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Harness · Basics

Harness basics

The model is the part everyone talks about. The harness is everything else, and it is often the reason one AI product works and another, using the same model, does not.

In one line: Agent = Model + Harness. The model is the engine. The harness is the car: steering, brakes, fuel line, dashboard and the road rules.

What a harness is

A harness is the system the model runs inside. It decides what the model sees, what it is allowed to touch, how its steps are organised, what checks run on its work, and what happens when something goes wrong.

Consider a model asked to fix a bug in a codebase. The model alone can only produce text. Add a harness and it can read the files, run the tests, see the failures, edit the code, try again and stop when the tests pass. Same model, different system, very different result.

The five parts

There is no official list, but most working harnesses have these five parts. Each one answers a question the model cannot answer for itself.

  1. 1ContextWhat does the model know right now? Instructions, documents it can search, memory from earlier steps, and summaries that keep long tasks from overflowing.
  2. 2ActionHow does the model affect the world? Tools, APIs, file access, a browser, code execution. The tool descriptions matter as much as the tools.
  3. 3OrchestrationHow is the work sequenced? Planning, keeping track of state, breaking a task into steps, handing parts to sub-agents, retrying.
  4. 4ControlWhat is the model allowed to do? Permissions, approval for risky actions, sandboxes, spending and time limits, output checks.
  5. 5FeedbackHow do you know it works, and how does it get better? Evals, logs and traces, tests the agent runs on its own work, and a process for turning each failure into a fix.

A quick test

For any AI system you use or build, ask:

Does it know enough? Can it act? Does it stay on track? Is it safe? Can we show that it works, and that it is getting better?

If you cannot answer one of these, that is the part of the harness to look at first.

What's next

The next article covers how to design a harness for a specific job, step by step: Designing a harness.

Sources

Compiled by Human Agency from the sources listed. The five-part breakdown is a synthesis, not an official standard. For the wider overview, see Harness Engineering.