How do people build something like a personal harness? Are there tools for that or is it done from scratch?

I like this tutorial for an agent in 50 lines:

http://minimal-agent.com/

And if you add one additional while loop, for user input, you can actually use it! :)

https://gist.github.com/a-n-d-a-i/5461a662ef8a7ee0a5eb7778c8...

Build it from scratch. Understanding fundamentals of how agentic coding harnesses is a must though if you gonna go that route. I think everyone should take time and learn these things, maybe reverse engineer Codex Cli or something like that as a starter. That info is very valuable in this day and age.

Not the comment author, but I use pi and customize it with my own extensions. Pi automatically tells models how to customize itself, so it's a pretty easy process.

I started mine from scratch in 2023 because I wanted to use LLMs from a terminal and there was nothing else compelling at the time (nowadays there is pi and opencode)

Harnesses are/can be incredibly simple things, not much more than a HTTP client that renders things in a way that suites your taste.

It’s not that difficult, it’s just a system prompt and a set of basic file edit/bash/etc tools.

Me, personally, I didn’t build it from scratch but I ported original CC from published sources into Python and extended it to match my own requirements.

Here is a video I made explaining it from absolute basics:

https://m.youtube.com/watch?v=_AgKuFGvJfI

And the repo:

https://github.com/abtinf/homunctor

Building something like this is the todo list of agents.

I found this one easy to understand:

https://ampcode.com/notes/how-to-build-an-agent

The real question is when do you transition from building it with codex/CC to the harness itself.

Why use a personal harness?

You have to pay API pricing, which is far more costly.

I'd either switch to GLM wholesale or just continue to use Opus within Claude Code as the blessed, subsidized path.