Agent harness, explained like you're five. Every big lab is shipping one, almost nobody says why you need it
So, imagine a horse, it's your model. Strong, fast, and it runs wherever it wants.
The harness is the reins, the saddle and the cart. Without it, all that power goes nowhere useful.
Claude in your browser and Claude Code run the same model. But one answers questions, and the other actually builds a whole projects on your computer. The difference is the harness.
In Claude Code, that's the CLAUDE.md file with your rules, the memory of your project, access to your files and terminal, tools and a loop where it runs the code, checks the result and fixes it.
Here is why you need one. Without it, you are the harness. You repeat the rules, paste the files, check every answer and decide the next step. Every day.
Google researchers kept the model and only changed its harness. On one real office task, the score jumped from 43 to 78 out of 100, just from adding a rule to double-check its own work.
Three things make a good one:
1. Rules written once. A CLAUDE.md or AGENTS.md file it reads before every task, so you stop repeating "use our template"
2. Only the tools the job needs. Every extra app you connect is another way for it to get confused
3. A check before "done". Tests for code, a checklist for everything else. If it can't verify the work, you will have to
Start small. Take the instruction you paste most often and save it as a rule. Take the mistake it makes most often and turn it into a check.
The model is not the bottleneck anymore, it got commoditized. The harness is where the results come from.