We updated all mods we currently run, per Uveso's advice. After that didn't solve the desync issue, we finally checked the Mod Manager on my brother's PC, and found some quite old mods that had no business being listed. It turns out there were old copies of mods found in the folder "\steamapps\common\Supreme Commander Forged Alliance\mods". I don't even know how those got there. He deleted that folder and we got everything working fine in multiplayer, no desyncs at all with any combination of mods.

Thank you all for your patience. (This is embarrassing. I swore I wasn't gonna be the guy who asked for help without first checking all of the extremely obvious solutions. I feel very stupid now -__- )

We updated all mods we currently run, per Uveso's advice. After that didn't solve the desync issue, we finally checked the Mod Manager on my brother's PC, and found some quite old mods that had no business being listed. It turns out there were old copies of mods found in the folder "\steamapps\common\Supreme Commander Forged Alliance\mods". I don't even know how those got there. He deleted that folder and we got everything working fine in multiplayer, no desyncs at all with any combination of mods.

Thank you all for your patience. (This is embarrassing. I swore I wasn't gonna be the guy who asked for help without first checking all of the extremely obvious solutions. I feel very stupid now -__- )

@Uveso said in Personal mod creating desyncs in multiplayer:

Please post a game.log

Here you go, the game logs for both my brother and me when our game desynced running both Uveso AI and my personal mod (called "WCT" in the files):

game_14139931(host).log
game_14139931(bro1).log

Hm... my log (host) reads "not safe" when checking the FAF gamedata files. I take this to mean they're possibly corrupted, and I should reinstall FAF.

If you have desyncs with the AI-uveso mod, check all mod files. A desync is caused almost in every case by different mod files.
Maybe you have installed the mod in 2 different folders.

I'll double-check again with them!

Update:

After a few hours of testing, one of my brothers and I narrowed the issue down to a problem with one specific other mod: Uveso AI.

We can do multiplayer, set up the way we always play, just fine with my mod, and any other of our favorite mods activated, using: the base game's AI, Sorian AI, and RNG Standard AI (with Uveso pathfinding turned on, since RNG Standard requires it), in any combination. But if we try to play with Uveso AI actually assigned a faction to control, the game immediately desyncs, exactly like I complained about earlier. I have no idea why this might be the case.

Obviously, this is not ideal, since Uveso's AI is pretty good, and we do enjoy playing against it. But Sorian and RNG Standard are enough to make the game fun, so that's what we'll be using in multiplayer from now on.

Thank you guys for helping! I greatly appreciate all the FAF community has done for Supreme Commander.

@Sprouto Hm, I'll have to double-check with my brothers. But I think it's unlikely: all mods are put into one zip file, uploaded to Google Drive, then downloaded, and unzipped into the same \mods folder. If my mod is placed wrong, then all of them should be. Furthermore, the desync hits so quickly we barely have time to give our first orders. We do usually play with the pre-built option activated, but the only extra units it gives you are some T1 mass extractors, some T1 power generators, and a T1 land factory—none of which my mod alters.

Okay, stupid question: I thought only the person hosting the game could turn a mod on or off, and as long as the other players had the same mod in the correct folder, it would work fine? Do the other players need to turn a mod on?

I have a modest mod that I've been making for myself and my brothers to use in multiplayer with each other. It works fine when I run it by myself versus AI in skirmish—no crashes or slowdowns. Unfortunately, any attempt to run it in multiplayer results in an immediate desync.

I always upload it to Google Drive, and have them download it to the standard \mods folder, so we're definitely all running the same version. We run several other mods in our games, too, but I've narrowed it down to my mod as the source of desyncs. (With any combination of other mods: no desync. With my mod, even without any other mod activated: immediate desync at the start of a game.)

I am at my wits' end trying to make this thing work without a desync. Do you guys have any advice? Here's a link to it on GitHub; maybe there's something obvious I'm missing?