The Steam client for Linux is designed to run on distributions from rolling-release distros like Arch, all the way back to distros such as Ubuntu 14.04 (2014). To achieve this, Steam uses its own library stack to avoid having to rely on the libraries provided by the host system. Valve uses classes from the video game 'Team Fortress 2' to version their runtimes.
This library stack is known as
steam-runtime version scout and is located at
The Steam client itself and many games use this particular runtime by adding its path to the
LD_LIBRARY_PATH environment variable. Steam also calls the 'scout' runtime the 'LD_LIBRARY_PATH runtime'.
Older versions of Proton (5.0 or earlier) use the same 'scout' LD_LIBRARY_PATH runtime as most native Linux games.
A new approach to providing a Steam Runtime is one that uses the Linux namespace (containers) feature. This allows games to run in a predictable and stable environment even on unusual Linux systems. Provided by Steam Play compatibility tools, this version of the runtime is known as the 'Steam container runtime' or 'steam-runtime' version soldier.
Newer versions of Proton (5.13 or newer) use the 'soldier' container runtime with newer library versions.