Sub Question : IS there a real successful agent product today that uses a library for harness(like langgraph etc)? Building our own worked for us. Works with our components(postgres, events ...) and scales naturally with our system.

I don't know about real successful. Since you mentioned Langchain, you could look at https://www.langchain.com/dcode which is a CLI harness build off Langchain deep agents.

We've also defaulted to just writing our own. We couldn't find anything that was suitable for the use case and had the features we wanted. So when you've got the ability to generate code, why not use it to make what you need?