How to make a co-op mod
Any game can get an online mode. You do not have to write the networking yourself: a coding agent (Claude Code, Codex, Gemini CLI, Cursor and others) does most of the work if you give it a good guide and name the game. Which guide depends on the kind of game.
Flash games
Use our Flash multiplayer skill
It reads the game's SWF file, maps its code, builds an online mod without changing the original file, tests it with copies of the game that play themselves, and packages it. Friends join with a five-letter room code through a relay server that comes with it, so nobody opens a port or types an IP address.
- Get the tools. Java 21, FFDec 26.x, Python 3.10 or newer, and the Ruffle desktop player. The agent asks before it installs anything.
- Tell your agent. In the folder with the game's
.swffile:Read https://github.com/multiplayereverything/flash-multiplayer-skill and follow AGENTS.md. I want to play <game>.swf online with my friends. - It picks a path. A game with two players on one keyboard becomes online by sending the second player's keys over the network. A one-player game becomes several players, with remote characters and enemies that target anyone. Or it fixes a mod that already exists.
- Run a relay. One small Python relay serves every game you make. The skill explains how to start it, and how to put it on a server. Want to use ours instead? Ask us on Discord.
It has already been used for Bad Ice-Cream, Gun Mayhem 2, Territory War, Bow Man, Stealth Hunter 2, Air Battle, Ultimate Crab Battle and Raft Wars.
PC games
Use Co-op Anything
Co-op Anything is a guide for any single-player game on your PC, made by cranebeaker. Your agent looks for what already exists, works out how the game and its engine behave if it has to, builds or fixes the co-op, tests it with bots inside the game, and packages it so friends can install it in one click and join over Steam.
- Tell your agent.
Read https://github.com/cranebeaker/coop-anything and follow AGENTS.md. I want to play <game> in co-op with my friends. - It picks a path. If a co-op mod already exists but is buggy or abandoned, it finds the last build, reads the bug reports and fixes it. If nothing usable exists, it builds one from zero: show the second player, sync enemies, the world and the story, handle level changes and saves.
- Add the MCP server (optional). It lets your agent search the co-op directory and detect a game's engine from its install folder. The setup for each agent is in the project's playbook.
- Look before you build. Every co-op mod people have made is listed on coopanything.app. Yours may only need finishing.
We make mods for PC games too: Baldi Together, Spooky Together and Berry Bury Berry Co-op are on our games list.
Then send it to us
When your mod works with friends, put it on a public GitHub page and send it to us. If we approve it, your project goes on this website, and the game too if we do not have it yet. If there are several mods for one game, the game's page compares them and says which one to start with.
- Keep the repository public, with a README that says how to install it and how to play.
- Tag your releases. The site shows your latest release, your stars and when you last worked on it, straight from GitHub.
- Do not put the game's files in the repository. Only your mod.
- Test it with real friends first, on different computers. The agent does the work, but you are the one who says it works.