Simon Willison’s Weblog

OpenAI Codex. Announced today, here's the documentation for OpenAI's "cloud-based software engineering agent". It's not yet available for us $20/month Plus customers ("coming soon") but if you're a $200/month Pro user you can try it out now.

At a high level, you specify a prompt, and the agent goes to work in its own environment. After about 8–10 minutes, the agent gives you back a diff.

You can execute prompts in either ask mode or code mode. When you select ask, Codex clones a read-only version of your repo, booting faster and giving you follow-up tasks. Code mode, however, creates a full-fledged environment that the agent can run and test against.

This 4 minute demo video is a useful overview. One note that caught my eye is that the setup phase for an environment can pull from the internet (to install necessary dependencies) but the agent loop itself still runs in a network disconnected sandbox.

It sounds similar to GitHub's own Copilot Workspace project, which can compose PRs against your code based on a prompt. The big difference is that Codex incorporates a full Code Interpeter style environment, allowing it to build and run the code it's creating and execute tests in a loop.

Copilot Workspaces has a level of integration with Codespaces but still requires manual intervention to help exercise the code.

Also similar to Copilot Workspaces is a confusing name. OpenAI now have four products called Codex:

• OpenAI Codex, announced today.
• Codex CLI, a completely different coding assistant tool they released a few weeks ago that is the same kind of shape as Claude Code. This one owns the openai/codex namespace on GitHub.
• codex-mini, a brand new model released today that is used by their Codex product. It's a fine-tuned o4-mini variant. I released llm-openai-plugin 0.4 adding support for that model.
• OpenAI Codex (2021) - Internet Archive link, OpenAI's first specialist coding model from the GPT-3 era. This was used by the original GitHub Copilot and is still the current topic of Wikipedia's OpenAI Codex page.

My favorite thing about this most recent Codex product is that OpenAI shared the full Dockerfile for the environment that the system uses to run code - in openai/codex-universal on GitHub because openai/codex was taken already.

This is extremely useful documentation for figuring out how to use this thing - I'm glad they're making this as transparent as possible.