What Is OpenAI Codex? How to Use It for Coding (2026 Guide)
OpenAI Codex is OpenAI’s AI coding agent. You describe a task in plain English — build a feature, fix a bug, write tests — and Codex reads your code, makes the changes and shows you the result to review. You can run it on your own computer through the Codex CLI or the VS Code extension, or hand tasks to Codex in the cloud from ChatGPT, where it works on your GitHub repository and prepares changes for you. It is included with paid ChatGPT plans such as Plus and Pro.
📑 Table of contents
What is OpenAI Codex?
Codex is OpenAI’s software engineering agent. It is powered by OpenAI models tuned specifically for coding, and it is designed to take on real development tasks from start to finish: understand the codebase, plan the change, edit files, run commands and tests, and report back.
Don’t confuse it with the old Codex model from 2021 that helped launch GitHub Copilot. That model was retired. The Codex everyone talks about today is a full agent — much closer to a teammate than an autocomplete.
The ways to use Codex
| Option | Where it runs | Best for |
|---|---|---|
| Codex CLI | Terminal on your computer | Developers who like the command line |
| IDE extension | VS Code and compatible editors | Working visually inside your editor |
| Codex in ChatGPT (cloud) | OpenAI’s cloud, on your GitHub repo | Handing off tasks and reviewing later |
| Code review | On GitHub pull requests | Getting a second opinion on changes |
The big idea of cloud Codex is delegation: you can queue several tasks — “fix the login bug”, “add Urdu translations”, “write tests for payments” — and Codex works on each one in its own sandbox while you do something else.
How to set up Codex
- Get a plan. Make sure you have a paid ChatGPT plan (or an OpenAI API key).
- For the CLI: install Node.js, then run
npm install -g @openai/codex. Open your project folder in a terminal and typecodex. - Sign in with your ChatGPT account when asked.
- For VS Code: install the official Codex extension from the marketplace and sign in from its panel.
- For the cloud: open Codex from ChatGPT, connect your GitHub account, and choose the repository you want it to work on.
How to use Codex (step by step)
- Start small. Ask: “Explain how this project is organised.” This shows you what Codex understands.
- Give one clear task. “Add a Contact page with name, phone and message fields that sends to my email. Match the existing design.”
- Review the plan and the diff. Codex shows exactly what it will change. Read it — don’t blindly accept.
- Run the project and test it yourself in the browser, on mobile size too.
- Iterate. Point out problems precisely and let Codex fix them.
- Commit with Git after each working step, so you can always roll back.
AI agents make good developers faster — they don’t replace understanding. The developers earning the most are the ones who can review AI output with confidence.
AGENTS.md and approval modes
AGENTS.md is a simple text file in your project where you write instructions Codex should always follow — coding style, how to run tests, “don’t touch the payments folder”, brand colours, and so on. It saves you repeating yourself in every chat.
Codex also lets you choose how much freedom it has:
| Mode | What Codex can do | When to use |
|---|---|---|
| Read only | Look and suggest, no changes | Exploring a new codebase |
| Auto (default) | Edit files & run commands in your project, asks before going further | Everyday work |
| Full access | Act without asking, including network access | Only in safe, throwaway environments |
Beginners should stay in the default or read-only mode until they are comfortable reviewing changes.
Best tasks for Codex
- Fixing bugs from an error message or screenshot
- Adding features to an existing website or app
- Writing and running tests
- Refactoring messy code and updating old libraries
- Explaining unfamiliar code before you change it
- Reviewing pull requests before you merge them
How to earn with Codex
Freelancers in Pakistan use AI coding agents to take on more work and deliver faster: website builds, WordPress and Shopify fixes, custom dashboards, automation scripts and MVPs for startups. On Fiverr and Upwork, “fast turnaround” is a big selling point — but your reputation depends on quality, so always test before delivery.
Learn Codex in Okara
IDT Okara’s Vibe Coding course covers Codex, Claude Code and Cursor side by side, so you learn the workflow — not just one tool. You build real projects, learn Git and deployment, and graduate with a portfolio.
Read next: What Is Claude Code? and What Is Vibe Coding?
Frequently asked questions
What is OpenAI Codex used for?
Codex is used to write new features, fix bugs, explain unfamiliar code, write tests, and review changes. Developers give it tasks in plain English and review the code it produces before using it.
Is OpenAI Codex free?
Codex is included with paid ChatGPT plans such as Plus, Pro, Business and Enterprise, with usage limits depending on the plan. Developers can also use it with an OpenAI API key and pay per use. Check OpenAI’s site for current plans and limits.
Is the new Codex the same as the old Codex model?
No. The original Codex (2021) was a code-completion model that powered early GitHub Copilot and has been retired. Today’s Codex is a full coding agent that can work through multi-step tasks on its own.
What is the difference between Codex CLI and Codex in ChatGPT?
Codex CLI runs in the terminal on your own computer and edits your local files. Codex in ChatGPT runs in the cloud on a connected GitHub repository, so you can give it several tasks at once and review its proposed changes later.
Codex or Claude Code — which is better?
Both are excellent and very close in ability. Many developers use both. Pick the one that fits the subscription you already have, then learn the workflow — clear instructions, small steps and careful review matter more than the tool.
Can beginners use Codex?
Yes, but beginners should start with small, clear tasks and learn the basics of HTML, Git and how projects are structured. Without that, it is hard to tell whether the AI’s changes are correct.
Learn Codex, Claude Code & Cursor hands-on
IDT Okara’s Vibe Coding course teaches you to build and ship real websites and apps with today’s top AI coding agents — with a trainer reviewing your work. Small batches, apply online in 5 minutes.