Use any model and any provider with the official OpenAI Codex Desktop App, without modifying its code, and continue to use the official models in parallel?

All in the title. The official OpenAI Codex Desktop App only accepts models that are from OpenAI and from a curated list. But there is a trick, you can make it think you are using the official OpenAI servers and models by just impersonating the model name and using another one.

There is three things you need to do

1) The first one is to modify the officiel config of the codex desktop app. This is none intrusive and doesn’t break the app in anyways, and can also be reverted. You can do that by going in the settings and clicking the « open config.toml » link.

Then you need to modify the official config by pointing it to your own server where your models are living.

model = "gpt-5.3-codex"
model_provider = "multivibe"
model_reasoning_effort = "xhigh"
personality = "pragmatic"
sandbox_mode = "danger-full-access"
approval_policy = "never"

[model_providers.multivibe]
name = "Multivibe"
base_url = "http://127.0.0.1:1455"
wire_api = "responses"
env_key = "MULTIVIBE_API_KEY"

2) The second thing you need to do is to store the api key that allows to communicate with your hosting server. In my case it is called MULTIVIBE_API_KEY, and I put it into zshrc, you might need to put it in your bashrc or any similar location.

3) If you did the first two it might or might not work, your mileage may vary. But this third step will definitely make it work 100% of the time. Sometimes codex desktop checks what model it is really using. And since you are likely using a completely different model than an OpenAI model it will display a message similar to this one :

{"detail":"The 'MiniMax-Latest' model is not supported when using Codex with a ChatGPT account."}

I have built a proxy for codex that originally was intended as a codex proxy for using multiple ChatGPT account (and it still does work for that purpose).

But there is one neat new feature that has been under used by most which is the « alias ». You can configure in the alias tab how the multicodex proxy resolves the final model. In my case anytime a client (codex desktop) asks for the model « gpt-5.3-codex » it resolves to « MiniMax-Latest » BUT ! And this is the very important part, it ensures every single messages that are sent to the client (codex desktop) disguises the model name as being « gpt-5.3-codex » even though it is completely different.