If you can express something deterministically with code, it's better to do that rather than have an agent do it, because it's faster, cheaper, and deterministic. E.g. you regularly copy file A to file B. You can ask the agent to do it, or you can write a script and have the agent call the script via skill. That's the beginning of a harness.
Eventually you arrive at building custom software that does a lot in the traditional way, but delegates certain tasks to the model where it makes sense or it's non-trivial/impossible to express via code.
If you can express something deterministically with code, it's better to do that rather than have an agent do it, because it's faster, cheaper, and deterministic. E.g. you regularly copy file A to file B. You can ask the agent to do it, or you can write a script and have the agent call the script via skill. That's the beginning of a harness.
Eventually you arrive at building custom software that does a lot in the traditional way, but delegates certain tasks to the model where it makes sense or it's non-trivial/impossible to express via code.