)#

Codex Cloud runs tasks on OpenAI's computers instead of yours. You connect a GitHub repository, set up an environment once, and then hand it tasks from the web, your phone or the desktop app. It keeps chugging along even while your computer is asleep. When it's done, you review the changes and open a pull request if you like what you see.

What Is Codex Good At?#

Codex handles the same kinds of everyday coding work you've watched Claude Code do.

• Explaining a project. "Tell me about this project" is a GREAT first question in a folder you don't understand yet.
• Focused changes. Fixing a bug, renaming something across files, adding a small feature.
• Running your tools. It can run your tests and read the output to see if its change actually worked.
• Reviewing changes. The CLI has a /review command that looks over your changes and reports problems it finds, without editing anything.
• Working in the background. With Codex Cloud you can hand off a task and come back later.

It's got some safety habits baked in, too. On your own computer, Codex runs inside a sandbox, which is a fenced-off area that limits what it can touch. By default it can only change files in your project folder, and it has no internet access unless you say so. We'll dig into that more in the setup lesson.

How Codex Relates to Claude Code#

Here's the simple version. Claude Code and Codex are the same kind of tool, made by two different companies.

• Claude Code is made by Anthropic and runs on Claude models.
• Codex is made by OpenAI and runs on OpenAI models.

Both are AI coding agents. Both read your project, make changes, run commands and ask your permission (each in its own way). Both can live in your terminal, your code editor, a desktop app or the web.

They also both read a special instructions file you keep in your project. Claude Code uses CLAUDE.md, which you met earlier in this module. Codex uses AGENTS.md. Same idea, different name. And here's the good news... Claude Code can read an AGENTS.md file too, which makes running both on one project a whole lot easier. There's an entire lesson on that coming up.

A Quick Example#

Say you open a project folder and start Codex. You type this.

Tell me about this project. What does it do, and which files matter most?

Codex reads through the folder and answers with a quick tour. Then maybe you ask this.

The contact form doesn't show an error when the email is blank. Fix that and run the tests.

Codex finds the form, makes the change, runs your tests and tells you what it did. You check the change before you keep it. If that feels familiar, it should, because it's the exact same loop you've been running with Claude Code.

TL;DR#

• Codex is an AI coding agent made by OpenAI, tied to your ChatGPT account
• You can use it in your terminal (CLI), your code editor (IDE extension), the ChatGPT desktop app or on the web (Codex Cloud)
• It reads your project, edits files, runs commands and can review changes
• On your computer it runs inside a sandbox that limits what it can touch by default
• Claude Code and Codex are the same kind of tool from different companies, and both use a project instructions file (CLAUDE.md and AGENTS.md)

What's Next?#

Next lesson you'll install the Codex CLI, sign in and give it its very first safe task in a real project folder...