# Codex or Claude Code: which coding agent should you use?

Date: 2025-05-27 · Author: Robert Scales · https://guidetoartificialintelligence.com/en/learn/codex-claude-code-coding-agent-use/

**Answer:** Use OpenAI's Codex when you want to hand off several well-defined tasks to cloud sandboxes and review the results later, and Anthropic's Claude Code when you want an agent working beside you in your own terminal or editor, asking before it changes things. Many developers will want both, for different kinds of work.

Two coding agents arrived in the same week. On May 16, OpenAI opened a research preview of Codex inside ChatGPT, running on codex-1, a version of its o3 model trained for software engineering. On May 22, Anthropic made Claude Code generally available alongside its new Claude Opus 4 and Claude Sonnet 4 models. Both read your code, edit files, run tests and report back. The difference that matters day to day is *where* they work and *how* you supervise them.

Codex works away from you. Each task runs in its own isolated container in the cloud, loaded with a copy of your GitHub repository, with internet access turned off while it works. You type a request, click "Code", and come back in one to thirty minutes. Claude Code works with you. It runs in your terminal, or through new beta extensions in VS Code and JetBrains editors, on your own machine, and by default it asks permission before writing files or running commands that change things.

## The same task, two workflows

Take a common job: a small web app's signup form accepts email addresses without checking them. You want validation added, with tests. Here is what each workflow looks like, based on how each company describes its tool. The descriptions are illustrative, not a benchmark.

| Step | Codex (cloud) | Claude Code (local) |
|---|---|---|
| Setup | Connect the GitHub repository; write a setup script that installs dependencies, because the sandbox has no internet during the task | Install it, open a terminal in the project folder, start `claude` |
| Project instructions | An `AGENTS.md` file in the repository: how to run tests, code conventions | A `CLAUDE.md` file: commands, style rules, test instructions |
| Giving the task | Type the request in ChatGPT's Codex panel and click "Code"; you can launch other tasks in parallel | Type the request in the terminal; Anthropic suggests asking it to explore and plan before writing code |
| While it works | Watch progress; no steering mid-task in this preview | It stops for approval on file edits and commands unless you allow them |
| Evidence | Citations to terminal logs and test output | You see each command and its output as it runs |
| Result | Review the changes, ask for revisions, or open a GitHub pull request | The changes are already in your working folder; you review the diff and commit |

A good prompt works for both:

```prompt
Add email validation to the signup form.
- Reject empty values and addresses without "@" and a domain.
- Show the existing error style; don't add new CSS.
- Write tests first in tests/test_signup.py, confirm they fail, then make them pass.
- Don't change any other test. Run the full test suite before finishing.
```

The "tests first" line follows Anthropic's own advice for Claude Code: agents do best with a clear target, and failing tests are one. It suits Codex equally well, since codex-1 was trained to keep running tests until they pass.

## How to choose

**Choose Codex when** the work is a batch of well-scoped jobs: renaming, adding tests, fixing several small bugs, drafting documentation. OpenAI says its own engineers use it this way, to clear repetitive tasks without breaking focus. The cloud sandbox also means nothing runs on your laptop. It suits teams on ChatGPT Pro, Team or Enterprise plans, the ones with access in this preview; Plus and Edu are due to follow.

**Choose Claude Code when** you want to stay in the loop: unfamiliar code you need to understand, changes that touch your local setup, front-end work where you want to show the agent a screenshot or a design mock (the Codex preview doesn't accept images yet). Its permission prompts slow things down, but they also catch mistakes before they happen. Its headless mode (`claude -p "..."`) and new GitHub integration let it run in scripts and on pull requests too.

Both are strong. Anthropic reports 72.5% for Opus 4 and 72.7% for Sonnet 4 on SWE-bench Verified, a test built from real GitHub issues, and OpenAI reports strong results for codex-1 on the same family of tests. Those numbers say both can do real work; they don't say which suits *your* habits.

> **Try it:** Pick one small, real issue in a project you know. Write the task as a prompt with a test to pass, as above. If you have access to both tools, run it once in each and note three things: how long you waited, how many times you had to intervene, and how easy the result was to review.

## Can you use both on the same project?

Yes, and it can be a sensible split: Codex for background tasks you queue up and review later, Claude Code for the interactive work you do at your desk. A few habits keep them from getting in each other's way.

Keep one set of instructions. Codex reads `AGENTS.md`; Claude Code reads `CLAUDE.md`. Write the shared content (test commands, conventions, what not to touch) once, and keep the two files in step, for example by making one a short pointer to the other. If they disagree, the agents will follow different rules in the same codebase.

Let Git be the referee. Codex works on its own copy and proposes a pull request; Claude Code edits your working folder. Have each work on its own branch, and merge through normal review, so two agents never edit the same files unseen.

Review everything the same way. OpenAI says plainly that people must check agent-written code before merging and running it, and Anthropic warns that letting an agent skip permission checks is risky outside an isolated container. The agent that wrote a change doesn't change the standard it has to meet: tests pass, a human reads the diff, then it merges.

## Sources

- Introducing Codex: https://openai.com/index/introducing-codex/
- Introducing Claude 4: https://www.anthropic.com/news/claude-4
- Claude Code: Best practices for agentic coding: https://www.anthropic.com/engineering/claude-code-best-practices