Mobile App Development with Codex

Codex is OpenAI's desktop coding app (with mobile companion via the ChatGPT app) designed for agentic development. It goes far beyond simple autocomplete or one-shot generation — it can edit files across your project, run builds and tests, use plugins and skills for deep tool integration, and participate in multi-agent workflows.

This makes it especially powerful for mobile app development, where traditional workflows involve heavy context switching between Xcode, simulators, design tools, and terminals.

The Game-Changing "Build iOS Apps" Plugin

Recent updates have dramatically improved native iOS development inside Codex.

As highlighted in this thread (shared via @minchoi quoting OpenAI developers), the new Build iOS Apps plugin lets Codex:

• View and test your iOS app directly in an in-app browser/simulator
• Open SwiftUI previews
• Hot-reload edits without leaving the Codex environment

This removes the painful context-switching that has traditionally made iOS development slow and fragmented.

@MengTo's demo in this post shows the iOS simulator running live inside Codex, with the caption "iOS dev is about to get way easier." Many developers are reporting that quick iterations now feel almost magical — "just one change" and you immediately see the result.

What Makes Codex Different for Mobile

Codex supports "vibe coding" — describing what you want in natural language and letting the agent handle scaffolding, iteration, debugging, and even deployment loops.

It can:

• Generate and maintain full project structures (Xcode projects, SwiftUI views, etc.)
• Run builds, launch simulators, and inspect UI
• Use plugins/skills for tight integration (simulator control, Figma, etc.)
• Participate in multi-agent setups (one agent for UI, another for data layer, etc.)
• Work alongside you via the ChatGPT mobile app for remote steering

This aligns perfectly with the shift toward loops, not prompts — treating the entire development process as an autonomous system rather than a series of chat messages.

Prerequisites

• Mac (strongly recommended) for the best native iOS simulator experience. Windows/Linux users can still use React Native, Flutter, or cloud-based builds.
• Codex app installed (available through OpenAI / ChatGPT desktop).
• Xcode installed (required for iOS builds and the simulator).
• Apple Developer account (free tier is enough for simulators and testing; paid account needed for App Store distribution).
• ChatGPT mobile app (for on-the-go monitoring and steering of Codex sessions — this is currently a preview feature).
• Enable the Build iOS Apps plugin in Codex settings (search for it in the settings panel as mentioned in the community threads).

Step-by-Step: Build Your First iOS App with Codex

1. Start a New Project

Open Codex and begin a new chat or session. Use a clear, structured prompt:

"Scaffold a new SwiftUI iOS app for a simple habit tracker. Use modern best practices, include a tab bar navigation, and set up a basic project structure with an AGENTS.md file for future workflows."

Codex will generate the Xcode project, ContentView.swift, and supporting files. Ask it to create planning documents like AGENTS.md or a project plan — this is excellent practice for agentic loops.

2. Enable the Build iOS Apps Plugin

Explicitly invoke the plugin in your conversation:

"Use the Build iOS Apps plugin to open the iOS simulator and show SwiftUI previews for this project."

The plugin integrates with xcodebuild, simulator control, hot-reloading, and live inspection. You should now be able to see your app running inside Codex.

3. Iterate with Natural Language

Describe features conversationally:

"Add a list of habits with the ability to add and delete items. Persist the data using UserDefaults. Support dark mode. Hot reload and show me the updated SwiftUI preview in the simulator."

Codex will edit the relevant files, rebuild the app, and display the results in the integrated simulator — exactly as demonstrated in the tweets linked above.

4. Debug and Test

Prompt for quality and verification:

"Write unit tests and UI tests for the habit list feature. Run the tests and report any failures."
"The list isn't scrolling smoothly on the simulator. Diagnose the issue and fix it, then show the updated preview."

You can also leverage Computer Use capabilities (if enabled) for even broader control of the environment. Many people monitor long-running Codex sessions from their phone using the ChatGPT app while the actual work happens on their Mac.

5. Add Advanced Features

Once the basics are working, expand:

• Design integration: Connect Figma via plugins or generate UI from descriptions.
• Backend & Data: Add Firebase, Supabase, or custom APIs. For agent-native projects, explore integrations with tools like those at HeadlessDomains or .agent domains.
• Agentic capabilities: "Make this app more agentic — add voice input using Whisper and the ability to export data via a structured format."
• Multi-agent workflows: Set up specialized agents (one focused on UI polish, another on data persistence, another on accessibility).

6. Build & Deploy

When you're ready to ship:

"Prepare this app for App Store submission: generate app icons, create placeholder screenshots, update Info.plist with proper metadata, and produce a build archive."

Codex can handle much of the preparation via the CLI. You'll still use Xcode or Transporter for final signing and upload in most cases, but the heavy lifting is dramatically reduced.

Pro Tips from the Community

Cross-Platform Development

Codex works well beyond native iOS:

React Native + Expo

One of the strongest cross-platform experiences right now. Prompt Codex to scaffold an Expo project and it can manage both iOS and Android simulators side-by-side.

"Create a cross-platform habit tracker using React Native and Expo. Set up navigation, local storage, and make sure it runs cleanly on both iOS and Android simulators."

Flutter

Codex handles Dart and Flutter scaffolding very effectively. The hot-reload experience is excellent.

Android Native (Kotlin)

Fully supported but requires Android Studio. Less seamless than iOS on a Mac because the simulator integration isn't quite as tight yet.

The real power for cross-platform work is the shared codebase — Codex can make coordinated changes across platforms while you guide it at a high level.

Full Workflow Enhancements

• Mobile Steering: Use the ChatGPT app on your phone to start sessions, approve steps, or give high-level direction while Codex runs on your desktop.
• Plugins & Skills: Beyond the Build iOS Apps plugin, explore Figma integration, design system tools, and even video generation for creating app demo videos.
• Agentic Loops: Structure your work using the patterns from Loops, Not Prompts and the Grok Build X Thread Workflow. Define clear roles for different agents and review gates.
• Start Small: The biggest wins come from shipping tiny, valuable MVPs quickly and then iterating based on real usage.

Example Project Ideas

• Simple habit tracker or todo app (great first SwiftUI project)
• Utility tool or mini-game
• Agent-powered mobile companion (e.g., an app that interacts with your headless domains, .agent infrastructure, or MCP servers)
• Internal tool for your team or community (for example, something tied to events like the Chiang Mai AI meetups)

Next Steps

This approach can turn what used to take weeks into prototypes you can build and test in hours or days.

The two tweets linked at the top perfectly capture why this moment feels different: the friction of switching between IDE, simulator, and terminal is being removed.

Try this today: Open Codex and run the habit tracker prompt above. Then come back and tell me what kind of app you actually want to build first — whether it's for personal use, a side project, or something that ties into the agentic and decentralized work happening around AI agents and Headless Domains.

I can help you refine prompts, debug specific issues, or even generate starter project structures and AGENTS.md files tailored to your idea.

Let's build something great. What app are you thinking about first?

Related Resources

• Loops, Not Prompts — Boris Cherny on Claude Code
• Grok Build X Thread Workflow (multi-agent patterns you can apply here)
• Claude Code and Claude Agent Skills (comparable agentic coding approaches)
• AI Agents Category — more experiments in orchestration and tool use
• Headless Architecture — relevant if you're building mobile frontends for agent-native backends