# Build an App with Codex: A Beginner's Guide > Build your first app project with OpenAI Codex: set up Expo, scope your prompts, test on iPhone and understand the steps before the App Store. Original page: [Build an App with Codex: A Beginner's Guide](https://devoria.fr/en/blog/build-an-app-with-codex-beginners-guide) Author: [Mehdi Azizi](https://devoria.fr/en/mehdi-azizi) Published: 2026-10-01 Content last revised: 2026-10-09 ## Can you build a mobile app with Codex? Yes. Codex, OpenAI's coding agent, can help you build an app by reading your project, editing files and running commands. For an iPhone app, you can use it in a React Native and Expo project, then test the result on your phone. Codex works on the code. How the app looks on iPhone, compiling a distributable build and submitting to Apple are steps you need to check separately. This guide walks you through a first run with one simple feature, before you add accounts or payments. ## 1. Start with an idea small enough Write three lines before you install anything: - The user: who the app helps. - The action: what they need to be able to do in the first version. - The check: how you'll know that action works. **Fictional example used here:** a hiker adds the items in their backpack to a list, then checks them off. The first version has no map and no social network. If your description still sounds like "an awesome AI app", the [guide to finding an app idea](https://devoria.fr/en/blog/how-to-find-app-idea-tests-before-coding) will give you a more concrete starting point. Then you can use the [one-page spec](https://devoria.fr/en/blog/mobile-app-requirements-document-one-page-template). ## 2. Create the mobile project and show a first screen To follow along, set up Node.js in the version Expo recommends, a code editor and a test environment. Follow the [official guide to creating an Expo project](https://docs.expo.dev/get-started/create-a-project/): it gives the current command and the compatibility caveats for Expo Go. Get the starter project running before you ask Codex for a feature. If the empty project won't start, it'll be hard to tell whether a later error comes from your setup or from the generated code. Expo Go lets you test the features that are compatible with its environment. When you add native code that isn't included in it, you may need a different test environment, and that code goes through an Apple check when you upload it, like ITMS-90338 for non-public APIs. Don't confuse this preview with the final version meant for the App Store. The [React Native and Expo](https://devoria.fr/en/expertise/react-native-expo-mobile-app) guide explains what these tools do. ## 3. Point Codex at the right folder Pick the surface that suits you: desktop app, terminal or editor extension. OpenAI's [official quickstart](https://learn.chatgpt.com/docs/quickstart) describes the available options and how to sign in. In the desktop app, open your app's folder and select Codex for development work. In the terminal, after installing the CLI following the [official documentation](https://learn.chatgpt.com/docs/cli), move into that same folder before launching the agent: ```bash cd my-app codex ``` `my-app` stands for your folder name here. Check it instead of copying the command into some random directory. Don't open your whole computer when the project folder is enough. Your first request can stay read-only: ```text Read this project without modifying any file. Explain where the screens, the navigation and the data are. List the available commands to start and check the app. I'm a beginner: explain the technical terms you use. ``` ## 4. Build one feature, then look at the result Ask for a limited change: ```text Add a screen to pack a hiking backpack. The user can type the name of an item, add it to a list and check it off. Stick to the project's conventions and use local data. Don't create an account, a server or a payment. Explain the plan first, then list the modified files. Check the project with its existing commands. Give me the actions to test on my phone. ``` After the change, look at the modified files and run the app. Ask for an explanation if you don't understand why the agent added a service or replaced a big chunk of the project. Test adding an item, an empty input, checking an item and opening the keyboard. On a small screen, the button must stay reachable. A successful technical command alone doesn't prove this flow is usable. ## 5. Fix things with precise observations "It doesn't work" leaves too many possibilities. Describe the action, the expected result and what actually happened: ```text On my iPhone, I add "water bottle", then I close and reopen the app. The list is empty again. I want to keep the items between launches. Suggest the simplest fix in this project. Tell me how to check saving and deleting an item. ``` Add the relevant screenshot or error message, without private data or secret keys. If several fixes fail, ask Codex to explain the observed cause before trying again. Save a working version so you can roll back. ## 6. Go from prototype to distributable app Once the main feature is usable, your next choices depend on what you need: local or synced data, required or optional sign-in, free version or subscription. Every addition creates a new thing to check. An account means testing each user's access to their data. A subscription means testing the purchase, the restore and access after expiration. A feature using a paid API means understanding where the call runs and who pays for its usage. Read the [pitfalls of an iOS app built with AI](https://devoria.fr/en/blog/build-ios-app-with-ai-pitfalls-checks) before these steps. To distribute a test build, use the [TestFlight guide](https://devoria.fr/en/blog/testflight-test-iphone-app-before-submitting). To go live, follow the [App Store guide](https://devoria.fr/en/expertise/publish-an-app-on-the-app-store). ## Frequently asked questions ### Does Codex automatically add AI to my app? No. Using AI to write the code doesn't mean the final app contains a chatbot or calls a model. Our example can run on a local list, with no AI in the product. ### Can you start without subscribing to every tool? Check your account's access and limits in OpenAI's current plans. Start with a single feature before piling on services. The guide to [building an app for free](https://devoria.fr/en/blog/make-iphone-app-free-real-cost) separates the prototype from publishing fees. ### Codex or Claude Code for beginners? Compare the workflow you can actually follow rather than a promise of speed. The [Codex vs Claude Code comparison](https://devoria.fr/en/blog/codex-vs-claude-code-iphone-app) offers a shared exercise to help you decide. ## Want to know what to build next? The free [diagnostic](https://devoria.fr/en/diagnostic) helps you assess where your project stands and pick the next step, then sends you my checklist for avoiding Apple rejections. The [iOS Kit](https://devoria.fr/en/kits/ios) gives you the starter code from my apps and a four-step path, from idea to App Store. For the full method, check out the [iOS app development training path](https://devoria.fr/en/expertise/ios-developer-course). If you're building with AI, the [vibe coding training](https://devoria.fr/en/expertise/vibe-coding-course-iphone-app) page follows the same path. ## Your project [iOS Kit](https://devoria.fr/en/kits/ios): Build your iPhone app with Claude Code: the starter code behind my apps, skills that check its work, and the Apple Review, Screenshots and ASO kits. 129,99 €.