Stop Focusing Only on the AI Model. Start Building AI Harnesses.

When most people build AI applications, they spend most of their time improving prompts or switching to a better model.

But after building more AI systems, I think there's something even more important: AI harnesses.

What is an AI Harness?

Think of a climbing harness.

A harness doesn't make the climber better—it keeps them safe and attached to something stable.

An AI harness works the same way.

It's everything around the AI model that makes it reliable. The model is just one part of the system. The harness makes sure it behaves consistently and safely.

What's Inside a Harness?

A harness can include things like:

Tools the AI can use (APIs, databases, browsers, files)

Context management to prevent conversations from getting too large

Guardrails like maximum steps or token limits

Verification to check if the AI actually completed the task

Retry and recovery logic when something goes wrong

For example, imagine an AI agent trying to book a flight.

The AI clicks the "Book" button but gets redirected to a login page. Without a harness, it might simply reply:

"Done! Your flight has been booked."

A harness would detect that the AI is actually on a login page, perform the login automatically, let the AI continue, and verify that the booking really happened before reporting success.

Why It Matters

Large language models are still non-deterministic. The same prompt can produce different results, and models change over time.

A good harness makes your application reliable regardless of which model is running underneath.

In many cases, a smaller and cheaper model with a solid harness will outperform a larger model without one.

My Take

I think we've spent the last couple of years talking about prompt engineering.

The next big shift is harness engineering.

The companies building the best AI products won't just have the smartest models—they'll have the best systems surrounding those models.

The model is the brain.

The harness is what makes it dependable.