Builder Journal · ARC Prize 2026
Every boxing movie has the sparring montage. The hero looks untouchable, gloves snapping, trainer grinning in the corner. Then the bell rings on the real fight and none of it holds, because sparring was never the fight. This month I built an AI agent that was crushing it in sparring. The real fight went differently, and the gap between the two finally taught me how this competition actually works.
This is a thread in my builder's log from inside the ARC Prize 2026, where my agent has to win small games it has never seen before, with no instructions, and the games it actually gets graded on are hidden from me. An earlier entry laid out what the competition is and why it might be the most important benchmark in AI. This one is the day I learned to stop trusting my own practice scores.
The gym every competitor has to build
To improve the agent I had to measure it, and I cannot measure it on the real test, because the real games are secret. There is no way around that wall. So, like everyone in this competition, I built a practice gym at home: the handful of example games the organizers do hand out, a scoring script, and a way to run two versions of the agent head to head and see which did better.
