How to use OpenAI Codex: your first task, checked and working

Codex is OpenAI’s coding agent. It can inspect a project, edit files and run checks using the tools available to it. You describe the result you need, then review the changes. Check that the feature works, the facts are correct and the rest of the project still behaves as expected.

This guide uses a workshop price calculator as a first project. You can check its output by hand without knowing how to code. The prices are fictional teaching examples; replace them with your own before using the calculator in a real business.

What can you ask Codex to do?

Pick a task whose outcome you can describe and verify. ‘Show the total for this number of participants using this formula’ gives you something to test. ‘Make the website better’ leaves most decisions unresolved.

• Add a calculator, a catalogue filter or an article’s table of contents.
• Investigate a broken button, invalid form submission or page that stopped loading.
• Explain where a project keeps its content and how to run it.
• Write a script to check links or clean up an export. Try it on a copy of the data first.

A chat may be enough for discussing an idea or editing a paragraph. Codex is useful when you need changes to a project. What it can access depends on the environment, tools and permissions you give it.

Choose the app, editor extension or CLI

If you are new to coding, start with the desktop app. The official setup guide currently directs users to the ChatGPT desktop app: choose Codex in the product selector and open your project folder. Older screenshots may show a different interface.

• Official setup instructions

• Desktop app: a place to manage a task and work with local project files.
• Editor extension: useful if you already work in a supported code editor and want to review changes alongside the code.
• CLI: runs in your terminal. You do not need it for the exercise below.

• Codex editor extension: compatibility and installation

• Image source: official OpenAI documentation

• Codex CLI: installation and first run

Do you need a paid plan?

At the time of checking, OpenAI lists Codex access with ChatGPT plans including Free. Usage is limited, and model availability and allowances vary by plan. Using an API key involves separate API billing.

Try a few real tasks with your current account before choosing a more expensive plan. Check current prices and your account’s limits directly; a price copied into a guide can become outdated.

• Current Codex plans and usage limits

First project: a workshop price calculator

Prepare the folder and the numbers

Create a folder called workshop-calculator and open it as a project. If you are experimenting with an existing site, save its current state in Git or make a backup first. For this exercise you only need a base fee, a price per participant and the allowed group size.

Check the selected folder and permissions. This exercise needs file changes inside the project. If Codex requests software installation or an external service, check the reason: the brief below requires no external dependencies.

• Permissions and access controls

Describe the behaviour and expected results

Give Codex the rules and a way to check them. You do not need to choose a framework or prescribe every line of code.

Build a workshop price calculator in this folder.

Example pricing:
- base fee: €120;
- €35 per participant;
- whole numbers from 1 to 12 participants;
- total = 120 + 35 × participants.

Include a participant input, the total and an explanation of the formula.
Show a helpful error for an empty field, a fraction, 0 or 13.
Never leave a previous total looking valid when the input is invalid.

Use plain HTML, CSS and JavaScript without external libraries.
Save the page as index.html. It must open without a server or internet
connection and work on a phone and with a keyboard.
Do not add payments, contact collection, analytics or invented reviews.

Check: 1 participant = €155, 4 = €260, 12 = €540.
Explain how to open the page, what you tested and what still needs
manual checking.

• Open the example calculator and try the inputs

Open the page and check it yourself

Open index.html in a browser. Enter 1, 4 and 12 and compare the totals with the brief. Then clear the field: the old price should not remain visible as a current result. The agent’s report is useful evidence, but it does not replace trying the page.

1. Try 0, 13, a fraction and an empty field. Each error should make the next action clear.
2. Narrow the window to a phone-sized width. Nothing should require horizontal scrolling.
3. Use Tab to move between controls. A visible outline or other indicator should show which control is selected, and you should be able to use the calculator without a mouse.
4. Reload and calculate again to check that previous input does not affect the result.

This is a local prototype, without a public URL. If you later connect it to checkout, the server must independently validate the price: visitors can alter calculations that run in their browser.

Ask for a specific fix

Describe the steps, what happened and what should happen. If clearing the input leaves an old total, use a request like this:

I enter 4 and see €260. After I clear the field, the total remains.
For an empty field, show “Enter the number of participants” and hide the total.
Keep the formula, the 1–12 range and the design.
Test these sequences: 4 → empty → 1 and 12 → 13 → 12.
Explain the cause and how you checked the fix.

Keep working in the same task until you accept the calculator. If a fix breaks something that worked, compare the file changes with your saved version. ‘Undo that’ cannot guarantee an exact restoration when no previous version exists.

Working on an existing website

Ask Codex to inspect the relevant part of the project before editing it. An established site may already have components, styles and checks that are not obvious from a screenshot.

Find how this project builds blog articles. Do not change files yet.
Explain where titles, body text, metadata and language links come from.
List the files needed for a new article and the existing validation commands.

Then request one bounded change. Bringing the calculator into the site’s existing design is one task; adding checkout is another, with different acceptance criteria.

Save recurring rules in AGENTS.md

Codex reads AGENTS.md instructions before working on a project. Use this file for lasting requirements such as how to run the project, where content comes from and which checks matter. For the exercise:

# Project rules

Workshop price calculator. All prices are examples.
Open index.html in a browser; no server or external libraries.
Change the formula or participant range only when explicitly requested.
Show an error and hide the total for invalid input.
After edits, check 1, 4, 12, 0, 13, fractional and empty input.
Distinguish completed checks from checks you could not run.

Keep one-off design requests in the task conversation. Update this file when requirements change; an outdated rule can steer later work in the wrong direction.

• How Codex finds and applies AGENTS.md

Common problems and next steps

• You get a plan but no files: check the project and write permissions, then explicitly request implementation.
• The project will not start: provide the exact error and how you launched it. Ask for the cause before replacing dependencies.
• Only one example works: add boundary values and invalid inputs to the checks.
• The agent invents prices or claims: provide the source of truth and ask it to check the remaining text for the same problem.
• You reach a usage limit: save your files and check reset times or continuation options in your account.

Before publishing a website, review its text, links, forms and search settings as well as the design. These guides cover useful next steps:

• SEO and GEO: what each approach addresses

• llms.txt: planning a guide to your site’s content

Product information and documentation links checked on September 6, 2026. The calculator is a teaching exercise, not a client case study or a promise of a finished product from a single prompt.