Codex

Codex is an AI coding agent made by OpenAI. It writes features, fixes bugs, answers questions about a codebase and proposes changes for review. OpenAI released Codex CLI in April 2025 and the cloud version inside ChatGPT in May 2025. The name was first used for an earlier OpenAI code model in 2021.

Codex works in the terminal, in an extension for VS Code and similar editors, and in the cloud, where it runs tasks in parallel in separate sandboxes. It can review pull requests on GitHub. It runs on OpenAI models tuned for coding. Codex CLI is open source. Developers can give Codex a task and come back later to review the changes it made, with logs and test results.

Codex is included in ChatGPT Plus, Pro, Business, Edu and Enterprise plans, with usage limits that depend on the plan. Developers can also use it with an OpenAI API key and pay per use. OpenAI is based in San Francisco.