Ah ha, I know what you're hitting.

5.3-codex is only available via the Responses API, not the Completions API. Two different APIs for model access. If you were using Completions you have to port to Responses. It's not that hard. I did this for my own agent the other week. I think it might be like that for all their new models from now on. Responses is a much more powerful API. It's more like a front to ChatGPT than the underlying models.