Lesson 9.1 · 40 minCodex in your course project

What you will have

Codex is installed and signed in, runs in the course project in a mode where it asks before every edit, and reads the project rules through a short AGENTS.md that points to CLAUDE.md. The student has made one change with Codex, reviewed it and committed it.

Video

The video for this lesson is not recorded yet.

Do along

Do these steps on your own project, pausing the video where a step says so.

1. Pause after Part 2. Install Codex with the command for your system, open a new terminal, and run codex --version.
2. Make sure your ChatGPT plan includes the Codex CLI, or prepare an OpenAI API key with billing.
3. cd into your project folder, run codex and sign in.
4. Pause after Part 3. Accept the trust question. Type /permissions and choose Read Only. Check with /status.
5. Pause after Part 4. In your editor, create AGENTS.md at the top level with the five lines from Part 4.
6. If you have no CLAUDE.md yet, create the starter from Part 4. Adapt: your own project name in the heading.
7. Type /exit, start codex again, set Read Only, and send the rules question from Part 4.
8. Pause after Part 5. Send the first prompt from Part 5. Adapt: your own opening hours, or another one-line change to your page. Read the approval prompt, then approve once.
9. Review with /diff, and with git status and git diff in a plain terminal. Reload the page in the browser.
10. Send the commit prompt from Part 5. Read the command before approving.

Check your work

1. Run codex --version. Expected: codex-cli and a number.
2. Run codex login status. Expected: it reports that you are signed in.
3. Start a new session and ask the rules question. Expected: Codex names CLAUDE.md and quotes a rule from it, without you naming the file.
4. Run git log --oneline -1. Expected: the commit "Bonus 9.1: Codex set up, opening hours added".
5. Open the repository on GitHub. Expected: AGENTS.md, CLAUDE.md and that commit.

Common problems

• codex is not found after installing. → The terminal was open during the install. → Open a new window. If it is still missing, install with npm, which you have from lesson 0.2: npm install -g @openai/codex.
• You sign in but Codex will not work. → The plan does not include the Codex CLI. → Check the pricing page; change plan or sign in with an API key.
• Codex edited a file without asking. → The session is in Auto. → Type /permissions, choose Read Only, and review the change with git diff.
• Codex does not know the rules. → AGENTS.md is misnamed, not in the top folder, or was created after the session started. → Fix the name and place, then start a new session.
• Codex knows AGENTS.md but does not open CLAUDE.md. → It did not follow the pointer. → Start the session with "Read CLAUDE.md first." Alternative while the project has no AGENTS.md: put the line project_doc_fallback_filenames = ["CLAUDE.md"] at the very top of ~/.codex/config.toml; Codex then loads CLAUDE.md itself. The setting is ignored in a folder that contains an AGENTS.md.
• The commit fails with index.lock: Operation not permitted. → The command ran inside Codex's sandbox, which cannot write to the Git folder. → Approve when Codex asks to run it with permission, or commit yourself.

Homework

About 20 minutes, after the lesson, on your own. Nothing later in the course depends on it.

1. A second change, briefed by you. Pick one more one-line change to your page that is true for your business (a phone number, the area you serve). Write the request yourself in the shape of the Part 5 prompt: what you want, where, what done means, and "do not commit". Approve the edit, review it, then ask Codex to commit and push, without a tag. Deliverable: a second commit made through Codex. Done when: the line shows on the page and git status reports nothing to commit.
2. One rule of your own, proven. By hand, add one rule to CLAUDE.md that holds for every page of your business, for example the exact spelling of its name. Start a new Codex session and ask the rules question from Part 4. Commit and push, without a tag. Deliverable: the new rule in CLAUDE.md. Done when: Codex quotes your rule without you naming the file.
3. Your start-of-session card. On paper or in a note outside the project, write the three or four things you will check each time you start Codex: the folder, Read Only in /status, a clean git status, anything else you want. Then open a new session and use it once. Deliverable: the card. Done when: you have run every line of it in a real session.