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What Is the Difference Between Codex and ChatGPT?

Codex is OpenAI's cloud-based coding agent inside ChatGPT. Here's how they differ.

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What Is the Difference Between Codex and ChatGPT?

Codex is a cloud-based software engineering agent that lives inside ChatGPT — it's a specialized tool for coding tasks, not a separate product. ChatGPT is OpenAI's general-purpose conversational AI. The core difference: ChatGPT handles open-ended conversations across any topic, while Codex is purpose-built to read your codebase, write code, run tests, and propose pull requests in isolated cloud sandboxes.

Context

The confusion is understandable because Codex is accessed through ChatGPT's sidebar. But under the hood, they run different models optimized for different jobs. ChatGPT uses models like GPT-4o for general conversation, while Codex is powered by codex-1, a version of OpenAI o3 specifically trained with reinforcement learning on real-world coding tasks.

When you use ChatGPT, you're having a conversation — asking questions, generating text, analyzing documents. When you use Codex, you're delegating a software engineering task. Each Codex task spins up an independent sandbox environment preloaded with your GitHub repository. The agent can read and edit files, run terminal commands (test harnesses, linters, type checkers), and commit changes — all without internet access during execution for security.

ChatGPT responds in seconds. Codex tasks take 1 to 30 minutes depending on complexity, and you can run multiple tasks in parallel. For a deeper look at Codex's capabilities, see our complete Codex guide.

Practical Steps

  1. Use ChatGPT for general questions, brainstorming, writing, and quick code snippets in conversation
  2. Use Codex when you need an agent to work directly in your repository — writing features, fixing bugs, refactoring, or generating tests
  3. Assign Codex tasks by clicking "Code" in the ChatGPT sidebar and linking your GitHub repo
  4. Ask codebase questions by clicking "Ask" — Codex will search your repo and return grounded answers with citations
  5. Guide Codex with AGENTS.md files in your repository to define coding conventions, test commands, and project standards

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