Forcing previous2021 Beta

Introduction

The Source SDK Base 2013 Multiplayer 2025 update breaks mods that ran on the older version of the engine. While some mods are technically playable in the new SDK version, they are bug-ridden. This guide is for modders who currently do not want to update to the new SDK and also wants to prevent players from opening the mod with the new SDK too.

Code

The method here is to check the engine's protocol version. It should be 24 in the old SDK, but due to class misalignment, calling this function in the new SDK Base with the old SDK code returns a bizarre number.

Detecting the New SDK

Open cdll_client_int.cpp and find:

void CHLClient::PostInit()

and add this right above it:

bool IsNewSDK()
{
	return engine->GetProtocolVersion() != 24;
}

Adding the Error

Now modify the CHLClient::PostInit function so that it looks like this:

void CHLClient::PostInit()
{
	IGameSystem::PostInitAllSystems();

	if ( IsNewSDK() )
	{
		Error( "The previous2021 beta must be selected for Source SDK Base 2013 Multiplayer to play this mod." );
	}

#ifdef SIXENSE
	// allow sixnese input to perform post-init operations
	g_pSixenseInput->PostInit();
#endif

	g_ClientVirtualReality.StartupComplete();

#ifdef HL1MP_CLIENT_DLL
	if ( s_cl_load_hl1_content.GetBool() && steamapicontext && steamapicontext->SteamApps() )
	{
		char szPath[ MAX_PATH*2 ];
		int ccFolder= steamapicontext->SteamApps()->GetAppInstallDir( 280, szPath, sizeof(szPath) );
		if ( ccFolder > 0 )
		{
			V_AppendSlash( szPath, sizeof(szPath) );
			V_strncat( szPath, "hl1", sizeof( szPath ) );

			g_pFullFileSystem->AddSearchPath( szPath, "HL1" );
			g_pFullFileSystem->AddSearchPath( szPath, "GAME" );
		}
	}
#endif
}

If a player tries to run the mod with the modern Source SDK Base 2013 Multiplayer version, the game will stop them and show the above error message.