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.

trivial with claude code, it's an md file in the agents directory. It has a format, you follow it and have the ai create your first agent with your ideas on what an agent should do, and concern itself with.

You write in plain English text, and put it .claude/agents. That is all.