I recommend a multi-LLM library like Vercel AI SDK. Anything more than that and you will work with lots of abstraction layers that will prevent you from learning how agents work, anything less than that and you are using the APIs by the providers - still fine, but too limiting.
If you are an experienced engineer, you should be able to build the necessary primitives yourself pretty easily.
If you are an experienced engineer, you should be able to build the necessary primitives yourself pretty easily.