What it is
The job it was built for, and the job it is not for.
GPT-5.1 is a general-purpose frontier model rather than a specialist one, intended to handle reasoning, long-document work and everyday conversation from the same endpoint. OpenAI describes it as using adaptive reasoning, so it varies how much thinking it applies to a request instead of treating every prompt the same way. It is not a cheap high-volume classifier and it is not an audio or video model, it takes text, images and files in and returns text.