[Feature] The ability to specify which model should be used per prompt

[Feature Request]

It would be cool if you could specify which model you want the system to use for a specific prompt. For example often the first major prompt of a project is complex and using the Opus might be helpful and the small tweaks after don't need the large model, or if sonnet has struggled on a fix you @Opus to have a go at it. Etc etc. I understand there would likely be some significant cost for having to load all the previous context into a new model, but that might still be worth it and as long as the user knows they could pick to use it or not.