installing codex

installation

Codex is OpenAI's coding agent running in the terminal. It installs via npm the same way:

npm install -g @openai/codex

Check:

codex --version

signing in

Start it in a project folder:

codex

On first start, choose signing in with a ChatGPT account – this uses your Plus subscription's allowance, no separate API key needed. After the browser approval, the terminal greets you with the same conversation interface as Claude Code.

same genre, different character

The two agents do the same work – write files, run commands, iterate – but their style differs noticeably. The practical difference you'll see in the lessons:

• Claude Code's strength is the long, multi-step session and the disciplined following of project rules (CLAUDE.md).
• Codex's strength is fast, focused task execution and a strong sandbox (an isolated execution environment – the agent works without access to the rest of your system) default.

As a beginner you don't have to choose "forever": the curriculum shows you both on real tasks, and you'll develop a feel for when to reach for which. What matters, though, is that one agent works on a project at a time – two agents on the same code overwrite each other's changes.

try it

Give it the same task you gave Claude Code:

Create a hello.txt file that introduces you in one sentence.

Watch the difference between the two tools' behavior: what it asks, what it approves on its own, how it reports. This comparative attention is a recurring exercise in the curriculum – it's how you build the judgment that is the real expertise of agentic work.

Both agents running, both signed in – everything on your machine is ready. The module's last lesson is the first real build: a working web page, from zero, from your own terminal.