Source SDK Base

Source SDK Base is a game/tool available to all Steam users which is locked to a particular engine branch. Compared to basing your mod on a "real" game, use of SDK Base massively increases your potential audience and prevents incompatible engine updates from making your code unusable.

Versions

There are currently four versions of SDK Base: the original Source 2006 version (AppID 215), the Source 2007 version (AppID 218), and two Source 2013 versions - SP (AppID 243730) and MP (AppID 243750).

Alien Swarm deserves mention here too, as it is also freely available and also provides source code.

In most cases, it is recommended to base your game on Source SDK Base 2013, as it's more up to date than previous branch. Source SDK Base 2013 has two sub-branches, with the multiplayer branch having some additional features such as BSP compression and static prop lightmaps. Meanwhile, the Alien Swarm engine branch add some new engine features (including features first introduced in Left 4 Dead) which were not present in Source 2013, but they can be backported. Older version of SDK Base (2006, 2007) are provided in order to play older mods.

Setting up the Source SDK Base

Install Source SDK Base 2006/2007 from Steam->Library->Tools or from the links below:

Install/Run Source SDK Base 2006 (AppID: 215)
Install/Run Source SDK Base 2007 (AppID: 218)

For Source 2013 version, see this page.

Content

The stock gamecode is a duplicate of Episode One's or Episode Two's, depending on which version (see below) you are using. But the only content available is that of Half-Life 2 and Half-Life 2: Lost Coast: while Zombines and Hunters exist in the code and can be spawned, they appear as errors because their models are not included. If you want to use episodic things, you will need to mount the appropriate game.
SDK Base might provide mods with Lost Coast's content, but dedicated servers do not. Multiplayer mods should either not use anything from Lost Coast, or include what they do use in their dedicated server download.
Tip:In Source 2007 or later you can remove the sourcetest SearchPath from gameinfo.txt and Lost Coast will disappear. Unfortunately, doing this under Source 2006 (Episode One) will make the game crash immediately!

Bugs and Fixes

There are two small glitches and things that need to be changed:

The main menu has a "Visual Stress Test" option instead of anything useful. Copy resource\GameMenu.res from another suitable game into your mod.

Episode Two's flashlight and weapon pickup icons do not appear. Open resource\ClientScheme.res and add this to the Fonts section:

		WeaponIconsSmall
		{
			1
			{
				name		HalfLife2
				tall		32
				weight	0
				antialias 1
				additive	1
				custom	1
			}
		}

In addition, Source SDK Base 2007 is unable to mount Half-Life 2: Episode Two content from provided depot VPKs, because of a missing config file and the VPKs being switched around with Base 2006.

The fix for it is described in the linked article.

Content (2007)

Source SDK Base 2007 (sourcetest)
Source SDK Base + Half-Life 2: Lost Coast (sourcetest)
Source 2007 shared (hl2)
Source shared (hl2)
Source 2007 engine branch

Content (2006)

Source SDK Base + Half-Life 2: Lost Coast (sourcetest)
Source shared (hl2)
Source 2006 engine branch