GTA IV Co-op
A co-op mod for Grand Theft Auto IV: The Complete Edition. Up to 4 players play the story together across
GTA IV, The Lost and Damned and The Ballad of Gay Tony.
One player hosts and plays their own save and story. Everyone else joins that world as their own character and plays
the missions alongside the host. Your own saves are never loaded or changed when you join someone else's game.
Website · Download the latest release · Report a bug
Testers wanted. The mod has only been tested on one PC against a test bot, so it needs people playing it with
real friends. Expect rough edges, and please report what you find (see "Reporting bugs").
About this project
I've wanted a co-op campaign for GTA IV since I was 11 years old. This is a side project I made for fun with spare AI
tokens: Anthropic's Claude wrote the code and I steered it. I'm not a reverse engineer and don't claim to be, just a
fan of the game. I am a good programmer, though, and the project is built on solid fundamentals: a clean structure,
every change reviewed, and automated tests that host, join and switch stories on the real game. Help, bug reports and
pull requests are welcome.
What you need
- GTA IV: The Complete Edition on Steam, version 1.2.0.59, on Windows.
- Every player owns their own copy of the game, and plays on their own PC.
- The Complete Edition includes both episodes, so nothing else is needed for The Lost and Damned or The Ballad of
Gay Tony.
Install
- Download
iv-coop-setup-<version>.exe from the releases page and run it.
- The installer finds your game automatically and checks it is the supported version.
- If you don't have TJGM's FusionFix pack yet, the installer downloads it from its official mirror and installs
it (anything it replaces is backed up first). The mod needs it to run.
- Tick Radio Restoration (strongly recommended). It brings back the original Vladivostok FM songs in GTA IV and
the club music in the Episodes.
- Click install, then start the game normally from Steam.
Everyone in the group should install the same version.
About the mod pack: I chose TJGM's pack because it gives the game a good, stable base to build on. It's the only
setup the mod is tested with: other mods haven't been tested and may not work alongside it. I'm not affiliated with
TJGM or the makers of FusionFix. I just consider their work a baseline for the game, and the installer fetches it
from their official downloads to save you downloading it separately.
Host or join a game
Use your in-game phone and open Multiplayer.
- Host: choose Host. You get a 6-digit password. Share it with your friends.
- Join: choose Join and enter the host's password. The connection goes through Steam, so nobody has to open
router ports.
- Leave when you are done, or Go to host if you got separated.
If the phone menu isn't available, you can connect directly by IP address instead: the host presses F10, and a
joiner presses F11 (it joins the address written in plugins/iv-coop.ini under [Net] JoinAddress). Pressing
the same key again leaves the session. The host's router port is opened automatically when the router allows it.
The host's story is what everyone plays. In The Lost and Damned or The Ballad of Gay Tony, everyone must be in the
same episode as the host. If you are in a different one, the mod tells you which to start.
Your character and the host settings
- Character: choose male or female and any clothes, hat or glasses, with a live preview. Everyone sees your look.
- Co-op Settings (decided by the host): Team fire (Off / On / No aim), Revive, Shared pay, Radio sync, Weather
sync, Partner blips, Scenes (host camera or your own), Ambient sync (shared pedestrians and traffic, off by
default) and Invasions.
Activities
Bowling, darts, pool, air hockey, golf and races (The Ballad of Gay Tony), arm wrestling and hi-lo cards (The Lost
and Damned) are played with everyone as their own character, with their own score and turns: stand near the host
when they start the activity. Bowling, darts, golf, cards and races take up to four players; pool, air hockey and arm
wrestling are one against one, and whoever is waiting plays the winner. In a race each player drives their own car.
What to expect
Partners are real characters: they take damage, ragdoll and catch fire. On missions a deadly hit leaves you down, and
a friend can revive you. Mission characters, cars, sounds, cutscenes, the radio and the weather are the same for
everyone. Each partner keeps their own money for each host. Press J to scream.
Invasions (optional)
When the host is on a mission or has three or more wanted stars, and Invasions is switched on in the settings,
another group of up to four players can invade as FBI agents and hunt the host's character. Invaders can't respawn
and can only revive each other; if they all go down, the invasion fails. Start one from phone > Multiplayer >
Invade.
Reporting bugs
Please attach the logs. They only contain the mod's own information.
- In the game: phone > Multiplayer > Save logs, or press Ctrl+Shift+L. A zip appears on your Desktop.
- If the game won't start: open the installer and click Collect logs.
Open an issue and attach the zip, with a short description of what you
were doing (which mission, what went wrong).
Uninstall
Run the installer again and click Uninstall. It removes the mod. The FusionFix files it installed are left in
place, and the backups it made stay in your game folder in case you want to restore anything.
Known limitations
- Early access: invasions, the multiplayer activities, the character creator, shared pedestrians and traffic, and the Episodes support
have had little play-testing.
- The Episodes' own minigames and some HUD meters aren't fully shared yet.
- The radio can hiccup briefly when a song changes.
Disclaimer
This is an unofficial fan project. It is not affiliated with, endorsed by or sponsored by Rockstar Games or
Take-Two Interactive, nor with TJGM or the makers of FusionFix. Grand Theft Auto and related names belong to their
owners. You need your own legal copy of
the game. Use the mod at your own risk.
For developers
See docs/FEATURES.md and docs/ARCHITECTURE.md.
Needs Visual Studio 2022 with the "Desktop development with C++" workload, and GTA IV CE with
FusionFix installed (it provides the ASI loader).
cmake --preset x86
cmake --build --preset debug
The post-build step copies iv-coop.asi into <game>/plugins/ (a running copy is renamed to *.old first). Use
-DIVCOOP_GAME_DIR=... if the game is elsewhere, or -DIVCOOP_DEPLOY=OFF to skip the copy. On first run the plugin
writes plugins/iv-coop.ini with all settings; check plugins/iv-coop.log. F8 toggles the debug overlay.
Only one game runs per PC, so build\Debug\iv-coop-bot.exe --name Alice --radius 3 --retry stands in for another
player after you host with F10. For local tests set [Net] BindAddress=127.0.0.1.
