I just put the varying parameters in a trailer prompt and have them change every time. It doesn’t matter because the cache is prefix keyed. You lose caching for the last 20 tokens or so but that’s not a big deal. Moving it to a tool call makes it too slow (needs full roundtrip).

If you’re constructing the prompt you don’t have to jam everything together you can arrange it appropriately.

Yes indeed! Mostly don't put changing data in the beginning or prepend.