ValveSoftware/source-sdk-2013

What it is

Source SDK 2013 is Valve's public release of the game code that sits on top of the closed-source Source 1 engine. It contains the game code for Half-Life 2, HL2: DM and, since the February 2025 update, Team Fortress 2 (README). Modders use it to change gameplay rules, weapons, entities and server behavior, then ship the result as a Source mod.

The important boundary: this is game code, not the engine. It does not include the renderer, physics or platform layer, so it cannot by itself port a game such as TF2 to a new platform. A commenter on the TF2 code release made this point directly (jsheard, 2025-02-18).

Quick start (Windows)

1. Install Source SDK 2013 Multiplayer through Steam.
2. Install Visual Studio 2022 with the Desktop development with C++ workload: MSVC v143 x64/x86 build tools, and Windows 11 SDK 10.0.22621.0 or Windows 10 SDK 10.0.19041.1.
3. Install Python 3.13 or later.
4. Clone the repository:

git clone https://github.com/ValveSoftware/source-sdk-2013

5. From inside the clone, go to src and run:

createallprojects.bat

This generates everything.sln (build instructions).

6. Open everything.sln, choose Build > Build Solution, and wait for all projects to build.
7. Set the Client (Mod Name) project as the startup project, then start the Local Windows Debugger. The default launch options for the Release configuration are already filled in.

Quick start (Linux)

Linux builds run through podman and target the Steam Runtime. You still need Source SDK 2013 Multiplayer installed via Steam. From src:

./buildallprojects

This builds all SDK and mod projects (Linux instructions). Then run your mod from the game directory with its launcher, for example:

./mod_tf

Mods distributed on Steam must be built against the Steam Runtime; the scripted build already does this.

Constraints

• Engine is not included. You get game code only; the Source 1 engine remains closed. Porting to new platforms or rebuilding engine-level systems is outside what this repository provides.
• License is non-commercial. The code is released under the SOURCE 1 SDK LICENSE on a non-commercial basis (LICENSE); review it before distributing anything.
• Steam mod distribution. When shipping a mod on or off Steam, follow Valve's distribution guidance (distributing your mod); Steam-hosted mods need Steam Runtime builds.
• Build tool versions are specific. The Windows path depends on VS 2022 with the listed SDK/toolchain components and Python 3.13+; older toolchains are not described in the README.
• Ecosystem churn is real. The 2025 SDK update broke existing setups; TF2 Classic told players to switch Source SDK Base 2013 Multiplayer to the previous2021 beta branch while porting (Lammy quoting the TF2C announcement, 2025-02-18). Expect to manage SDK revisions alongside your mod.

Community context

The TF2 code release drew substantial discussion. Server-side modders described moving from binary analysis in IDA to reading the code directly (sevenf0ur, 2025-02-18), and others noted the release may legally enable derivative projects such as TF2 Classic (turbobrew, 2025-02-18).

HotRepo's take

On 2026-11-01 the repository held the 6th spot on the GitHub daily C++ list with 7 stars gained that day — steady interest years after creation, not a spike. The real signal is the HN thread on the TF2 release: 1,863 points and 387 comments (discussion), with the strongest practical takeaway being that this is modding code layered on a closed engine, not an engine release. That matches the repository's own scope and should set expectations for anyone arriving from the announcement.