Half-Life Coop is a mod for Half-Life 1 that adds a cooperative mode to the game's campaign. Unlike other coop mods like Svencoop or DC coop, it does not change important aspects about the game, because the objective of this mod is to be compatible with the servers of Half-Life 1 and keep it as vanilla as possible.
This is a small addon for my mod, so you should first download the version 1.0 of my mod in the "files" section.
To download this addon, all you have to do is execute the exe file from the zip folder and install it where you have your Half-Life installed (Example: C: \ Program Files \ steam \ steamapps \ common \ Half-Life .. .)
I give some credit to "Half-Dead" that despite not borrowing his Unity Half-Life maps, his files were a great friend in this addon to fix them completely.