(Sorry for the french voices, and long-ass write-up. TL;DR is : just go play it if you have friends)

After a few evenings spent playing it (25+ hours on our current run), I'm very happy to announce that the co-op mod currently in development for STALKER Anomaly is finally, as of its latest version, fully playable with STALKER GAMMA, and it is EXCELLENT.

We initially tried getting it to work with the Zona modpack (more lightweight so easier to share), but unfortunately some of the mods it uses won't be compatible until the next version, and the limitations were a bit too significant. So we ended up turning back to GAMMA, which has been tested by the dev and is supposedly fully functional.

We're playing a vanilla+ version of GAMMA (to make sure everything stays compatible). For once, I haven't added 200 mods on top of it, just a few config changes to restore some of my usual settings, a handful of QoL mods, and the Alife Plus suite, which adds a lot to the game.

The mods I added that I consider essential QoL improvements for GAMMA

Drop Disassemble on Press: Lets you drop and disassemble items directly from your inventory with a keybind. Pretty much essential given the amount of crafting you do in GAMMA.
RNG Attach & Kits: A mod made by streamer Eroktic that changes weapon attachments and kits so that you have to loot them from stalkers or stashes, just like weapons and armor. I modified it so that it only adds the drops while keeping the ability to buy them, mainly to reduce the grind a bit. The grind is cool in single-player, but I find it less enjoyable in co-op.
Seamless Inventory Sort: Adds some QoL to inventory management. Really convenient.
LNCT Campfire Healing: Speeds up healing at campfires a bit. Same reasoning as above, having to sit around doing nothing for 5 minutes while playing co-op isn't exactly the most exciting thing.
Custom iTheons PDA Taskboard Gamma: A more up-to-date version of the task board included in GAMMA, with yet more QoL improvements.
Random Pile of Addons: I use this with a modification I coded myself that lets me bind weapon canting to the same key as quick melee (MB4) while aiming, which helps reduce the number of keybinds. I only use the "Road to Vostok mouse weapon controls" module to change weapon stance with the mouse wheel, and "Inventory UI Context Menu Colors" + "Don't Stack Weapon and Armor" for a bit more inventory QoL.
Quick Action Wheel Overhaul: A more up-to-date version of the quick-access wheel included in GAMMA, with more QoL improvements (and STALKER 2 visuals).
AmmoCheck Enhanced v3 REWORK: More QoL for binding everything related to reloading, switching ammo types, checking magazines, and weapon inspections with fewer keys. It also allows you to perform a more immersive ammo check (inspired by Tarkov), which I personally prefer.
Free Zoom ADS-only addon: A small modification I made that limits free zoom to when you're aiming, which also lets me bind it to Shift alongside sprint.

All these mods bar the last one can be found on the GAMMA discord or ModDB.

Even though this is now probably the best way to experience GAMMA thanks to the absolutely massive amount of work modders have been putting into it recently, I'm not using magazines or wearable devices for a completely HUD-less experience, because they were causing issues with Zona. Those issues have already been fixed in the dev version of the co-op mod, though.

A bit about Alife Plus

I want to make a separate section for Alife Plus, because the suite absolutely deserves it.

It's a collection of four modular and fully configurable mods that completely change the gameplay experience and make the Zone feel much more alive:

Alife Plus: The original mod. It works on both the online and offline sides of A-Life to add as many possibilities as possible and create natural events throughout your playthrough. It's excellent, but you need to give the game a few in-game days to really see its effects : factions taking over positions from each other, dynamic traders appearing, ambushes around corpses & stashes, etc. It completely shakes up the metagame and adds a lot of randomness, and therefore replayability, to the game.
Alife Tactics: The newest addition to the collection and the one with the most immediately noticeable impact. It's a complete combat AI overhaul. It's excellent and fixes one of the biggest flaws of the original game IMO: stalkers strafing left and right at absolutely ridiculous speeds. AI behavior differs from faction to faction. Stalkers use flanking tactics, suppressive fire, run for cover while reloading, use consumables, and their equipment/artifacts affect their stats, etc. It's fantastic, but be warned, the base game is already brutal, and this mod makes it extremely difficult. I've tweaked it a bit to make it fairer, but you still get punished very quickly for making mistakes : staying in one place, standing in the open, not watching your back, etc. In particular, I increased enemy weapon dispersion because I was getting laser-beamed by suppressed pistols from 150 meters away, limited AP ammunition to high-rank stalkers, and made bushes opaque to NPCs since we can't see through them either. This doesn't prevent them from shooting into bushes if they know or suspect you're there.
Alife Balance & Alife Guard: These two work together on the offline side of A-Life to maintain a substantial population of stalkers and mutants while keeping performance under control, since performance has a nasty tendency to deteriorate the further you get into a save.

As for the CO-OP itself:

It works extremely well.

Everything is very smooth, with no lag or stuttering, akin to xrMPE for Call of Pripyat, which we played through with that same friend and although we had a lot of fun with it (especially in the first half, the game falls apart after getting to Pripyat imho, but that's often the case with the series), I find the freeplay focus of GAMMA much more suited for a coop run.

You can join directly through Steam without any configuration (the game shows up as Call of Pripyat, but you can very easily change the AppID in case not everyone owns it), or connect through IP over a local network if you prefer / play offline.

Everything is synced flawlessly between players. We're currently playing as a duo, but it can support up to 4 players at the moment.

Even quests are properly synchronized: one player can accept a task and another can turn it in, and everyone gets the rewards.

The mod also adds several co-op-specific features:

You can ping locations and enemies.
You can directly transfer items or money through your inventory when you're close to another player.
Clients can quicksave/quickload just like the host.
When your HP gets critically low, you fall to the ground and start crawling. Your buddy can use a medkit on you to revive you and apply its effects.
A text chat but I didn't try it since we play on Discord

Configuration

I only adjusted a few sliders to balance things for co-op:

Trader restocks made 2x faster, otherwise you run out of medical supplies pretty quickly when playing as a duo, especially in early game before getting a drug-making kit. (in Progression difficulty settings, you have to be in-game to alter it since it changes depending on your difficulty choices for your run)
NPC population doubled for more action (in MCM > ZCP settings)
Friendly fire enabled (in xrRazom settings).

We're playing on Medium difficulty + Hard survival, which feels very good for co-op without having to grind for dozens of hours (in solo I usually play in the hardest settings with stuff like Fatal Error mod that make it even more miserable, but that's another mood)

We're already 25+ hours in and haven't even reached the Miracle Machine yet, although we take our time with way too long crafting sessions and have had quite a few combat reloads because the game is really hard (and I didn't tweak the difficulty of Alife Tactics on the couple first evenings yet).

Bugs

All the bugs we've encountered so far have been pretty minor, and they're already fixed in the current dev version.

For example, if my friend loots a mutant or takes parts from a weapon before me while I'm the host, we'll end up with duplicate parts. Or stats from skill levels not applying to the client when loading a save until we change regions.

The most problematic bug was one that prevented you from reviving your teammate if they'd had their torso or head blown apart. That's already been fixed in a hotfix available on the mod's Discord and will be included in the next release.

Other than that, the only bugs we've encountered have been extremely rare and unrelated to multiplayer, they're just base-game bugs, and a quickload fixes them every time.

During our last session, which lasted 5.5 hours (this game is really hard to put down), I don't think it happened even once.

Restrictions / things to keep in mind

The biggest requirement is that everyone needs to have exactly the same modlist, ideally with the same configuration as well. I handled everything on my end and simply shared my installation with my friend. That's probably the easiest solution for now (that's why you can see on the above clips that the graphics are not as good as you often see on youtube, It's mostly stock settings whereas when I play solo I use the latest beta SSS shaders, Melancholy Weathers or latest Atmos & various texture mods on top that put Stalker 2 to shame).

You can't talk to the same stalker simultaneously, so sometimes you have to take turns with traders. I'm pretty sure that's intentional. Everything else works fine, though: containers, corpses, even crafting stations. So it's barely an inconvenience.

Everyone has to be on the same map, you can't have one dude in Yantar while the other is in Rostok, but you can be at the opposing sides of the same map without any issues. You have to be 10m apart to change zones (I bumped it up to 30m, feels better), and you can also fast travel / use guides if near each other.

Finally, you need to be a little careful when saving/loading in unsafe areas, because clients take a few seconds to spawn in after the host finishes loading. I'd say it's generally somewhere between 3 and 10 seconds.

Installation

Setting it up is pretty straightforward:

Install a vanilla GAMMA setup. It's really quick, the longuest part is waiting on the mods to update through the launcher.
Install xrRazom Slim 1.3 on top of it. There's a guide on the mod's Discord, and I strongly recommend using the ModOrganizer method.
Install the revive fix from xrRazom Discord.
After launching the game, don't forget to change your name in the xrRazom settings so host & client(s) are not the same.
Start a new game on host, clients can simply join through steam, and voilà !

The upcoming version

I've mentioned the dev version a few times. The lead dev announced around a week ago that 1.4 would probably arrive within 1–2 weeks, so I don't think we'll have to wait much longer. As I understand it's mainly there to fix all the stuff that's been reported & add compatibility to a lot of additional mods. I've also seen some guys on the Discord work on VOIP with positional, occlusion and ray traced audio + working radio filter & interferences when you get too far apart, which sounds amazing, and should be available in the next few days.

Hopefully it'll be there for ROMEO's release in a week, the modpack by youtuber Clikrad focused on maximum immersion and tacticool play, and will remind me of old school Ghost Recon if playable in coop.

And finally, regarding performance: thanks to the multithreading engine recently introduced by the community, the game finally runs incredibly well.

u/ap0k41yp5

STALKER GAMMA Multiplayer with xrRazom Anomaly Co-Op - Thoughts after extensive testing