Eight months ago, did I know the words agent harnessing? No. I was using the agent. I was doing the job. I did not know what I was riding.
In the beginning I had no rules file. The agent could delete my folder. It could delete my project. It could push unfinished code to production. It could send an email to my customers. It could create a coupon and bankrupt my company, and nothing on my machine would have stopped it.
What I learned about harnessing, I learned through punches to my face. Every session it made a mistake, and every mistake made me feel stupid, humiliated by my own agent. Every time, I went back and asked one question: this is the mistake it made in this session, so how do I avoid it in the next one? The answer went into a rules file, a plain text file the agent reads before it does anything. I call mine the iron rule. It starts with plain context about what project the agent is working on, and then one rule for every mistake that cost me.
Since January 1, 2026, I have made more than four thousand commits. About fourteen a day.
Before one class, with my rules in place, I asked the agent to go through my local disk and reverse-engineer how I do agent harnessing. It did, from what was on my machine, and I showed that architecture to the class. A prompt that says what you want. Context, what the agent reads. The loop, where it tries and fails and tries again. Gates, checks that can say no. A sandbox around all of it, and a log at the end of every session. I did not sit on top of a mountain and get inspired. I did not write it as a theory on paper. Those commits taught it to me, and the thing that made the mistakes drew the picture.
Your agent made a mistake in nine of your last ten sessions. A teammate says it is getting worse and you should switch tools. What do you check before you agree?
Whether the mistakes are new. Mine still makes mistakes in almost every session. A new mistake is expected, and it tells you the next rule to write. The same mistake twice means you saw the lesson and did not write it down, and a new tool will not write it for you. And remember that a rule in a file only asks. It does not stop anything, which is why the picture does not end with the file.