Since when?

https://learn.chatgpt.com/docs/codex/cli

https://github.com/openai/codex

Looks like it's still a more-or-less-open-source product called Codex CLI.

FWIW, there have been issues with various versions of ChatGPT not knowing what Codex is -- at least in the (IIRC) GPT-5.3 timeframe, there were serious issues with the knowledge cutoff.

They did it about a month ago for the desktop Codex app. It is actually really confusing and there are many discussions on reddit.

First, the app was called Codex, then they combined them and now the app is called chatGPT, but recently inside the app you can switch between ChatGPT and Codex. lol.