Claude Code built a splitscreen co-op mod for 2006's Oblivion
RecordingTechnical86 · reddit · 2026-10-25
A modder used Claude Code to build a splitscreen co-op mod for Oblivion (2006), now in free beta on Nexus. The engine never supported two players, so most work was reverse engineering: Claude read game code via a Ghidra server and wrote C++ hooks as an xOBSE plugin, giving player 2 everything only player 1 had—menus, leveling, crafting, riding—plus new mechanics like knockdowns and revives.
Key lessons:
- Testing made it work: Claude built a harness launching the game with two virtual controllers and checking what each player actually sees. After a false pass where player 1's camera lay on the floor, every check inspects the picture, not engine values—179 scenarios now
- A 150+ entry decision log keeps new sessions oriented on why things are the way they are
- Every bug is reproduced live before fixing, since code-reading conclusions were often wrong
- The human did extensive playtests to shape how co-op feels
- Scale: 36k lines of C++, 28k lines of Python tooling, 247 commits; first bug reports two days after release, fix shipped the next day
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