The setup around the model
An AI model can produce an answer from a prompt. An AI harness manages the work around that call. For a coding assistant, it might read project files, send relevant context to the model, let it use approved tools, and show the results.
The harness may decide when to ask the model again, limit what tools can do, and run checks on the output. It does not make the model correct by itself. The word “harness” is used loosely, so it helps to ask which tools, instructions, and checks someone means.