Old school Gemini used to do this. It was super obvious because mid day the model would go from stupid to completely brain dead. I have a screenshot of Google's FAQ on my PC from 2024-09-13 that says this (I took it to post to discord):

> How do I know which model Gemini is using in its responses?

> We believe in using the right model for the right task. We use various models at hand for specific tasks based on what we think will provide the best experience.

> We use various models at hand for specific tasks based on what we think will provide the best experience

... for Google :)