What they did
The poster, u/Any-Explanation3814, wanted Valheim to switch into a city builder at the press of a key: settlers who move in, pay taxes, need food and mead and leave when they go without, plus boss camps you raid with your own army. Claude wrote most of the C# for it as a BepInEx plugin with Harmony patches into the game's own systems, covering settler behaviour, the economy and houses that assemble themselves from real Valheim building parts. The poster set up a small command file the agent could write to, so it could steer the running game: build a town, take screenshots, judge whether things looked right and fix them. The poster says they did the design and the testing and kept pushing.
How it went
The mod shipped free on Nexus and Thunderstore and the poster says it reached #1 on r/valheim. When a tester reported bosses chasing players into town and scouts getting stuck, Claude found and fixed both causes in one pass. It also wrote an in-game benchmark, measured frame rates with 10 to 150 houses, and found that merging finished houses for rendering helped most, taking the game from 36 to 56 fps.
Worth knowing
The harness was not named; the poster only says Claude. The work was one long chat with plenty of unrelated context, so there is no single prompt to share. The giveaway is the feedback loop: letting the agent act in the game and look at screenshots is what let it debug itself. Expect pushback in gaming communities; the r/valheim thread was mostly arguments about AI.