Hello. In this brief guide, you will learn about the issue many people and I have had for at least since the past 9 years of joining/hosting Left 4 Dead 2 Local Servers.
In absolute demise from how I used to have this problem myself from since the time I began using a lot of workshop mods (April 2020), I—just like many others—assumed that this problem was just caused by having too many workshop mods. BUT, I'd notice some other players who also use lots of mods are able to host Local Servers that are joinable. (By that, I mean that the game actually loads and you can play in the server). So, in about January 2021, a friend and I have done some experiments to discover what the exact culprit of this problem is, and we were very surprised and happy at the same time to find it out:

So, you attempt to join some individual's Local Server game (so that you can play a custom campaign in lieu of an official one), and you realize the loading bar had went blank, frozen, reset, reversed, disappeared, empty, whatever you call it, and it never loads after that. If this happens to you, you don't have to force quit L4D2. You could just press the tilde " ~ " key to open up the console, which opens up the pause menu for you to leave back to the main menu.

No matter how many mods you have, you could still load some other people's Local Servers and official servers.

You may have been informed already of needing to type this console command: However, this still doesn't work. You do need to do it as the host so that the players joining your local server do not need to download the exact same mod collection as you.

Here is the culprit my friend and I found out: Special Infected model mods.


UPDATE: Turns out Weapon HUD image mods do not cause this problem, so you can leave those enabled.


You—assuming you're the host—need to disable all of those that you have , and then try hosting a local server.
Remember to still type sv_consistency 0 into the console before you host.

If the problem still occurs to those joining you, you need to check your add-ons again. You may have missed something. I did too and that's how I discovered it. I disabled all my mods in my first experiment and my friend still got the blank loading bar. I double checked to find out I still had a Jockey model mod enabled. It worked after I disabled that, and then after a few more experiments, (survivors, weapon models, sounds, other models, scripts) they all worked.

We concluded that Special infected models are the only things keeping you from hosting Local Servers. With those disabled, you could host and while you're using as many mods as you want. AND Those joining the hosted game could still use Special infected mods of their own. Only the host needs to disable those specific mods.

My professional judgment says that this problem occurs due to the Special Infected models changing where the hitboxes are. Because in Left 4 Dead 2, the hitboxes are not in fixed locations; they're in parented locations relative to bones of the armature. Most special infected model mods change the sizes and local positions of the bones, and so, the players joining are unable to compute this if they don't have the same special infected mod (even with sv_consistency 0), and so, the game empties out the loading bar—it's trying to load something it doesn't have and can't receive.

BUT, for some strange reason, the host could still use survivor mods (also has the hitbox differences), and yet the players joining don't have this problem at all! The hitboxes on the survivors are that of the where they are on the host's survivor mods, and the game is able to compute that throughout the entire local server. Strange, but that's how it works.

Hope this was helpful to you. It's fun to picture it like I do, where in your group of friends one of you has to be the one that sacrifices their special infected mods so that they can be the group's designated host, while the others could still use their SI mods. It feels like getting a drivers licence! Or, like being that one friend that doesn't drink that much because they're driving.

I'm that friend, and that's why I don't use special infected mods anymore. Have fun and good luck!