What a harness is and how to build one with Claude Agent SDK

**Claire Vo** (0:00)
A harness is some code around an AI agent that makes it more effective. Why we've seen people build these specific use case harnesses, is sometimes with a specific job, you just want to micromanage a little bit. You just want to be more prescriptive about how that job gets done. I'm going to show you how it works, and then we will talk about how I built it. So the interface I built for my harness is a terminal UI. The Harness Core is run on Claude Agent SDK, and then it's connected to real tools. So it's connected to Sentry, Vercel, and then it's connected to Linear and GitHub in terms of getting tasks done. I think we all have done good work, but then now I've realized that these agents can help us solve very, very specific problems by constraining that work. It's really like changed my mind about how work gets done.
Everybody's saying it's not the model, it's the harness, but you know what not everybody is saying?
What is a harness? In today's How I AI episode, I'm going to demystify the idea of a harness, write my own harness and show you how you can do the same, and explain to you why a custom harness makes sense and could be better than using Claude Code or Codex alone. Let's get to it. This episode is brought to you by Bolt.new, the AI app builder for people who have ideas and want to ship them.
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Before I get into how to build a harness, let's talk about what a harness is, and I'm going to make it as simple as I can for all of you. A harness is some code around an AI agent. Yes, you heard it here first. A harness is just code around an AI agent that makes it more effective. Can that code have AI in it? Sure. Does that code have to have AI in it? Not necessarily. What is the goal of a harness? To make the AI better. It is so simple, and I feel like the way that people have been talking about this have made it such a mystery that I wanted to make it just very clear to you all. It is just writing more code around your AI to make it more useful for a specific use case. So what are the parts of a harness? Well, a harness is going to have specific context, it's going to be able to take specific actions, and it's going to have a goal of specific outcomes. It's just as simple as that.
I want to talk about when it makes sense to build a harness and when it doesn't. And I think you want to build a harness when the same workflow needs the same setup and the same outcomes. And so, it's kind of similar to when you would build an AI agent, and in fact, harness agent, sometimes you can interchange some of these concepts, but really it's when there is a sort of combination of deterministic and non-deterministic workflow, step-by-step process tools use cases, you want your AI to follow up to do a specific job. Usually, those jobs are like slightly more complex, and this is why you've seen these coding harnesses come out, like coding is a job to be done, it needs specific tools, it typically goes through kind of a standard workflow, and so coding harnesses are very popular, but you could also do things like managing production incidents where you need to go through a specific process, getting PRs ready for release, handling support escalations, managing migrations, even non-technical use cases like doing research in a very specific way or consolidating docs in a very specific way. That's how you and why you would use a harness.
So how did I decide what kind of harness I would build? Well, I looked across my business at ChatPRD and I thought, what am I doing sort of repeatedly and consistently that I think AI could be good at, that I think we can be doing better if we were more structured about the AI and how we used it.