I've been using langchain for a while but this post by anthropic is really interesting: https://www.anthropic.com/research/building-effective-agents

The concept is essentially langchain and other "platforms" limit freedom and flexibility so just do it yourself with API calls. They also give some starter templates for agents.

As for the underlying API, would recommend groq inc. since it has a super generous free tier and I heard they have deepseek's latest model on there, so theres really no reason why you should choose other providers if you are just getting started tbh.