> Isn't it open source?

No, its not even source available,.

> Or is there an open source front-end and a closed backend?

No, its all proprietary. None of it is open source.

No, it was never open source. You could always reverse engineer the cli app but you didn't have access to the source.

The Github repo is only for issue tracker

You may have mistaken it with Codex

https://github.com/openai/codex

No