If you want to play Co-op in Half-Life: Source, you will have to install the fixed version, because that is the only version that has working multiplayer.

Valve has hidden Multiplayer mechanics in single player games running Source 1 (Source 2013 and older only). You can play Cooperative with your friends or other people online.

Because of how HL2 was intended to only be a single player game, it may not always work correctly. For instance, some map features may not work correctly without modification, it may cause the game to crash or not work correctly at all. Example: Going near the end of the level in d1_trainstation_02 crashes the game.


Enable Developer Console by going to options,

https://steamuserimages-a.akamaihd.net/ugc/1010438039195856995/4F7EFF9061E8B55BCF370A8FA5E09F1D84F1935E/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Open up advanced options, and check the box that says "Enable Developer Console". Make sure you apply the new options and click the OK button.

https://steamuserimages-a.akamaihd.net/ugc/1010438039195859901/34495DEC75DC2DDC0ED151F1FEF3194644778608/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Press the "`" key, and type this command in console.


https://steamuserimages-a.akamaihd.net/ugc/1010438039195863568/614A696717D00CEDEF18C1536E61F3C03A823FAC/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

And you should be able to find a public server that you can join in.


This method is simple and you can easily do it in console.

You need to disconnect from the background map first. To do that, type in console.

In console, type this command.
https://steamuserimages-a.akamaihd.net/ugc/1010438039195868031/1D7782B9D81A7771B9B87213299BB6F8BE739B0A/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false



and then type this other command.
https://steamuserimages-a.akamaihd.net/ugc/1010438039195869264/45CC3C71AABE8BC11C89F94D906738BC61532C67/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false



You can set the map to any map you want. Type this final command in console.
https://steamuserimages-a.akamaihd.net/ugc/1010438039195872067/FFBF6E24913DC901698936CB74B28A94CEF0577D/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false



The server should be on. For other players to join their game can automatically download the map file from the server owner's computer.

After you've set the map, you must heartbeat the server, by typing this command in console.


If you have port forwarded, you're good to go! Players should now be able to join your server with this method of Cooperative. Just remember, try not to shoot your teammates!

You can also set the server's name! Type this command in console.


Unlike in portal, you can do text chat, but in Half-Life 2 and it's episodes, you can't because the chat itself is hidden from regular gameplay, but you can do voice chat.

Type this command in console and you'll be able to voice chat.

No bullet holes? No Problem! This can be fixed by disabling predicted weapons via console.


Be careful of using the SMG1 Grenade! Shooting it at a zombie can crash the server.

You can hear other people's voice chat, but they can't hear you if you are the server host.