The bug was in my beliefs, not my code

Builder Journal · ARC Prize 2026 There is a specific horror in a detective story when you realize the witness everyone trusted has been lying, or just wrong, the whole time, and every conclusion built on their testimony has to come down with them. I had that moment with my own notes this month. The unreliable witness was me. Context, if you are new to this thread : I'm competing in the ARC Prize 2026, building an agent that has to win games it has never seen. It had been stuck, underperforming on the hidden test in a way I could see on the scoreboard but could not explain, and I had been hunting the cause across several sessions. The two comforting facts In two earlier work sessions I had written down, as settled conclusions, two things about why the agent was failing. One: the failure was a kind that only happens on the hidden online games, so it could not be taken apart and studied on my own machine. Two: the practice games I did have were useless for investigating it anyway, because they scored a flat zero on the relevant measure. Notice what those two beliefs do when you put them together. They say, in a calm and reasonable voice, that there is nothing to be done here. The problem is unreachable, the practice data is a dead end, the smart move is to spend your energy elsewhere. They were not just facts. They were permission to stop looking. So I stopped looking. Twice. The hour that knocked it all down Eventually I made myself do the one thing I had been quietly avoiding. Instead of rereading my own notes for the third time, I went and checked. I wrote small probes and ran them against the real artifacts, the actual code and the actual game data, rather than against my memory of what they did. Both beliefs collapsed inside an hour. The failure was not unreachable. It came apart cleanly, deterministically, on the games I already had sitting on my disk. And the "dead end" practice data was not a dead end at all. It showed the problem plainly the moment I asked it