Well, the reason is simple: over the past several months, it has become very difficult to use Codex with non-OpenAI models. They removed the old edit tool that didn't require OpenAI's free form tool calling (that no other LLM host supports), they are adding tools to every request of a type that break most LLM hosts unless you use a proxy to filter them out, they add a "developer" role to some messages which breaks some chat templates, etc.
If someone wanted to fork Codex and make a community-maintained version that supports third party models, that would be great, because I liked Codex better than OpenCode for the most part.
Maybe you've found workarounds. Maybe you're using an old version of Codex. Maybe you have your own soft fork. I don't know. But I used to be able to use Codex with self-hosted models, and I gave up on that about a month ago as they kept breaking that.
Ah, I wasn't aware things regressed there. Yea certainly workarounds n soft fork sorts of things definitely would work- but thats a bummer than things have changed.
From watching Pr's and issues- seems like openai at least wants to come across as if theyre supporting non-oai models :/
Yeah... one of the relevant issues: https://github.com/openai/codex/issues/11940#issuecomment-45...
You would think they would support their own GPT-OSS model, but, not really anymore. I wish they would release a GPT-OSS 2, but this doesn't fill me with confidence.