Why do gpt-5.1 and gpt-5.4-mini behave so differently in production chatbot use cases?

The mini models are almost always less capable because they have significantly lower precision. The trade-off is speed and price. I personally find the nano models unusable for my use cases for this very reason. Mini models can be a good compromise for certain purposes and you will have to make that judgement for yourself. For my personal chatbot tasks I use the full size frontier models. It saves tokens in the end of the day because the model is more likely to one shot. Same goes for many production tasks. I don’t want to have to review and rerun things because a smaller model failed.