What they did
Ryan Madden, who had earlier got Claude Code playing Factorio, pointed GPT-5.6 Sol running in Codex at a fresh peaceful map, meaning no enemies to fight. The model was on its low thinking level. It did not see the screen like a player. A game manager exposed a small Python library that let it move, mine, craft and build by code, while holding it to the same limits a human has in the GUI, such as how far the character can reach and fog of war. The run manager recorded the screen and logged production numbers for the whole game. Madden meant to stop after blue science but let it carry on to the end.
How it went
Sol launched the rocket after 13.5 hours of game time. Madden says it did better than he expected early on, and clearly better than GPT-5.5, which in an earlier attempt never managed to use transport belts. The scaling trouble he expected at blue science arrived later and slowed it a lot. In the late game it failed to wire some production lines together and covered for it by carrying supplies between parts of the factory by hand, over and over, which is how it reached the launch.
Worth knowing
This only works with a harness that turns the game into an API; the model is not reading pixels. The map had no biters, so defence was never tested. Madden has posted a timelapse and says a commentary video is coming. A Reddit share of the post drew the question of what the method was, and the post answers it only at the level of the library.