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Half-Life: Decay Installation for SP and Online Co-op Guide
By Miyumi Ayase
This guide aims to provide an updated (2015) guide on how to successfully install and play Half-Life: Decay in Singleplayer and Online Co-op on the PC.
Most of the guides I encountered were very informative but they were published back in 2010 with some dating back 2008, since then, the Steam Client had undergone several updates and changes that makes the old guides outdated.

Image from Wikipedia[en.wikipedia.org]

Half-Life: Decay[half-life.wikia.com] is a first-person action shooter by Gearbox Software and Valve and serves as the third official expansion to the critically acclaimed 1998 game Half-Life. Unlike its predecessors (or even the main game), Half-Life: Decay is exclusive to the PS2 and the first in the Half-Life franchise to be designed for co-operative play.

Although Gearbox Software successfully created a port of the game for a PC release, it never saw the light of day "due to powers beyond Gearbox's control" thus prompting some dedicated fans to take matters in their own hands. A Fan made port was released in 2008 and is made available at Half-Life Creations Website[decay.half-lifecreations.com]

Patrick Deupree, a developer and member of the Gearbox team that made Half-Life: Decay, had this to say:
Originally posted by Patrick Deupree:
I work for Gearbox and did most of the coding for the Decay work we did on the PS2. First I'd like to say that there is no interest or desire on our part to fox this project. Everyone here thinks it would be great to see this ported to Source.

We even had a port of the game ready on HL1 for the PC, but due to powers beyond our control, were unable to release it at the time. So it's always been of interest to us to see Decay make it to the PC since, as someone else here has mentioned, it does deliver a part of the Half-Life storyline so it's always been a shame to prevent a large portion of the Half-Life players out there from seeing that part of the story.
When this mod is done I will probably fire it up with a friend of mine that never had the chance to play Half-Life on the PS2. It's been a while since I played through Decay (though I certainly played through it quite a bit while we were working on it). -Patrick Deupree (Originally posted on the VERC)
From the Valve Developer Community Wiki
Installation Guide and Singleplayer
Half-Life: Decay was made for Co-op so anyone wanting to play singleplayer must constantly switch between both characters by pressing the “G” Key on their keyboard. The bot controlling the other character is just basic AI and is only capable of defending itself and nothing else, movement must be made by the players themselves.

You must have Half-Life (not Half-Life: Source) installed in your Steam Library and directory.


Download the fan made port of Half-Life: Decay (the file name should be decay_eng.exe)
File: Half-Life: Decay Fan Created Port[decay.half-lifecreations.com]

Double click decay_eng.exe and follow the installation wizard. If asked to Choose where your Half-Life directory is, simply choose “common” from the 'Steam account:' dropbox field in the installer.

'Half-Life' should appear under 'Game folder to install Decay to:'

After this, a new folder called 'decay' should show up on your Steam Directory INSIDE your Half-Life folder.

Run Half-Life and from the main menu, click 'Change game' (if you can't find 'Change game' in your Half-Life menu, see STEP 5) and choose Decay. The game should now be available for Singleplayer.

STEP 5: (OPTIONAL: Follow this if 'Change game' is not in your Half-Life Menu)

Go to: \Steam\steamapps\common\Half-Life\valve\resource\GameMenu.res, open it with notepad and paste the entire code from this guide (not the CS 1.6):
Change Game from Main Menu Guide.[github.com]

STEP 6: How to start a Singleplayer Game
Click 'Play Decay' and select dy_accident1 in the map options. All other maps are locked and can only be unlocked by playing through the game.

It should look like this:
Setting Up Dedicated Server and Online Co-op
Follow STEP 1, 2 and 3 on the Installation Guide and Singleplayer section as seen above. Disregard if you've already done the above steps.

Make a folder called 'dedicated server' anywhere you want.
ex. C:\dedicated server

STEP 3: Setting up a Dedicated Server
Download SteamCMD and then unpack its contents preferably on its own separate folder to keep things clean.

Double click Steamcmd.exe (it should have a Steam Icon). A command prompt will appear and will attempt to update. Just let it do its thing and wait until it finishes updating.

After the update, you should now see the command line “Steam>”
Now type the following commands IN ORDER:
  • login anonymous
  • force_install_dir C:\dedicated server (or wherever you created your dedicated server folder earlier)
  • app_update 90 validate
The first attempt will ALWAYS FAIL! Don't panic and proceed to STEP 6 =)

After it FAILS to download the first time, just type app_update 90 to validate it again. This time, it should finish downloading all the proper files.

COPY (not cut), your 'decay' folder from your Half-Life directory (Steam\SteamApps\common\Half-Life), into your dedicated server folder (c:\dedicated server)
Note: Be sure to copy the entire folder and not just the contents

Inside your 'dedicated server' folder, double click an exe file called 'hlds.exe' (should have a red Half-Life (Lambda) icon). It will open a GUI where you can setup your own server.

Put the following entries:
Game = Decay
Server name = (whatever you want your server to be called)
Map = dy_accident1
Network = Internet
Max. players = 3 (no more no less)
UDP Port = 27015
RCON Password = (whatever you want it to be)
Check “Secure (Valve Anti-Cheat)” -or not, your call

Click 'Start Server' and your server should now be up and running, all we have to do now is set up the map cycle order.

STEP 10:
Under the 'Main' tab, double click Map Cycle and enter the maps in this order:

STEP 11:
Run Half-Life and from the main menu, click 'Change game' (if you can't find 'Change game' in your Half-Life menu, see STEP 5 on the SINGLEPLAYER section above) and choose Decay.

Click 'Find Servers' (you should, if you followed the Steps correctly, find your newly set up Server there)

If you're the host, the server should appear in both your Internet Tab and your LAN tab. For your friends, it will appear only on their Internet Tab (unless you're in the same network, in that case, it will also appear on his/her LAN tab)
Server not showing up on Friend's Internet Tab?
If you've followed the guide to the letter and was able to setup a dedicated server and you yourself see the server but your friends don't, then that means you need to do some Port Forwarding with your router but that's a matter outside my guide's scope.

This should not be a problem when you and your friends are playing in LAN.
Steam Library Icon and Name Fix
A 'new' game with no Icon should have appeared in your Steam Library called 'Decay'.

This is a Shortcut made by Steam for you to quickly access Half-Life: Decay.

If you want to give the shortcut an Icon and rename it to Half-Life: Decay, simply download this file: Half-Life Decay Steam Icon Patch[www.2shared.com]

Unzip it and copy paste the contents to: "Steam\SteamApps\common\Half-Life\decay"

Your half-life/decay folder should look something like this:
Taken with DiamonD Half-Life Final mod Weapon and hand models.

Thanks to Azrak for helping me test this out.
Thanks to soulsynapse and ♥ Sora ♥ for helping me set up my Rounter Ports

Credit to DarkIce for his excellent Complete Lost Packs Guide where I found out about Half-Life: Decay and several other great mods to improve your Half-Life (1) experience.

Credit to 'y0ursource' for his very helpful SteamCMD and Dedicated Server Guide[github.com].

Credit to 'MaxKorz' for his Change Game from Main Menu Guide.[github.com]

Credit to 'kamilo' for his Steam Icon fix

Credit and a big thank you to the guys at Half-Life Creations[decay.half-lifecreations.com]for porting this game to the pc. Again, thank you for all your hard work!
Other Related Contents
Shameless self-promotion
If you're interested, also see my:

1. Curator's Page: Miyumi's Review Corner
2. Half-Life Review
3. Half-Life: Opposing Force Review
4. Half-Life: Blue Shift Review
5. Black Mesa Review
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Mr. Cuckoo 17 hours ago 
I can't get the single player working. My character is the only one to appear so I can't progress at all.
Ramzej Nov 9 @ 8:45am 
Steam Library Icon and Name fix link is not working... :/ Can you reupload this?
diddlydarnman Oct 26 @ 6:21pm 
I got stuck on the "Open the GUI of the dedicated server menu by clicking on the HLDS Icon"
TheRetrodude Sep 28 @ 4:57pm 
This is a good mod, but I'm having 2 problems. For one my mouselook aiming is acting like an analog stick for whatever reason and is throwing my aim way off. Also my brother and I are having trouble getting past a few parts and we want to know how to enable cheats, since we've tried to activate them numerous times but it seems they're disabled by default.
TryHardGamer Sep 23 @ 11:47am 
Hey, I have no idea if people help here but I get random crashes as well as the other person I'm playing with. We're on dy_hazard and playing on LAN. Is LAN the fault? Do I have to setup a dedicated server to stop the random crashes? Thanks for the help!
Pip Boy Sep 16 @ 5:10am 
i also had that problem PHO3NIX. I ended up going with hamachi, clearly OP has given up on this tutorial
Pho3nix_Pro Sep 16 @ 2:24am 
i try to select the game and map option on the dedicated server menu and nothing comes up what do i do?
"Under the 'Main' tab, double click Map Cycle and enter the maps in this order" After I hit create server it justs exits out I don't see the map thing
Hammer0311 Aug 23 @ 5:09pm 
Have you perhaps tried verifying the integrity of tool files?
Pinsplash Aug 23 @ 3:10pm 
after i click start server (for the dedicated server) the little window disappears andi dont see anything about maps come up