Coding agents that add co-op to games

Co-op Anything is a playbook for AI coding agents, not a mod. You point Claude Code, Codex, Gemini CLI, Cursor or OpenCode at the repo, name a single-player game, and the agent tries to make it playable with friends. It looks at what already exists. If it has to, it reverse engineers the game and its engine. Then it builds or fixes the co-op, tests it with bots inside the game, and packages it so friends can join over Steam without port forwarding. The repo hosts no game files. What it gives you is a process and the tools to back it up.

How it works

The agent picks one of two paths. Path 1 is for games where a co-op mod already exists but is buggy, stalled or abandoned. The agent finds the last build, digs bug reports out of issues, forums and Discord, audits the code and ships fixes as a separate package. Path 2 is for games with nothing usable. That means reverse engineering, hooking in, rendering the second player, and syncing enemies, world state, story, level changes and saves.

There's a table that maps engine families to approaches. Unity gets BepInEx and Harmony patches. Bethesda games get script extender plugins. Closed binaries with nothing else get full reverse engineering, which the README calls the hardest path.

The tools/ folder is the part I like most. It's a set of game-agnostic sync testing tools. One is a UDP relay that adds lag, loss and reordering between real clients. Another is a flight recorder that finds the first desync across machines and writes a regression test for it. There's also a seeded co-op fuzzer and a small C++ library for NPC ownership handoff. Networking goes through Valve's free Spacewar test app, so nobody has to buy anything beyond the game.

Try it

Read https://github.com/cranebeaker/coop-anything and follow AGENTS.md.
I want to play <game> in co-op with my friends.

claude mcp add coop-anything -- python <clone>/tools/mcp/coop_mcp.py
python -I tools/test_all.py

The first lines are the prompt you give your agent, the MCP command is for agents that don't read the repo's .mcp.json, and the last one runs the tool test suite.

My take

The strength here is the testing discipline. Most "let the agent do it" projects stop at "it compiles." This one requires automated tests against real game clients, and every directory entry carries a QA level that shows how it was verified. The track record is real too: Deus Ex is released, and there's work on Fallout: New Vegas, Ocarina of Time, Dungeon Keeper and S.T.A.L.K.E.R.

The FAQ answers "will it work for my game?" with "Yes," and I'd be more careful. The same FAQ says path 1 projects take days at most and path 2 depends on the engine. A closed binary means a lot of agent hours and a lot of tokens. You should also review every draft PR your agent opens.

If you mod games, or you want a hard, measurable test of how well your coding agent holds up, this is worth a look. Start with the 230+ entry directory before you spend any compute.