Lesson 0 / 7 · ~8 minutes
Codex: an agent,
not a chat
You know ChatGPT: you type a question, you get an answer. Codex is a different animal. It doesn't advise you on how to do things — it does them. It reads your files, writes code, runs it, tests it and hands you a finished result to review.
While classic vibecoding has you sitting in a conversation with AI, building the thing together step by step, Codex is a delegate: you brief a task, it goes off to work, and you come back to a finished result. It will happily work on three things at once — and happily while you're at lunch.
The delegation loop
Brief the task so the agent can handle it without you. The agent works — minutes, sometimes half an hour. Review the result: the diff, the logs, the tests — and decide: accept, adjust, or re-brief differently.
Where Codex runs
- The desktop app — since July 2026, Codex is part of the ChatGPT app for Windows and Mac. This is the most comfortable way to start.
- The cloud — tasks run in an isolated environment on OpenAI's servers with a copy of your project. Your computer can sleep meanwhile.
- Mobile — you brief and review tasks straight from your phone; via Codex Remote you can even drive a paired computer from afar.
- The terminal (CLI) — for command-line lovers. You'll see it a few times in this course's replays.
What it's good for
The same rule as all vibecoding: brilliant for websites, calculators, internal tools, scripts and changes to existing code — things whose results you can verify. Not for critical systems without experienced oversight. And with Codex it applies doubly: the better you can describe the task up front, the more you get out of delegation.
Test yourself: myth or reality?
Five claims about Codex. Click what you think.
"Codex works even while I sleep."
Reality — cloud tasks run on OpenAI's servers in an isolated environment. Brief in the evening, review in the morning.
"I need an API key and to pay per token."
Myth — Codex is part of the regular ChatGPT subscription. ChatGPT Plus is enough to start.
"Codex is just better code completion for programmers."
Myth — it's a full agent: it reads the project, writes, runs, tests and returns a finished solution. You write the brief in plain sentences.
"I can brief a task from my phone on the way back from lunch."
Reality — from the mobile ChatGPT app you can create, watch and fork tasks. You review the result when you're back at your desk.
"If the tests passed, I can accept the result without looking."
Myth — reviewing the diff is your half of the job. Tests verify nothing broke; whether the app does what you want, only you can tell.
Summary
- Codex = a delegate: BRIEF → AGENT WORKS → REVIEW.
- It runs in the ChatGPT app, the cloud, on mobile and in the terminal.
- ChatGPT Plus is enough, no API keys.
- Reviewing the result is your job — the agent does, you decide.