This is the classic blog post, by Thorsten Ball, from way back in the AI Stone Age (April this year): https://ampcode.com/how-to-build-an-agent

It uses Go, which is more verbose than Python would be, so he takes 300 lines to do it. Also, his edit_file tool could be a lot simpler (I just make my minimal agent "edit" files by overwriting the entire existing file).

I keep meaning to write a similar blog post with Python, as I think it makes it even clearer how simple the stripped-down essence of a coding agent can be. There is magic, but it all lives in the LLM, not the agent software.