A Beginner’s Guide to Codex – Even Total Beginners Can Learn

I’m writing a truly hands-on beginner’s guide for all Codex newcomers.

No esoteric concepts, no jargon overload – just how an ordinary person can go from zero to using Codex to read code, modify code, fix bugs, and write features.

1. What is Codex?

In one sentence:

Codex is OpenAI’s “code agent.”

It’s not an ordinary chatbot, nor a plugin that only autocompletes a few lines of code.

More accurately, it’s like an AI programmer that can participate in your project. You can give it a task, for example:

“Help me understand this project.” “Help me fix this error.” “Help me add a login page.” “Help me write tests for this API.” “Help me refactor this code.” “Help me review whether these changes have any problems.”

It reads your code structure, understands the context, proposes a plan, modifies files, runs commands, and even explains why it made those changes.

The most important thing for beginners to remember:

Don’t use Codex like a “search engine.” Use it like a “junior pair programmer.”

You are responsible for clearly stating the goal, checking the result, and deciding whether to accept it; Codex is responsible for reading, writing, modifying, checking, and running.

2. Who is Codex for?

I think it’s best for these types of people:

First: People just learning to program. You can’t understand project structure, you don’t know where a function comes from, you don’t understand error messages. Codex can explain code in plain language.

Second: People who know some code but aren’t familiar with engineering projects. You might be able to write Python, JS, HTML, but when you open a real project you get overwhelmed. Codex can guide you through directories, dependencies, startup methods, and main logic.

Third: Developers who want to improve efficiency. Repeatedly writing CRUD, changing styles, adding tests, fixing lint, migrating configs – these tasks are well-suited for Codex.

Fourth: Solo developers. When building a product alone, the biggest missing piece is a partner. Codex can temporarily act as a “code buddy.”

3. Where should a beginner start?

If you’re a pure beginner, I recommend this sequence:

1. First use Codex to read code.
2. Then have Codex make small-scope modifications.
3. Then have it fix bugs.
4. Finally have it write complete features.

Don’t start with:

“Help me build a complete SaaS.”

Such requests are too large, and beginners can’t easily judge if it’s doing it correctly.

The right approach is:

“First explain the project structure.” “Then tell me how to start it.” “Then find a minimal feature point to modify.” “Then let me see the diff.” “Then run the tests.”

Step by step – this is the safe way.

4. What do you need to prepare?

You’ll need at least:

• A ChatGPT / OpenAI account
• A code project
• A local development environment
• Basic knowledge of Git (preferable)

If you don’t know Git yet, just remember these three commands:

git status
git diff
git checkout -b codex-test

Explanation:

• git status: see which files have changed.
• git diff: see exactly what changed.
• git checkout -b codex-test: create a new branch to avoid polluting the main branch.

Beginners must form the habit:

Never let AI directly make large changes on the main branch.

First create a branch, modify, check, and only merge if everything is fine.

5. Common ways to use Codex

There are several common entry points:

Codex App: Suitable for people who like a graphical interface. You can manage multiple tasks in the app and see Codex’s progress and changes.

Codex CLI: Suitable for people comfortable with the terminal. You open a terminal in your project directory, type codex, and it can read the current project.

IDE Plugin: Suitable for asking questions while coding in editors like VS Code, Cursor, Windsurf.

Codex Web: Suitable for handing tasks to a cloud environment, especially for working with GitHub repositories to run tasks, create PRs, and review code.

How to choose for beginners?

• If you’re a complete beginner: Use the App or IDE plugin first.
• If you’re comfortable with the terminal: Learn CLI directly.
• If you have a GitHub project: Try Codex Web.

6. Codex CLI installation for beginners

For macOS/Linux, the official installation looks like this:

curl -fsSL https://chatgpt.com/codex/install.sh | sh

After installation, type in the terminal:

codex

The first time you run it, it will prompt you to log in to your ChatGPT account or use an API key.

After logging in, go to your project directory:

cd your-project
codex

Then you can start chatting with it.

If you’re a Windows user, I recommend checking the official Windows guide first, or setting up a Linux-style development environment using WSL2.

7. First time using Codex – don’t rush to modify code

The first time you open a project, I suggest you send this:

Please do not modify any files yet. Please read this project and tell me:
1. What does this project do?
2. What is the main directory structure?
3. What commands are needed to start the project?
4. Which files should a beginner start reading?
5. What do you suggest I do next?

Why add “do not modify any files”?

Because for beginners, the most important thing on the first use is to build trust and understanding. You want to see if it correctly understands the project, not let it change things randomly.

If its explanation seems reliable, then continue to the next step.

8. Let Codex help you understand code

You can ask like this:

Explain the purpose of src/main.ts. Requirements:
1. Explain in plain language that a beginner can understand.
2. Describe what each main function does.
3. Highlight 3 concepts I need to focus on.
4. Do not modify the code.

Or:

Help me trace the execution flow of this request from frontend to backend. Don't change code, just explain.

Or:

I don't understand this error. Please explain the cause in the context of the project and tell me which files might be involved. Don't modify anything yet.

As a beginner, Codex’s most valuable ability is not “writing code” but “reading code + explaining code.”

9. Let Codex make small-scope modifications

When you’re ready to have it modify code, never make the request too broad.

Don’t say:

Optimize the whole project for me.

That’s too vague.

Instead, say:

Please only modify the button text on the login page:
1. Change "Submit" to "Login"
2. Don't change any other styles
3. Before modifying, tell me which file you plan to change
4. Show the diff after modification

Or:

Please add an empty state to the user list page:
1. When the user array is empty, display "No users"
2. Don't affect existing list rendering
3. Minimize changes
4. After modification, explain what you changed

Key points:

• Goal must be specific.
• Scope must be small.
• Constraints must be clear.
• Result must be verifiable.

10. The right way to let Codex fix bugs

Many beginners just throw in:

Fix it for me.

That’s not enough.

A better way:

I encountered this error after running npm run dev:

[Paste error]

Please:
1. First explain what this error means
2. Find the most likely cause
3. Tell me which files you plan to check
4. Do not modify code yet – wait for my confirmation

If you want it to fix directly:

Please fix this error. Requirements:
1. Minimal changes
2. Don't refactor unrelated code
3. After modification, run relevant tests or start commands
4. Summarize the reason for the change

The core of bug fixing is:

Locate first, then modify, then verify.

Don’t let Codex make large-scale changes without boundaries.

11. Standard prompt for letting Codex write features

Here’s a general template:

Please add [feature name] to this project.

Background:
[Explain what problem this feature solves]

Requirements:
1. [Requirement 1]
2. [Requirement 2]
3. [Requirement 3]

Constraints:
1. Follow the existing code style as much as possible
2. Don't introduce unnecessary new dependencies
3. Don't modify unrelated files
4. If anything is uncertain, list it before making changes

After completion, please:
1. Summarize which files were modified
2. Show the key diff
3. Explain how to test
4. If tests fail, explain why

Example:

Please add a "filter by completion status" feature to this Todo project.

Requirements:
1. Add three filter options: All / Active / Completed
2. Default to All
3. Clicking Active shows only unfinished tasks
4. Clicking Completed shows only finished tasks

Constraints:
1. Don't change the data structure
2. Don't introduce a new UI library
3. Keep the current style

After completion, explain how to manually test.

12. You must learn to review the diff

After Codex modifies code, don’t just accept it blindly.

At minimum, check three things:

First: See which files were changed.

git status

Second: See exactly what was changed.

git diff

Third: Run the project or tests.

npm test
npm run dev
pytest
go test ./...

Different projects have different commands, but the principle is the same:

AI writing code doesn’t mean it’s done. It’s only done if it runs, the logic is correct, and changes are minimal.

13. 7 pitfalls beginners must avoid

Pitfall 1: Asking it to refactor the entire project right away. The larger the refactoring scope, the harder it is to check.

Pitfall 2: Accepting changes without reviewing the diff. This is the most dangerous habit.

Pitfall 3: Pasting API keys, passwords, or tokens directly to it. Never send sensitive information to any AI tool. Use fake or anonymized data when debugging.

Pitfall 4: Letting it delete large amounts of code without confirmation. Any operation involving deletion, migration, or database changes must first have the plan explained.

Pitfall 5: Not providing context. Just saying “it’s broken” may not be enough. Attach the full error, reproduction steps, and expected outcome.

Pitfall 6: Giving it multiple objectives at once. For example, “optimize styles, fix bugs, add login, refactor directory.” This can cause the task to spiral out of control.

Pitfall 7: Blindly trusting AI results. Codex is powerful, but it can still make mistakes. You are ultimately responsible.

14. Recommended fixed workflow for beginners

I suggest you follow this process each time:

Step 1: Create a branch

git checkout -b codex-task

Step 2: Let Codex read the project without modifying code

Please understand this project and tell me how you would accomplish the task. Do not modify files yet.

Step 3: Confirm the plan

If the plan seems good, say:

You can start implementing. Keep changes minimal and summarize the diff after completion.

Step 4: Check the changes

git status
git diff

Step 5: Run tests

npm test

Or run the project:

npm run dev

Step 6: Let Codex review

Please summarize this modification:
1. What was changed?
2. Why was it changed that way?
3. How to test?
4. What are the potential risks?

This workflow is great for beginners.

15. Writing a project description for Codex: AGENTS.md

If you often use Codex on a project, consider adding an AGENTS.md file to the project root.

Think of it as a “project manual for the AI programmer.”

Example content:

# AGENTS.md

## Project Description
This is a Next.js project for managing personal tasks.

## Common Commands
- Install dependencies: `npm install`
- Run locally: `npm run dev`
- Run tests: `npm test`
- Check formatting: `npm run lint`

## Code Style
- Uses TypeScript
- Components are functional components
- Do not introduce new UI libraries unless explicitly requested by the user
- Prefer minimal changes

## Notes
- Do not modify .env files
- Do not commit node_modules
- For database migrations, explain the plan before executing

With this file, Codex can better follow your project rules.

16. 20 most useful prompts for beginners

1. “Please do not modify code. Help me explain this project structure.”
2. “Tell me how to start this project and what dependencies are needed locally.”
3. “Explain the purpose of this file in plain language.”
4. “Explain the input, output, and core logic of this function.”
5. “Help me find the most likely cause of this error. Don’t modify code yet.”
6. “Fix this bug with minimal changes, and explain why you made the changes.”
7. “Please modify only this component without affecting other files.”
8. “Please add unit tests for this function.”
9. “Review this diff for potential bugs.”
10. “Make this code more readable without changing its behavior.”
11. “Help me find where this page’s data comes from.”
12. “List the files that need to be modified to implement this feature; don’t start yet.”
13. “Give me an implementation plan. I’ll confirm before you start writing code.”
14. “Implement using the current project style; don’t introduce new dependencies.”
15. “After modifying, run the tests and tell me the results.”
16. “Explain the diff of this modification.”
17. “Check for security risks, such as token leakage, SQL injection, or XSS.”
18. “Break this complex function into smaller functions while keeping tests passing.”
19. “Help me write the ‘Getting Started’ section of the README.”
20. “Act as a code reviewer and point out the top 5 areas most worth improving in this code.”

17. A complete example: Let Codex fix a frontend bug

Suppose the button on your page does nothing when clicked.

You can ask:

In the current project, the login button is not responding when clicked.

Please:
1. Find the component corresponding to the login button
2. Check whether the click event is bound
3. Check whether the request function is called
4. First summarize the cause
5. Do not modify code yet

After Codex analyzes, if you agree, continue:

Based on your analysis, please fix this problem. Requirements:
1. Minimal changes
2. Don't change UI styles
3. After modification, list the files involved
4. Tell me how to manually verify

After it finishes, you check:

git diff
npm run dev

Open the page and manually click the button to verify.

Finally, have Codex review:

Please summarize the cause of this bug, the fix, and how to avoid it in the future.

This way you not only fix the bug but also learn something.

18. A complete example: Let Codex add a new feature

For example, you want to add a search box to your blog project.

Prompt:

Please add an article search feature to the current blog project.

Requirements:
1. Add a search box at the top of the article list page
2. When the user types a keyword, filter articles by title and summary
3. Display "No articles found" when there are no results
4. Search logic should be done on the frontend – no new backend API needed

Constraints:
1. Keep the current UI style
2. Do not introduce new dependencies
3. Do not change the article data structure
4. Only modify files related to the article list if possible

After completion:
1. Summarize which files were modified
2. Explain how to test
3. Remind me of any edge cases

What makes this prompt good?

It specifies the page location, functional logic, empty state, technical constraints, scope of changes, and acceptance criteria.

Codex’s biggest enemy is when “you don’t know what you want yourself.”

The more specific you are, the more reliable it becomes.

19. Codex doesn’t think for you – it amplifies your thinking

Many beginners fall into this misconception:

“With Codex, do I still need to learn programming?”

No.

More accurately:

You still need to learn basic concepts like variables, functions, components, APIs, databases, Git, testing.

But Codex can speed up your learning.

Before, if you couldn’t understand a project, you might be stuck for a day. Now you can have it explain file by file.

Before, when fixing an error, you had to search everywhere. Now you can have it locate the issue with project context.

Before, you were afraid to change code. Now you can have it propose a plan first, then make small changes.

It doesn’t let you skip learning – it lets you learn by doing on real projects.

20. I suggest beginners get started with Codex in 3 days

Day 1: Read only, don’t modify

Tasks:

• Have Codex explain the project structure
• Have Codex explain 3 core files
• Have Codex tell you how to start the project
• Have Codex draw a feature flow

Goal: You can roughly describe how the project works.

Day 2: Make small modifications

Tasks:

• Change a button’s text
• Change an empty state on a page
• Update the README
• Add a simple test

Goal: You can read git diff and know what the AI changed.

Day 3: Fix bugs or add small features

Tasks:

• Fix a clear error message
• Add a small feature
• Have Codex run tests
• Have Codex do a code review

Goal: You form a closed loop: “plan → modify → check → test → review.”

21. My final advice

If you’re a Codex beginner, just remember these 5 rules:

1. Have it explain first; don’t rush to have it modify.
2. The smaller the task, the more reliable the result.
3. Always create a Git branch before any modification.
4. Always review the diff after every modification.
5. Never paste secrets; never blindly trust results.

Codex’s real power isn’t “helping you write fewer lines of code.”

It’s that it can take you from “staring at a project alone” to “having someone read code, break down tasks, try solutions, and run tests with you.”

For beginners, that is the greatest value.

I suggest the first time you use it, don’t work on a large project.

Just find a small project, and say to Codex:

“Please do not modify code yet. Walk me through understanding this project.”

That sentence is the best start.