A change can be only a few lines long and still leave you wondering what it might break. Codex can help you work through that question in the project itself: read the surrounding code, find the tests, make a proposed edit and run the tools you already use. It is OpenAI’s coding assistant, and this guide covers its command-line interface, or CLI, which runs in a terminal. We will start with the connection between a change and its tests, because understanding that connection makes the next edit easier to judge.
01Fit
02Requirements
Check the account you will use before starting a long session. CLI access, included usage and limits depend on that account; billing through an API key is separate from a ChatGPT subscription. The current access table explains what is included and when another charge may apply.
Check current access and pricing03Setup
On macOS or Linux, the command below downloads and runs the official installation script. Review the script before running it. Windows has its own instructions on the same setup page.
curl -fsSL https://chatgpt.com/codex/install.sh | shSave your current work in Git, open the project directory in a terminal and run codex. Follow the sign-in flow available to your account. Having a saved starting point lets you see what Codex changes and return to the original code if you decide against the patch.
codexUse /permissions and /status to inspect the session’s access and settings. For an initial look around, the security guide also describes launching Codex read-only. A request to explain the code tells Codex what you want; read-only filesystem permissions prevent an unwanted edit while you are still discussing it.
04Using Codex
Use a change you are already considering, preferably in a part of the project small enough to read yourself. Perhaps a bug fix looks straightforward, but you are unsure which test would catch the old behavior. Codex can help connect the reported problem, the code that produces it and the checks already in place. Name the function or file and explain the behavior you want before asking the question below.
What to ask
Before we change this code, help me work out how to check it. Find the relevant source and tests, explain what they already cover, and show where the behavior I described is tested or missing. Suggest a test that would distinguish the current behavior from the intended result. Wait for me before editing.
You have a way to check the change and an explanation of why that check matters. The useful contribution may be a new test, or it may be discovering that the right test already exists. Either result gives you a firmer basis for the edit than asking Codex to change the code first and explain it later.
05Next steps
If each new task begins with missing dependencies or the same setup commands, a prepared cloud environment can save that repeated work. The Codex Cloud guide explains what belongs in reusable setup and what remains specific to an individual task. That distinction matters when a successful run depends on files left behind by an earlier session.
Read the guidanceThe person reviewing your patch will need the same connection you just established between the behavior, the edit and the test. Keep the tested revision, commands and results with the change. The release-engineering guide follows that information into a release review, where a passing result is useful only if everyone knows what was actually checked.
Read the guidance06Alternatives
07Evidence and scope
Based on official documentation reviewed September 30, 2026. The example is illustrative; this guide does not report an installation, benchmark or production test.
Documentation checked