What is ‘Harness Design’ and why does it matter

AI coding starts to feel a lot more useful when you stop expecting one agent to do everything. The real leverage is not just the model. It is the system around it. Clear roles, clear context, clear rules, and clear validation. That is what makes the whole thing more predictable, easier to trust, and much easier to scale. Once you start thinking this way, the workflow changes. You stop asking for magic and start designing responsibility. The support layer matters more than people think A lot of the quality does not come from the prompt itself. It comes from the files and structures around the workflow. These support files are what keep the system from turning into chaos. They define how work should happen, what should be respected, and how the agent should behave when the task gets bigger or more ambiguous. You can think about them like this: Agent files define the default behavior and the overall boundaries Skills define reusable capabilities for specific kinds of work Subagents split