Fan-made co-op mod for Mewgenics lets you play Adventure with a friend

An unofficial mod called MGMP has arrived for Mewgenics, adding co-op play to the Adventure mode. The host starts a run, a second player joins, and the two then split the cats between them and make their way through the map, battles, shops, events and leveling together.

Mewgenics has no official multiplayer: on Steam the game is still listed as Single-player. MGMP is an independent fan project with no connection to Edmund McMillen, Tyler Glaiel or the game’s developers.

In battle, each player gets their own share of the squad. When it’s the turn of a cat under a given player’s control, that player commands it. The mod syncs dice rolls, damage taken, enemy actions and the state of the fight, and also shows your partner’s cursor and the target they’ve selected.

The author of MGMP on GitHub says the mod has already been used to complete a full co-op run online. That said, the project remains experimental, so desyncs and other bugs are possible.

You can rejoin the fight after a disconnect

One of the most interesting features is the session recovery system. If one player loses their connection, they can reconnect even in the middle of a battle.

MGMP replays previous turns up to the current state of the fight, after which the player continues the run alongside their partner. Both copies of the game also compare the state of the run on a regular basis. If the data diverges, the mod halts the session instead of letting the two versions of the run drift even further apart.

Outside of battles, a different scheme is used: the host’s decisions about the map, shops, events, inventory and progression are sent directly to the second player.

At the same time, the home portion of Mewgenics stays single-player. Cat breeding, furniture and other activities at home are not synced between players. It’s best to connect after the host has moved into Adventure.

For now, the developer recommends planning for two players. The code allows for three or four people to connect, but those sessions have effectively not been tested.

Playing online requires a direct connection

MGMP runs on 64-bit Windows and currently targets Mewgenics version 1.1.21239. Both players need to be on the same build of the game.

By default it uses TCP port 27600. On a local network, the host computer’s IP address is enough, while for online play you can set up port forwarding or use a virtual network such as Tailscale, ZeroTier, Hamachi or Radmin.

Another option is the free Porthole on Steam. The program shares a selected local port with friends through Steam’s infrastructure, removing the need to configure your router or use a public IP.

The project’s most unusual detail concerns how it was made. The author of MGMP states outright in the README: “The human didn’t write even a single line of code”, thanking Claude. In other words, according to him, all of the mod’s code was created with AI, though testing, configuration and reverse engineering of Mewgenics were still carried out within the project itself.