How to Build an Android App Using OpenAI Codex

We recently shared a detailed guide on how to build Android apps using Google Antigravity. That approach is best for someone who wants an affordable way to build AI apps that work.

Current Google models in Antigravity offer far better rate limits than models from ChatGPT or Claude. However, Gemini isn’t the best choice for building advanced AI apps or for including other advanced features in your app.

For advanced apps, using Claude or ChatGPT Codex is the most reliable option. OpenAI’s ChatGPT models in Codex are not free — you need a minimum ChatGPT Plus subscription to access top models. Rate limits are also quite restrictive and can be exhausted quickly during active use.

That said, these models are among the leading frontier models available today. Thanks to various connections, plugins, and MCP servers, building Android apps using Codex is more convenient and requires fewer steps compared to Google Antigravity.

Prerequisites

To build an Android app in Codex, you need:

1. ChatGPT Plus or Pro subscription — get it here
2. An Android phone with Expo Go installed — download the Expo Go app
3. An Android phone
4. A free account on supabase.com — sign up here
5. A free account on expo.dev

Once you have everything set up, follow the steps below to build your Android app using OpenAI Codex.

Set Up Codex

Step 1: Download and install the ChatGPT app on your Mac, Windows, or Linux machine.

Step 2: Once installed, sign in with your ChatGPT Plus or Pro subscription account.

Step 3: Switch to Codex.

Step 4: Tap New Chat, then tap Choose Folder.

Step 5: Create a new folder or select an existing folder that will store all your app files.

Create Your App

Step 1: Enter the following prompt. Edit it to match your own app idea. For this guide, we are building a simple Pomodoro timer app.

Build a simple Pomodoro timer app using Expo and TypeScript.

The app should let users start, pause, resume, reset, and skip Pomodoro sessions. Include focus, short break, and long break modes. Show the remaining time clearly, track completed focus sessions, and save the timer settings and session history locally using SQLite.

Create a clean, polished mobile UI that works well on Android. Do not create an Android build yet. First, implement the app, check for errors, and explain how I can preview it using Expo Go.

Step 2: In the model selector, choose GPT-5.6 Sol or GPT-6 Astra.

The latest Codex GPT-6 Astra model is very capable at frontend development. Even without reference images, it can generate a reliable UI from a text prompt alone for simpler apps.

If you want something more personalized, however, it is worth submitting UI reference images alongside your prompt. Codex will match the design closely, including font choices. You can use Google Stitch to generate those UI references.

Design the UI With Google Stitch (Optional)

Google Stitch is an AI-focused UI design tool similar to Figma. It lets you generate UI designs through prompts and refine them with basic editing tools.

To quickly create UI designs using Google Stitch, go to stitch.withgoogle.com and follow these steps:

Step 1: Paste your app prompt into the prompt area.

Step 2: Wait for Google Stitch to generate your UI designs.

Step 3: Tap the prototype icon at the top to preview the design.

Step 4: Export and save the designs as a zip file, then attach them to your Codex prompt so the model can use your custom UI as a reference when building the app.

With your UI reference ready, you can return to Codex and include the exported designs alongside your prompt to get an app that looks exactly the way you envisioned it.