What is a Harness? | EARENDIL

What is a Harness?

Date:Thu, 20 Aug 2026

From:Earendil Product rfc@earendil.com>

To:You

Subject:What is a Harness?

Harness – definition by the Cambridge Dictionary

Noun. a piece of equipment with straps and belts, used to control or hold in place a person, animal, or object

Verb. to control something, usually in order to use its power

When I think of a harness, I think first of the set of straps and belts that I put on in middle school before scrambling up the walls of my school. I was a mediocre climber at best.

Royal Robbins on El Capitan, his harness racked with the tools of the ascent. Photo by Tom Frost.

If you’re main-lining into the AI newsfeed these days however, your archetypal harness may already be an agent harness. And, this post was not written for you.

This was written for those who may be curious to know what an agent harness is, but don’t, and have been too embarrassed to ask.

Let’s get back to climbing.

Why do you strap on a harness when you go climbing? Well, firstly, the harness supports you and keeps you safe. It does that by connecting you to carabiners and ropes that secure you from falls, moderate your pace, and govern your route. You can also attach other tools to your harness like a chalk bag, nut tools and quickdraws.

And when you go climb different mountains or make different ascents you can take your harness with you. Depending on the terrain, you can even modify your harness and what goes on your gear loops. Climbing harnesses are adaptable. They are used by acrobats and arborists. The people who own them can make them their own.

There are similarities between climbing harnesses and agent harnesses both in terms of structure and function.

Agent Harnesses

Others have written (simplistically) that Agent \= Model + Harness. Here the word Harness refers to an Agent Harness. But what is an agent harness? Agent harnesses use AI models to create AI agents, and their first application was for coding. Now, agent harnesses sit at the core of all types of AI agents and understanding how an agent harness works will help you understand what an AI agent is.

An agent harness is a piece of software that provides an environment for an AI model to operate within. Unlike most AI models, you as an end user can own your own agent harness.

Often, users like software engineers interact directly with harnesses like Pi using the Terminal application on their computer. But, harnesses like OpenClaw also use different user interfaces like iMessage, a chat app, or email. Our harness Lefos was built primarily to interact via email. Regardless of the interface, harnesses generally do four things: Firstly, they provide a set of instructions that help govern how the AI model responds. This set of instructions is typically called a “system prompt”. Secondly, they describe and provide a set of tools that are made available to the AI model to use in service of responding to requests from the user. Thirdly, the harness establishes a framework that governs how the model behaves. This framework does a lot of different things, but one of the main things it does is establish the “agentic loop”. Finally, most harnesses provide a crucial translation layer that enables the harness to work with a variety of different AI models.

I. System Prompt

Most AI models come with an embedded set of rules and guidelines that has been refined and arrived at during the training process. Most famously, Claude Opus 4.5 had a widely publicized “soul document” that explained to the AI model what it was and how it should act. The System Prompt in an AI harness is similar to this but is less embedded into the model. It’s more like a set of instructions a new employee might get on their first day of a job. It hasn’t internalized the instructions but it knows it should follow them when performing that work. System prompts are injected into the conversation together with every prompt and play an important role in ensuring that the AI model acts appropriately in the context of that harness.

II. Tools

Tools are a set of capabilities, written in code, that the model can “call”. The harness describes the tools and also provides the software that is the tool itself. Examples of these tools might include a web search tool, a tool that allows the model to write and execute software code, or a tool that allows the model to compose an email. Critically, the harness usually does not dictate when and how the AI model should use the tool. Instead, it simply makes the tools available, describes them clearly, and allows the AI model itself to decide when and how it should use them.

III. Agentic Loops

Now we have an AI model sitting within an agent harness with a set of instructions and a set of tools. Let us assume our harness was built to work within email, had the tools we described above (WebSearch, WriteCode, ComposeEmail), and that the user has asked the agent to compare rankings and test scores...