Mythos

Codex is an 📝OpenAI product that works as an AI software-engineering agent, writing, reviewing, and running code in a developer's terminal, in their editor, or in cloud sandboxes launched from 📝ChatGPT. The name first belonged to a 2021 model derived from 📝GPT-3 that powered the original 📝GitHub Copilot; OpenAI revived it in April 2025 for the open-source Codex CLI and in May 2025 for a cloud agent, reaching general availability in October 2025, and since September 2026 it runs the GPT-6 models. It competes most directly with 📝Anthropic's 📝Claude Code. Codex is included with ChatGPT Plus, Pro, Business, Enterprise, and Edu plans and is also available to developers through the OpenAI API.

I use Codex every day alongside Claude Code through the 📝Codex Integration for Claude Code skill. After Claude Code drafts a plan in Plan Mode, I have Codex review it for optimizations; the two models catch different things, and that second opinion makes me more confident taking on complex builds.

Key Features

  • Codex CLI — an open-source terminal agent, released in April 2025, that reads a repository, edits files, and runs commands locally under configurable approval and sandbox settings.
  • Cloud agent — runs many tasks in parallel inside isolated cloud sandboxes launched from ChatGPT, returning diffs and pull requests to review rather than touching the developer's machine.
  • IDE extension — brings the same agent into code editors such as VS Code and 📝Cursor, so developers can hand off tasks and review changes without leaving the file they are working in.
  • Code review — reviews pull requests on GitHub, flagging bugs and risky changes with comments, and can be triggered automatically on every pull request or on demand from a comment.
  • Model choice — lets developers pick among OpenAI's current models, from GPT-6 Astra for the hardest engineering work to GPT-6 Luna for fast, high-volume tasks, and set reasoning effort.

Getting Started

  1. Sign in to ChatGPT on a Plus, Pro, Business, Enterprise, or Edu plan to unlock Codex access.
  2. Install the CLI with npm install -g @openai/codex, then run codex inside a repository.
  3. Or open Codex in ChatGPT and connect a GitHub repository to run tasks in the cloud.
  4. Choose a model and reasoning effort, then describe the change you want in plain language.

Related

Contexts

Created with 💜 by One Inc | Copyright 2026