As far as I know, OpenAI did not release 5.3 Codex in their API. You can only use it with Codex CLI or app.
It's there, you just need to use it with the responses API. Set model field to 'gpt-5.3-codex'
alt Hacker News
It's there, you just need to use it with the responses API. Set model field to 'gpt-5.3-codex'