What is an "agentic harness," actually?

I've been hearing the word "harness" thrown around a lot lately. I assumed it just meant "the IDE" or "whatever app is running your agent." Turns out, it goes a little deeper than that.

So I sat down with @greggyb and asked him to explain it from the ground up: what an LLM actually does, what turns it into an agent, and where the harness fits into all of it.

What's in the video

Simon Willison's definition of an agent: an LLM with tools, running in a loop to accomplish a goal
What "tools" really means under the hood, and how function calling lets an LLM pull in context it wasn't trained on
The loop part: how an agent programmatically checks its own output to decide if it's actually done
Why the harness is everything after the LLM, and why that's a different thing than the interface
Why you can swap interfaces without touching the underlying harness, and why some agents don't need a UI at all

The point that stuck with me is the harness isn't the app you're looking at. It's the invisible plumbing deciding whether the agent keeps going or calls it done.