Proton prefix — compatdata

Updated: 2026-08-25

What is in there

Proton is Valve's build of Wine. Wine does not emulate Windows so much as re-implement it, and a Windows program still expects a C: drive, a registry and the usual user folders. Each game therefore gets its own prefix: a directory tree that looks like a fresh Windows install.

That tree holds the registry hives, whatever redistributables the game installed into itself (DirectX runtimes, Visual C++, .NET), and the contents of the virtual user profile — Documents, AppData, Saved Games.

The last one is the important part. A Windows game that saves to My Documents\My Games\<Game> writes into the prefix, because as far as it can tell that is where My Documents is.

Where it lives

Every library folder has its own compatdata, so a Deck with a microSD card has at least two of them. The folders are named by app id, exactly like the shader cache sitting next to them — which is how the two get confused.

How big it gets

A prefix starts around 300–400 MB before the game has stored anything, because a usable virtual Windows is not small. With redistributables and a few years of save data, individual prefixes run to several gigabytes.

They are not removed when the game is uninstalled. On a Deck that has been through a lot of games this is frequently the single largest category of leftover — larger than the shader caches people usually go looking for first.

Is it safe to delete?

For a game that is still installed, deleting the prefix resets it. The game will usually still start — Proton builds a fresh one and reinstalls the redistributables — but local saves, settings and any mods installed into the prefix are gone.

For a game that is no longer installed, the prefix is dead weight, with the same caveat: if you ever reinstall and the saves were never in the cloud, they are not coming back.

To check before deleting, open pfx/drive_c/users/steamuser and look in Documents, AppData/Roaming and Saved Games. If there is nothing but empty folders, nothing is at risk.