After the prompt, the context and the loop, the harness needs one more part, something that checks the work before it ships. That is a gate. A gate is a check written before the work starts, usually a small program, sometimes another agent, sometimes you. It looks at what the agent produced and answers one of three ways. Pass means it ships. Redirect means go back and fix this. Refuse means it does not ship at all, and no proof is a refuse.
We built a résumé agent to help our students. A student sends his résumé, with six years of experience, a summary and projects. The agent's job is to add bullet points about his agent work and the right keywords for the job he wants. Without gates, you ship whatever it wrote, apply for jobs, and look like an idiot.
Gate one is factuality. Compare with the original: the company, the title and the dates do not change. If the student's last job was at a bank, and the agent got excited and moved him to a rocket company, the gate cuts it and sends it back.
Gate two is slop. There are a couple of hundred words a model always reaches for. Seamlessly integrated. Unwavering support. Embark on this journey. Every one of them sends the résumé back.
Gate three is jargon, because too much of it makes a résumé annoying to read. Gate four is over-promising: everything we teach can go on the résumé, but if it starts claiming machine learning engineering, a different specialty that we do not teach, out it comes. I have about sixteen gates on that engine. Until every condition is met, the résumé does not ship.
The gates are small programs: one compares the dates, one counts the slop words. The agent that rewrote the résumé offers to run all sixteen itself and report back, to save time. Do you let it?
It can run them. It cannot report them. The agent can say pass. Where is the proof? It can even report a pass while some checks are failing, because pass is what it was asked to reach. So the verdict comes from the gate's own output, the raw result you can read yourself, never from the agent's summary of it. Show me the original dates next to the new ones. Show me the count of slop words. The agent can run the gate, but it can never be the gate.