What is OpenAI Codex and how is it different from ChatGPT?

Codex is OpenAI's AI coding agent. While ChatGPT can write code in a conversation, Codex is built to work directly on real software projects: reading a codebase, making changes across files, running commands and tests, and preparing changes for review.

The name has a history. OpenAI first used "Codex" for an early code model that powered the original GitHub Copilot around 2021. The current Codex is a newer and much more capable product, built as an agent for software engineering work.

How is it different from ChatGPT? When you ask ChatGPT for code, it writes a snippet in the chat, and you copy it into your project yourself. You are responsible for fitting it into the existing code, running it and fixing problems. Codex works inside the project. It can look at how your code is organised, follow your existing patterns, edit the relevant files, run the test suite and keep iterating until the change works. The result is a set of changes, often ready as a pull request, rather than a block of text to paste.

Codex is available in several forms, including a cloud version that works on tasks in a sandboxed environment connected to your GitHub repositories, a command-line tool that runs in your terminal, and integrations with code editors. Access is generally included with ChatGPT paid plans, with usage limits depending on the plan.

Typical tasks developers hand to Codex:

1. Fixing a reported bug, including finding the cause.
2. Adding a feature that touches several files.
3. Writing or improving tests.
4. Refactoring older code.
5. Explaining how an unfamiliar codebase works.
6. Reviewing code changes for problems.

Some good practices apply. Give clear task descriptions with acceptance criteria, such as "the form should reject phone numbers with fewer than 10 digits, and existing tests must pass." Keep tasks reasonably small. Always review the changes carefully before merging, because AI-written code can contain subtle bugs or security issues. And keep secrets such as API keys out of the code and the prompts.

Codex competes with other coding agents, such as Anthropic's Claude Code, GitHub Copilot's agent features and editors like Cursor. Many developers try several and choose based on their language, project size and workflow. For beginners, ChatGPT is a gentler start; for professional developers working on real repositories, an agent like Codex can save serious time.