Getting Started with OpenAI's Codex App

OpenAI's Codex app represents a significant leap forward in AI-assisted development, offering a dedicated desktop experience for building software with intelligent agents. Unlike traditional coding assistants that simply respond to prompts, Codex operates as a full-fledged development partner—managing parallel tasks, handling Git operations, and even automating recurring workflows.

In this guide, you'll learn how to set up the Codex app, understand its core features, and build a complete Next.js 16 project with TypeScript and Tailwind CSS. Whether you're exploring AI-powered development for the first time or looking to integrate Codex into your existing workflow, this guide will get you productive quickly.

What You'll Learn

• How to install and configure the Codex app on macOS
• Understanding the Codex interface: threads, worktrees, and automations
• Creating a Next.js 16 project with TypeScript and Tailwind from scratch
• Running projects locally and deploying to the cloud
• Best practices for effective prompting and iteration
• Advanced features like Skills and Automations
• Common pitfalls and how to avoid them

Prerequisites

Before diving in, make sure you have:

• A Mac with Apple Silicon (M1, M2, M3, or newer)
• A ChatGPT account (Free, Plus, Pro, Business, Enterprise, or Edu)
• Node.js 20+ installed on your machine
• Git configured with your credentials
• Basic familiarity with terminal commands

Pro Tip: While Codex is available on free plans for a limited promotional period, Plus and Pro subscriptions offer higher rate limits that make iterative development smoother.

Installing and Configuring the Codex App

Step 1: Download and Install

Head to openai.com and download the macOS installer. The Codex app is built with Electron, which means Windows support is coming soon, but for now it's macOS-exclusive.

Once downloaded, drag the Codex app to your Applications folder and launch it.

Step 2: Sign In and Select Your Plan

You have two authentication options:

1. ChatGPT Account (recommended): Sign in with your existing ChatGPT credentials. This gives you access to all features including cloud threads.

2. OpenAI API Key: For developers who prefer API-based billing. Note that some features like cloud threads may be limited with this option.

Step 3: Initial Setup

After signing in, Codex may ask a few personalization questions about your development experience and project interests. These help tailor suggestions but can be skipped if you prefer to jump straight in.

Understanding the Codex Interface

The Codex app organizes work around several key concepts that differ from traditional chat-based AI tools.

Threads and Projects

A thread is a conversation with Codex about a specific task. Unlike ephemeral chat sessions, threads persist and can be resumed later. The sidebar shows all your active threads organized by project folder.

The Main Workspace

The workspace consists of:

• Prompt Input Area: Where you communicate with Codex using natural language
• Model Selector: Choose between different Codex-optimized models
• Local/Cloud Toggle: Decide whether Codex runs on your machine or in the cloud
• Branch Indicator: Shows which Git branch Codex is working on

Worktrees for Parallel Development

One of Codex's standout features is built-in Git worktree support. This allows you to run multiple threads simultaneously, each working in isolated branches without conflicts. When you start a new thread, Codex can automatically create a worktree so changes don't interfere with your main branch.

Skills and Automations

Skills are reusable capability packages that extend what Codex can do. Think of them as plugins that teach Codex specialized workflows—like how to deploy to Vercel or run specific testing frameworks.

Automations let you schedule recurring tasks that run in the background. For example, you could set up an automation to check for dependency updates every morning or validate tests on a schedule. Currently, automations require your laptop to be powered on, but cloud-based automations are on the roadmap.

Pro Tip: Codex stores automation state in a SQLite database at ~/.codex/sqlite/codex-dev.db. You can explore this with tools like Datasette if you're curious about the internals.

Creating a Next.js 16 Project with TypeScript and Tailwind

Let's put Codex to work building a real project. We'll create a modern Next.js 16 application with TypeScript and Tailwind CSS.

Step 1: Set Up Your Project Folder

First, create a directory for your project:

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mkdir ~/projects/my-nextjs-app
cd ~/projects/my-nextjs-app

In the Codex app, click "New thread" and select this folder as your project directory.

Step 2: Initialize the Project with Codex

With the folder selected, make sure Local is selected (so Codex runs on your machine) and enter this prompt:

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1Create a new Next.js 16 project with TypeScript and Tailwind CSS. Set up the project with:
2- App Router (not Pages Router)
3- TypeScript strict mode enabled
4- Tailwind CSS with a custom color palette for a SaaS dashboard
5- ESLint and Prettier configured
6- A basic folder structure following Next.js best practices

Codex will analyze your request and likely ask clarifying questions about your preferences. Once confirmed, it will present a plan showing the files it intends to create.

Step 3: Review and Accept the Plan

Codex operates on an approval-based workflow—it shows you exactly what it will do before making any changes. Review the proposed file structure:

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1my-nextjs-app/
2├── src/
3│   ├── app/
4│   │   ├── layout.tsx
5│   │   ├── page.tsx
6│   │   └── globals.css
7│   ├── components/
8│   │   └── ui/
9│   └── lib/
10├── public/
11├── tailwind.config.ts
12├── tsconfig.json
13├── next.config.ts
14├── package.json
15└── .eslintrc.json

If the plan looks good, accept it and Codex will execute the commands and create the files.

Step 4: Examine the Generated Code

Let's look at what Codex typically generates for the core files:

src/app/layout.tsx:

typescriptCopy
1import type { Metadata } from 'next'
2import { Inter } from 'next/font/google'
3import './globals.css'
4
5const inter = Inter({ subsets: ['latin'] })
6
7export const metadata: Metadata = {
8  title: 'My SaaS Dashboard',
9  description: 'A modern dashboard built with Next.js 16',
10}
11
12export default function RootLayout({
13  children,
14}: {
15  children: React.ReactNode
16}) {
17  return (
18    <html lang="en">
19      <body className={inter.className}>{children}</body>
20    </html>
21  )
22}

tailwind.config.ts:

typescriptCopy
1import type { Config } from 'tailwindcss'
2
3const config: Config = {
4  content: [
5    './src/pages/**/*.{js,ts,jsx,tsx,mdx}',
6    './src/components/**/*.{js,ts,jsx,tsx,mdx}',
7    './src/app/**/*.{js,ts,jsx,tsx,mdx}',
8  ],
9  theme: {
10    extend: {
11      colors: {
12        primary: {
13          50: '#f0f9ff',
14          100: '#e0f2fe',
15          500: '#0ea5e9',
16          600: '#0284c7',
17          700: '#0369a1',
18        },
19        dashboard: {
20          bg: '#0f172a',
21          card: '#1e293b',
22          border: '#334155',
23        },
24      },
25    },
26  },
27  plugins: [],
28}
29
30export default config

Step 5: Run the Project Locally

Codex includes a built-in terminal for each thread. Click the terminal icon or use the keyboard shortcut to open it, then run:

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npm install
npm run dev

Your Next.js app should now be running at http://localhost:3000.

Alternatively, you can ask Codex directly:

Install the dependencies and start the development server. Tell me when it's running.

Codex will execute the commands and confirm when the server is ready.

Building Features Iteratively

The real power of Codex emerges when you iterate on your project. Let's add a dashboard page with components.

Adding a Dashboard Layout

Prompt Codex with a specific, detailed request:

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1Create a dashboard layout with:
2- A responsive sidebar navigation with icons (Dashboard, Analytics, Settings, Profile)
3- A top header bar with a search input and user avatar dropdown
4- A main content area that accepts children
5- Use the custom Tailwind colors we defined
6- Include subtle hover animations on navigation items
7- Make the sidebar collapsible on mobile

Codex will generate multiple files—likely a DashboardLayout.tsx component, a Sidebar.tsx component, and possibly a Header.tsx component. Review the diff view to see exactly what's being added.

Adding Interactive Components

Continue building with focused prompts:

typescriptCopy
// After Codex creates components, you might ask:

textCopy
1Create a stats card component that displays:
2- A title, value, and percentage change
3- An icon prop for customization  
4- Green/red coloring based on positive/negative change
5- Smooth number animation when the value updates
6
7Then create a dashboard home page that shows 4 stats cards in a responsive grid.

This iterative approach—starting broad and refining—produces better results than trying to specify everything upfront.

Running Projects in the Cloud

Codex offers two execution modes: Local and Cloud. Understanding when to use each is key to an efficient workflow.

Local Mode

When Local is selected, Codex runs directly on your machine with full access to your file system, environment variables, and local services. Use Local mode when:

• You need access to local databases or services
• You're testing against local API endpoints
• You want immediate file system access
• You're working with sensitive credentials stored locally

Cloud Mode

Cloud mode runs Codex on OpenAI's infrastructure. Select Cloud from the thread options to enable this. Cloud mode is ideal when:

• You want tasks to continue while your laptop sleeps
• You're running long-running operations like large test suites
• You want consistent, reproducible environments
• You're collaborating and want others to see thread progress

To deploy your Next.js project to a cloud platform, you can prompt Codex:

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Set up deployment to Vercel:
- Configure the vercel.json if needed
- Set up environment variables for production
- Create a GitHub Actions workflow for CI/CD
- Add preview deployments for pull requests

Pro Tip: For cloud deployments, ensure your project has a proper .gitignore and doesn't contain hardcoded secrets. Codex can help audit this with a prompt like "Review the project for any hardcoded secrets or sensitive data."

Advanced Codex Features

Using Plan Mode

For complex features, use Plan Mode by typing / in the prompt bar and selecting "Plan mode." This instructs Codex to break down your request into discrete steps before executing anything.

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1/plan
2Add authentication to this Next.js app using NextAuth.js with:
3- GitHub and Google OAuth providers
4- A PostgreSQL database for session storage
5- Protected API routes
6- A sign-in page with provider buttons
7- Session management in the dashboard layout

Codex will return a numbered plan you can review, modify, or approve step-by-step.

Creating Custom Skills

Skills make Codex smarter about your specific workflows. To create a skill for your Next.js project:

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Create a skill that describes how to add new pages to this dashboard:
- The file naming conventions we use
- How to register routes in the sidebar
- The layout wrapper pattern
- TypeScript interfaces for page props

Codex will generate a skill file in your project's skills/ directory that it (and other developers on your team) can reference in future threads.

Setting Up Automations

Automations run on schedules to handle recurring tasks. From the sidebar, click Automations and create one:

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Name: Daily Dependency Check
Prompt: Check for outdated npm dependencies. If any have security vulnerabilities, create a summary. If all dependencies are current, archive this run.
Schedule: Every day at 9:00 AM
Project: ~/projects/my-nextjs-app

Codex will check dependencies daily and only surface findings when action is needed.

Common Pitfalls and How to Avoid Them

1. Vague Prompts Lead to Vague Results

Avoid:

Make the homepage look better

Instead:

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Redesign the homepage hero section with:
- A gradient background from primary-500 to primary-700
- Centered headline text with the Inter font at 4xl size
- A CTA button with hover scale animation
- Responsive padding (more on desktop, less on mobile)

2. Skipping the Review Step

Codex shows you exactly what it will change before committing. Always review diffs, especially for:

• Changes to configuration files
• Modifications to existing logic
• Dependency additions

3. Not Using Worktrees for Experiments

When trying something risky, start a new thread with worktree isolation enabled. This keeps experimental changes separate from your working code.

4. Overloading Single Prompts

Instead of asking for an entire feature in one prompt, break it down:

1. First prompt: Create the data model and types
2. Second prompt: Build the API routes
3. Third prompt: Create the UI components
4. Fourth prompt: Wire everything together

This approach makes debugging easier and gives you checkpoints to verify progress.

5. Ignoring the Built-in Terminal

The integrated terminal lets you verify changes immediately. After Codex makes modifications, run your tests or start the dev server to catch issues early.

Best Practices for Effective Codex Usage

1. Provide Context: Start threads by describing your project's tech stack and conventions
2. Use Concrete Examples: When asking for UI changes, describe specific colors, sizes, and behaviors
3. Iterate Quickly: Don't aim for perfection on the first prompt—get something working, then refine
4. Leverage Git Integration: Use the built-in diff viewer and commit functionality to maintain clean history
5. Save Useful Skills: When Codex does something well, capture it as a skill for reuse
6. Review Security: Never include passwords or API keys in prompts; use environment variables

Conclusion and Next Steps

The Codex app transforms AI-assisted coding from a simple chat interface into a full development environment. With features like parallel threads, worktree isolation, and automations, it's designed for real-world development workflows rather than one-off code generation.

You've learned how to:

• Set up and navigate the Codex app
• Create a complete Next.js 16 project with TypeScript and Tailwind
• Run projects locally and configure cloud deployments
• Use advanced features like Plan Mode, Skills, and Automations
• Avoid common mistakes that trip up new users

Where to go from here:

• Explore the Codex Skills library for pre-built capabilities
• Set up your first automation to handle a recurring task
• Try the IDE extension to sync Codex with VS Code or Cursor
• Experiment with cloud threads for longer-running operations

The more you use Codex, the better you'll understand how to prompt effectively and leverage its full capabilities. Start with small features, build confidence, and gradually tackle larger challenges as you develop an intuition for what Codex does best.