Overview
An agent harness is the operating system around the model. It includes the prompts, instructions, available tools, reusable skills, context, execution environment, and the codebase in which the agent works. Model capability matters, but the harness determines how effectively that capability is converted into reliable work.
The practical mistake is to treat model selection as almost the entire problem. A more balanced view gives substantial attention to both the model and the harness. The model supplies reasoning and generation capacity. The harness supplies direction, constraints, access, feedback, and an environment that is easy to operate in.