Artificial intelligence is changing the way software is built. Developers no longer have to manually write every function, search through documentation for every error, or spend hours debugging repetitive problems.
AI coding agents can now work directly with software projects, inspect files, write and modify code, run tests, investigate errors, and help developers move from an idea to a working application much faster.
One of the most important tools in this area is OpenAI Codex.
But if you are new to Codex, you may be wondering:
- What is Codex?
- How do you use Codex?
- How do you install Codex?
- Can beginners use Codex?
- Can Codex build a website?
- Can Codex create an app?
- Can Codex fix bugs?
- Can Codex work with GitHub?
- Can Codex use the terminal?
- How do you write good Codex prompts?
- Can Codex test your code?
This guide explains how to use Codex step by step, from your first task to more advanced coding workflows.
What Is Codex?
Codex is OpenAI’s AI coding agent designed to help you write, review, and ship code.
The important word here is agent.
A traditional AI chatbot might give you a piece of code when you ask for one.
Codex is designed to work more directly with software-development tasks.
For example, instead of asking:
“Write a JavaScript function for a login form.”
you can give Codex a complete engineering task:
“Inspect this project, find why the login form isn’t working, fix the problem, add a regression test, run the tests, and explain what you changed.”
This type of workflow allows Codex to work with a project rather than simply generating an isolated code snippet.
OpenAI currently describes Codex as being useful for features, refactoring, migrations, testing, code review and other engineering work.
What Can You Do With Codex?
Codex can be used for many different programming tasks.
For example, you can use it to:
- Build websites
- Create web applications
- Write Python programs
- Create JavaScript applications
- Build APIs
- Debug applications
- Fix programming errors
- Refactor existing code
- Write automated tests
- Review code
- Explain unfamiliar projects
- Update dependencies
- Create documentation
- Work with Git repositories
- Run development commands
- Investigate failed tests
- Implement new features
- Review pull requests
- Automate repetitive development tasks
The major advantage is that Codex can work across a project instead of treating every question as a completely separate coding problem.
Codex vs ChatGPT: What Is the Difference?
ChatGPT and Codex are closely related, but their primary workflows are different.
ChatGPT
ChatGPT is useful for general-purpose tasks such as:
- Learning programming
- Asking technical questions
- Brainstorming ideas
- Explaining concepts
- Generating code examples
- Planning applications
- Troubleshooting individual problems
- Writing documentation
- Understanding programming concepts
Codex
Codex is focused specifically on software development.
It is designed to:
- Work with codebases
- Modify files
- Run commands
- Run tests
- Review changes
- Work with repositories
- Complete multi-step coding tasks
OpenAI currently separates the experiences this way: Chat is for conversational assistance, while Codex is for writing or debugging code, running tests and commands, reviewing changes and working with repositories.
A simple way to understand the difference is:
ChatGPT helps you talk about code.
Codex helps you work on software.
Where Can You Use Codex?
OpenAI currently supports Codex through several environments, including:
- Codex in the ChatGPT desktop app
- Codex CLI
- Codex IDE extension
- Codex web
Codex is included across ChatGPT plans, including Free and Go, although usage limits vary by plan.
Codex is also available across different development environments so that developers can work in the way that suits them best.
How to Start Using Codex
The basic process is straightforward.
Step 1: Sign in
Sign in using your ChatGPT account.
Step 2: Choose a Codex environment
You can use the Codex experience in the supported ChatGPT desktop application, CLI, IDE extension, or web environment depending on your setup and availability.
Step 3: Give Codex access to your project
Open the relevant project or repository.
Step 4: Describe the task
Tell Codex exactly what you want it to accomplish.
Step 5: Review its work
Look at the changes before accepting important modifications.
Step 6: Test the result
Ask Codex to run the relevant tests or checks.
Step 7: Continue iterating
If something isn’t correct, tell Codex what needs to change.
This creates a simple workflow:
Project → Prompt → Plan → Code → Test → Review → Improve
How to Use Codex in the ChatGPT Desktop App
For many beginners, the desktop application is one of the easiest ways to get started.
Open the current ChatGPT desktop application and select the Codex experience.
OpenAI’s current documentation says Codex is available as a dedicated experience in the desktop app.
You can then create a new Codex conversation and provide the project or task you want it to work on.
For example:
“Inspect this project and explain how the application is structured. Don’t change any files yet.”
This is a good first task because you can see how Codex understands your project before asking it to make changes.
How to Give Codex Access to a Project
The quality of an AI coding agent depends heavily on the context it has.
Suppose you have this project:
my-website/
│
├── index.html
├── style.css
├── script.js
│
├── images/
│ ├── logo.png
│ └── banner.jpg
│
└── assets/Instead of copying every file into a normal chat, Codex can work with the project in supported environments.
This allows it to understand relationships between files.
For example:
index.html
may load:
style.css
which may depend on:
script.js
and the JavaScript may interact with elements defined in the HTML.
Understanding that relationship is extremely important when debugging.
How to Use Codex CLI
If you prefer working in a terminal, Codex also has a command-line interface.
OpenAI currently lists the Codex CLI as one of its supported clients.
A typical workflow looks like:
Open terminal
↓
Open project directory
↓
Start Codex
↓
Describe task
↓
Codex inspects project
↓
Codex makes changes
↓
Run tests
↓
Review changesThe CLI is especially useful for developers who already spend most of their time in a terminal.
OpenAI’s current Codex page also documents the CLI alongside the ChatGPT and IDE experiences.
How to Use Codex in an IDE
If you prefer writing code inside an editor, Codex can also be used through a supported IDE extension.
This means you can work inside your normal development environment instead of constantly switching between applications.
For example, imagine you are working on:
src/
├── components/
├── services/
├── api/
├── utils/
└── pages/You could ask:
“Inspect the API service and determine why requests are failing intermittently.”
Codex can then investigate the relevant parts of the project.
OpenAI currently lists the Codex IDE extension as one of its supported ways to use Codex.
How to Write a Good Codex Prompt
Your prompt is one of the most important parts of the workflow.
A weak prompt gives Codex very little direction.
For example:
“Fix my website.”
There is no information about:
- What is broken?
- Which page?
- What should happen?
- What shouldn’t change?
- How should the result be tested?
A much better prompt is:
“Inspect the mobile navigation menu. The hamburger button appears, but clicking it doesn’t open the menu. Find the root cause, fix the smallest necessary part of the implementation, preserve the desktop layout, and test the result.”
Now Codex knows what you want.
The Four Parts of a Great Codex Prompt
A useful prompt can contain four important components.
1. Goal
What are you trying to accomplish?
Example:
“Add a search feature to the website.”
2. Context
What should Codex know?
Example:
“The website currently uses plain JavaScript and does not use a framework.”
3. Constraints
What should Codex avoid?
Example:
“Don’t change the existing navigation or introduce a new framework.”
4. Definition of done
How do you know the task is complete?
Example:
“The search should filter results instantly and work on mobile and desktop.”
This structure makes instructions much easier for an AI agent to follow.
Use Codex’s Planning Ability for Large Tasks
If you’re working on a large project, don’t immediately ask Codex to start changing hundreds of files.
First ask it to inspect the project and create a plan.
For example:
Plan this task before making changes.
I want to add user authentication to this application.
Requirements:
- Email and password login
- Registration
- Logout
- Protected pages
- Error handling
- Tests
First inspect the existing architecture.
Identify which files and systems are involved.
Create an implementation plan.
Do not modify files yet.This gives you an opportunity to understand the proposed solution before implementation begins.
OpenAI Academy’s current Codex guidance also recommends planning for complex tasks before implementation.
How to Ask Codex to Build a Website
Codex can be extremely useful for website development.
Imagine you want to build a gaming website.
You could provide this prompt:
Build a responsive gaming website.
Requirements:
- Modern gaming design
- Dark interface
- Responsive layout
- Homepage
- Search bar
- Game categories
- Game cards
- Featured games section
- Mobile navigation
- Fast loading
- SEO-friendly HTML
- Accessible buttons and navigation
Technology:
Use HTML, CSS and JavaScript unless the existing project requires another technology.
First inspect the project and create a plan.
Then implement the website.
After implementation, test the major interactions and fix any problems.This gives Codex both the creative direction and technical requirements.
How to Use Codex to Fix a Bug
Bug fixing is one of the most useful Codex workflows.
Suppose a registration form isn’t working.
Instead of:
“Fix registration.”
try:
The registration form does not submit when the user clicks
the Submit button.
Investigate the problem and identify the root cause.
Requirements:
- Preserve the existing design.
- Do not rewrite unrelated components.
- Make the smallest safe change.
- Add a regression test if appropriate.
- Run the relevant tests after the fix.
Explain the cause and summarize the files changed.This encourages Codex to investigate before modifying the project.
How to Use Codex for Debugging
When you encounter an error, give Codex the actual error message.
For example:
TypeError: Cannot read properties of undefinedThen ask:
Investigate this error.
Do not immediately patch it.
First:
1. Trace where the error originates.
2. Identify the underlying cause.
3. Explain why the problem occurs.
4. Propose the smallest safe fix.
Then implement the fix and run the relevant tests.This is much better than asking an AI to randomly change code until the error disappears.
How to Use Codex for Code Review
Codex can also act as a code reviewer.
For example:
Review the current changes as a senior software engineer.
Look for:
- Logic errors
- Security vulnerabilities
- Performance problems
- Error-handling issues
- Compatibility problems
- Missing tests
- Unnecessary complexity
Do not modify anything yet.
Rank your findings as:
Critical
High
Medium
LowThis can help identify problems before code is released.
OpenAI currently describes Codex as supporting code review as part of its engineering workflows.
How to Use Codex for Testing
Never assume that code is correct simply because Codex says it is finished.
Ask it to test the implementation.
For example:
Run the existing test suite.
If any tests fail:
1. Determine the cause.
2. Identify whether the failure is related to the changes.
3. Fix the problem if appropriate.
4. Run the tests again.
At the end, report:
- Tests passed
- Tests failed
- Tests added
- Any unresolved problemsTesting creates an important feedback loop.
Write → Test → Find problems → Fix → Test again
How to Use Codex for Refactoring
Suppose you have a large JavaScript file containing repetitive code.
Instead of:
“Refactor this file.”
try:
Analyze this module for unnecessary duplication and complexity.
Before modifying anything:
- Identify the main problems.
- Explain the proposed refactoring.
- Identify potential risks.
- Explain how you will verify that behavior remains unchanged.
Then implement the safe refactoring and run the existing tests.This reduces the chance of unnecessary changes.
How to Use Codex With Git
Git is an important part of professional software development.
Codex can assist with repository workflows where supported.
For example:
“Inspect the current Git changes and summarize what has been modified.”
Or:
“Review the current branch and identify incomplete implementation or missing tests.”
You can also ask Codex to help prepare changes for review.
However, you should always understand potentially destructive Git operations before allowing them.
How to Use Codex With GitHub
Codex can be useful in repository-based workflows.
A common process can look like:
GitHub issue
↓
Codex investigates
↓
Codex modifies code
↓
Tests run
↓
Developer reviews changes
↓
Pull request
↓
Code review
↓
MergeFor example:
Fix the bug described in issue #245.
First inspect the issue and relevant code.
Requirements:
- Preserve existing API behavior.
- Add a regression test.
- Run the relevant test suite.
- Explain the root cause.
- Summarize the final changes.How to Use Codex for Python
Codex can help with Python projects ranging from small scripts to larger applications.
For example:
Inspect this Python project.
Find why the CSV import fails when a row contains an empty value.
Requirements:
- Preserve the existing CSV format.
- Handle empty values safely.
- Add tests for empty fields.
- Don't change unrelated functionality.
Run the tests after implementing the fix.How to Use Codex for JavaScript
For JavaScript applications, you can ask Codex to investigate frontend or backend problems.
For example:
The search button works on desktop but does nothing on mobile.
Inspect the JavaScript event handlers and responsive UI behavior.
Find the root cause.
Fix the issue without changing the desktop behavior.
Test the search functionality on both layouts.How to Use Codex for HTML and CSS
Codex can also modify website layouts.
For example:
The website looks correct on desktop but the cards overflow
the screen on mobile.
Inspect the HTML and CSS.
Fix the responsive layout.
Requirements:
- No horizontal scrolling.
- Preserve desktop appearance.
- Keep the existing design.
- Test common mobile widths.This is much better than asking:
“Make my CSS responsive.”
Can Codex Build an App?
Yes, Codex can help build applications.
But large applications should be divided into smaller tasks.
For example, instead of:
“Build a complete social media app.”
break the project into:
- Project setup
- Database structure
- Authentication
- User profiles
- Posts
- Comments
- Likes
- Messaging
- Notifications
- Administration
- Testing
- Deployment
Then ask Codex to implement each major component systematically.
Can Beginners Use Codex?
Yes.
You don’t need to be an expert programmer to start experimenting with Codex.
For example, a beginner could say:
“Create a simple webpage with a title, image, button and a counter. Explain each file before creating it.”
Codex can help generate and explain the implementation.
However, learning basic programming concepts is still valuable.
You should understand at least the basics of:
- HTML
- CSS
- JavaScript
- APIs
- Databases
- Git
- Authentication
- Hosting
The more you understand, the easier it becomes to identify whether an AI-generated change is actually good.
How to Use Codex for Learning Programming
Codex doesn’t have to write everything for you.
You can use it as a programming teacher.
For example:
I'm learning JavaScript.
Explain this function line by line.
Don't rewrite it.
Then give me three small exercises that test
the same programming concept.Or:
I don't understand asynchronous JavaScript.
Teach me using a simple example.
Don't assume I know promises.
Explain the concept progressively.This approach can help you learn rather than simply copy code.
How to Use Codex for Performance Optimization
Don’t simply tell Codex:
“Make my website faster.”
Instead:
Analyze this application for performance problems.
Do not make changes yet.
Identify the five most important bottlenecks.
For each one explain:
- What is slow?
- Why is it slow?
- How significant is the impact?
- What would you change?
- How can we measure the improvement?
Then implement only the highest-impact safe improvements.This creates a measurement-based workflow rather than unnecessary optimization.
How to Use Codex for Security
Security deserves additional caution.
You can ask Codex to perform a security review:
Perform a security review of this application.
Check for:
- Authentication problems
- Authorization problems
- SQL injection
- XSS
- CSRF
- Unsafe file uploads
- Exposed secrets
- Weak input validation
- Insecure API endpoints
- Dependency vulnerabilities
Do not modify files yet.
Explain each finding and recommend a fix.AI-based security review should complement appropriate professional security testing for important applications.
Never Give Codex Your Secrets
Avoid putting sensitive credentials into prompts.
Do not casually expose:
- Passwords
- API keys
- Private keys
- Database passwords
- Authentication tokens
- Production credentials
- Secret environment variables
Use your project’s normal environment and secret-management mechanisms instead.
How to Tell Codex What NOT to Change
This is an extremely useful technique.
For example:
Do not:
- Change the database schema.
- Change the public API.
- Replace the framework.
- Modify authentication.
- Remove existing features.
- Upgrade dependencies unless necessary.
- Change unrelated files.This helps keep the scope under control.
Define “Done”
A good Codex task should have a clear definition of completion.
For example:
The task is complete when:
- The bug is fixed.
- Existing tests pass.
- A regression test exists.
- The application builds successfully.
- No unrelated files were changed.
- The desktop layout remains unchanged.
- The mobile layout works correctly.
- You provide a summary of the changes.This is much better than simply saying:
“Fix it.”
The Best Codex Workflow
For most projects, a good workflow is:
1. Understand
Ask Codex to inspect the project.
2. Plan
Create an implementation plan.
3. Implement
Make the required changes.
4. Test
Run relevant tests and checks.
5. Review
Inspect the changes.
6. Improve
Fix problems discovered during review.
The complete process is:
Understand → Plan → Implement → Test → Review → Improve
This is much more reliable than:
Prompt → Generate everything → Hope it works
Example: Build a Gaming Website With Codex
Suppose you want to build a website called GameHalt.
Start with:
I want to build a gaming website called GameHalt.
Goal:
Create a modern gaming website where users can discover
gaming content.
Requirements:
- Responsive design
- Dark gaming interface
- Homepage
- Search
- Categories
- Game cards
- Featured section
- Mobile navigation
- SEO-friendly HTML
- Accessible UI
Technology:
Use HTML, CSS and JavaScript unless the existing project
requires another technology.
First inspect the project.
Create an implementation plan.
Do not modify files yet.After reviewing the plan:
Implement the approved plan.
Keep the code modular.
Don't introduce unnecessary dependencies.
After implementation:
- Run available tests.
- Check for JavaScript errors.
- Check responsive behavior.
- Summarize all changed files.Then:
Review the website for mobile layout problems.
Check common mobile widths.
Fix any overflow or navigation issues.
Don't change the desktop visual design.Finally:
Perform a final review.
Check:
- Responsiveness
- Accessibility
- SEO
- JavaScript errors
- Broken links
- Performance
- Unused CSS
- Missing metadata
Fix safe issues and report anything that remains.This is a practical example of how to use Codex for a real project.
10 Useful Codex Prompts
1. Understand a project
Analyze this project without changing any files.
Explain:
- Architecture
- Entry points
- Main components
- Data flow
- API connections
- Authentication
- Testing setup2. Fix a bug
Find the root cause of this bug.
Explain the cause before making changes.
Implement the smallest safe fix.
Run the relevant tests.3. Add a feature
Add [FEATURE].
First inspect the existing architecture.
Create a plan.
Do not modify unrelated functionality.
Implement the feature and add appropriate tests.4. Review code
Review the current implementation for:
- Bugs
- Security problems
- Performance issues
- Missing tests
- Maintainability problems
Don't modify anything yet.5. Refactor
Identify unnecessary complexity in this module.
Propose a safe refactoring plan.
Preserve existing behavior.
Implement the refactoring.
Run the tests afterward.6. Write tests
Inspect this module and identify important untested behavior.
Add focused tests.
Do not change production code unless you find a genuine bug.7. Improve performance
Analyze this application for performance bottlenecks.
Identify the highest-impact problems before making changes.8. Investigate an error
Investigate this error and trace it to its root cause.
Explain why it happens before proposing a solution.9. Review changes
Review the current changes as a senior software engineer.
Rank findings:
Critical
High
Medium
Low10. Final review
Perform a final engineering review.
Check:
- Functionality
- Tests
- Security
- Performance
- Error handling
- Maintainability
Report anything that should be fixed before release.A Reusable Codex Prompt Template
Save this template for future projects:
Goal:
[What I want to accomplish]
Context:
[Project information]
Requirements:
- [Requirement 1]
- [Requirement 2]
- [Requirement 3]
Constraints:
- Don't change [X]
- Don't introduce [Y]
- Preserve [Z]
Process:
First inspect the existing project.
Create a plan if the task is complex.
Then implement the changes.
Validation:
- Run relevant tests.
- Check for errors.
- Review the changes.
- Report unresolved issues.
Definition of done:
[Describe exactly what successful completion means]Advanced Codex Workflows
Codex is not limited to simple code generation.
OpenAI currently describes Codex as supporting more advanced workflows, including multiple agents, worktrees, cloud environments, skills, automated workflows and longer-running engineering tasks.
For example, a larger project could involve:
Agent 1 → Investigate bug
Agent 2 → Write tests
Agent 3 → Implement fix
Agent 4 → Review changesThe advantage is that different tasks can potentially be handled in parallel rather than sequentially.
OpenAI’s Codex app was specifically designed around managing multiple agents and longer-running development work.
Can Codex Work in the Background?
Codex is increasingly designed for longer-running development workflows.
OpenAI currently describes background workflows for tasks such as issue triage, monitoring and CI/CD-related work.
This means Codex can move beyond:
“Answer this coding question.”
toward:
“Work on this engineering task and report the result when you’re finished.”
That is one of the biggest differences between a coding chatbot and an AI coding agent.
Common Mistakes to Avoid
Mistake 1: Vague prompts
Bad:
“Make my website better.”
Better:
“Improve the mobile layout without changing the desktop design.”
Mistake 2: Asking for everything at once
Bad:
“Build an entire social network.”
Better:
“Start by implementing authentication.”
Mistake 3: Not reviewing changes
Always inspect important modifications.
Mistake 4: Skipping tests
A successful-looking implementation can still contain bugs.
Mistake 5: Allowing unrelated modifications
Tell Codex what it should leave alone.
Mistake 6: Trusting security code blindly
Authentication, payments and permissions require careful review.
Mistake 7: Not defining success
Tell Codex exactly what “finished” means.
Is Codex Good for Beginners?
Yes, but beginners should use it as both a coding assistant and learning tool.
Instead of:
“Write everything for me.”
try:
“Build this feature and explain why each important part is necessary.”
That approach helps you understand what Codex is doing.
Over time, you can move from small tasks to:
- Websites
- Scripts
- APIs
- Mobile applications
- Full-stack applications
- Automation
- Testing
- Code review
Final Thoughts
Codex is more than an AI that generates programming code.
It is designed as an AI coding agent that can participate in real software-development workflows.
You can use it to inspect projects, plan features, write code, modify files, debug problems, run tests, review changes and work with repositories.
The most effective workflow is:
Understand → Plan → Implement → Test → Review → Improve
The quality of your results also depends heavily on how clearly you describe the task.
Instead of saying:
“Build an app.”
tell Codex:
“Here is what I want to build, here is the existing project, these are the requirements, these are the things you must not change, and this is how we will know the task is complete.”
That small change in the way you communicate with the AI can make a huge difference.
Whether you’re a professional developer, a student, a website owner, or someone learning programming, Codex can become a powerful part of your development workflow when you combine AI assistance with proper testing and human review.
Frequently Asked Questions
What is Codex?
Codex is OpenAI’s AI coding agent for writing, reviewing and shipping code.
How do I use Codex?
Sign in with your ChatGPT account, launch Codex through a supported client, provide access to your project when needed, describe your task, review the changes and run appropriate tests.
Is Codex free?
Codex is included across ChatGPT plans, including Free and Go, but usage limits vary by plan.
Can Codex build websites?
Yes. Codex can be used for website and web-application development.
Can Codex fix bugs?
Yes. Debugging existing code is one of the primary ways to use a coding agent.
Can Codex write tests?
Yes. You can ask Codex to create tests and run the relevant test suite.
Can Codex use GitHub?
Codex can participate in repository-based development workflows where the appropriate environment and permissions are available.
Can I use Codex from the terminal?
Yes. OpenAI provides a Codex CLI.
Can I use Codex inside an IDE?
Yes. OpenAI currently provides a Codex IDE extension.
Is Codex the same as ChatGPT?
No. ChatGPT is the general conversational experience, while Codex is focused on software development and engineering workflows.
Should I review Codex-generated code?
Yes. Important code should always be reviewed and tested, especially when it involves authentication, payments, databases, security or production systems.