Using the model gpt-5-bench-chatcompletions-gpt41-api-ev3 via the Chat Completions API will give you what is supposedly GPT-5.
Conjecture: The "gpt41-api" portion of the name suggests that there's new functionality to this model that will require new API parameters or calls, and that this particular version of the model is adapted to the GPT-4.1 API for backwards compatibility.
Here you can see me using it via curl :
https://preview.redd.it/glxute607egf1.png?width=1181&format=png&auto=webp&s=a8a6928801e2d7bf0f6a30122471a33eb3fa092d
And here's the resulting log in the OpenAI Console:
https://preview.redd.it/wagioy837egf1.png?width=1350&format=png&auto=webp&s=1144c1f27086857e97fb8ada942e254dfafeb414
EDIT: Seems OpenAI has caught wind of this post and shut down access to the model.