You will need a strong grasp of of C++, build systems like Visual Studio or Make (depending on the platform you're developing for), CMake, command line interfaces, version control systems (Git in particular) and package managers (vcpkg in particular).
Various configuration files use JSON so an understanding of its syntax is recommended.
Tools and scripts are written in C# so you will need to have an understanding of its syntax to modify them.
This project uses vcpkg in manifest mode to acquire most third party dependencies, with the exception of dependencies shipped with the game itself (vgui1 and SDL2). You should use vcpkg to add any dependencies you need. If a dependency is not provided by vcpkg's own registry you can add it to the local filesystem registry in vcpkg-configuration.json.
Note that the OpenAL library is a custom build that renames the dynamic library name to avoid conflicting with the engine's use of OpenAL on Linux. If you update OpenAL you must also update the patch used to rename the library.
Also note that Github Actions performs a shallow clone of the vcpkg repository. When updating vcpkg make sure all dependencies are using the latest version, otherwise Github Actions will fail to locate the package information.
You can use the local registry to reference an older version by copying the older version package definition from the vcpkg registry to the local registry, but do make note of transitive dependencies to avoid using multiple conflicting versions of the same package (e.g. spdlog references fmtlib which this project also directly references).
Consult the vcpkg documentation before changing anything to ensure you understand how it works and how your changes will affect the project.
Resources to learn these things:
Make sure to learn these things first so you understand how this SDK works!
The source code for the mod SDK is in the repository that this file is included with. This also includes the source code for the original tools included with the Half-Life SDK as well as delta.lst which is used to configure networking settings for the mod. liblist.gam is also included.
Mod installations are available on the releases page of this repository.
The source files for game assets are stored in the Asset Source Files repository. This includes sources for all types of models, configuration files, scripts, fgds; anything that's not mod source code.
See the README for more information on what exactly is stored there.
The source code for tools written in C# are stored in the C# Tools Source Code repository. These tools are used to automate some parts of development such as the copying of source files from the assets repository to the game installation, as well as the packaging of mods and installation of game content taken from the original games.
The tools are included with mod installations.
See Setting up the source code
Historically the name of a mod's server library defaults to hl because this is what it is called for Half-Life. Some games and mods change the name to match the game name and others change it to just server.
To keep things simple the name has been changed to server.
In order to have the game appear in the Steam game list you will have to add a liblist.gam file.
The SDK includes liblist.gam, located in the config directory and named liblist.gam.in. This file is a template used to generate the actual file which is copied to the mod directory when you build the INSTALL target.
When creating a new mod, change the game value to change the display name for your mod. This will be used in Steam and as the window title for the game.
Because there is no repository for the mod installation itself assets need to be copied from the assets repository to the game installation.
The AssetSynchronizer tool automates this. When you run it it will copy any files that are different and will then monitor the assets repository for changes to automatically mirror to the mod directory.
This tool uses a configuration file to control which files are watched and where they are copied.
The Packager tool is used to create the mod installation archive. This tool uses a configuration file to control which files are included and excluded and creates an archive containing the installation.
Archive filenames include a UTC timestamp to uniquely identify them. On successful completion old archives are automatically removed.
The ContentInstaller tool is used to copy, convert and upgrade maps from Half-Life, Opposing Force and Blue Shift.
Original maps can't be redistributed, and changes made to SDK code require changes to be made to these maps. This tool does that automatically.
The MapUpgrader tool is a standalone version of the installer's map upgrade functionality. It can be used to upgrade a map to the Unified SDK, provided that the map was made for the original version of Half-Life and not another mod or game.
Most modders won't need to use these tools, but they are there to streamline the development process as much as possible.
The Half-Life Uplink demo isn't available on Steam so the maps are included with the Unified SDK's mod installation.
When packaging the installation the packager looks in the mod directory named valve_uplink for the Uplink maps. You will need to take the Uplink maps and place them in valve_uplink/maps for this to work.