Codex is an AI coding agent from OpenAI: a program that uses a large language model to plan, write, change and review code for a developer, from a task given in plain language. OpenAI describes its scope as real engineering work, from planning and building features to refactors, reviews and releases.
Codex comes in four forms, all connected by one ChatGPT account: Codex in the ChatGPT app, Codex cloud on the web, an extension for code editors such as VS Code and Cursor, and the Codex CLI, which runs in the terminal on the developer's own computer. Codex cloud keeps a reusable environment for each repository and keeps working after the laptop is closed. Codex can run many agents in parallel, which use computer and browser tools to check their own work. It also reviews pull requests automatically, reports what it finds and answers questions about the changes. Plugins connect it to other tools.
Codex is included in ChatGPT plans, from the free plan upward, and higher plans give more usage; it can also run with an OpenAI API key. The Codex CLI is open source under the Apache-2.0 license.