Identify the failing layer
Proton stores game-specific Windows-like state in compatibility prefixes. An update of a game, compatibility tool or launcher can change files inside that state, so changing Proton back is not necessarily a complete rollback. Start by establishing whether the same executable ran before the change and whether the failure occurs before a game window, during launcher authentication, or at Vulkan device creation. A missing Windows DLL message is not sufficient evidence that the prefix is corrupt.
Collect discriminating evidence
Capture PROTON_LOG for exactly one reproducible startup, then locate the game-specific AppID, Steam library and compatdata directory. Check whether a second account or an external installation has another prefix. Before Winetricks, Protontricks or manual DLL changes, save the entire relevant prefix and check for unsynchronized local saves. Do not recommend deleting compatdata as a routine first step; recreating it can silently lose per-game settings and launch credentials.
Compare safely and record limitations
A controlled trial changes only the selected compatibility-tool version while preserving the prefix backup. If the issue persists across versions, compare launcher logs, graphics initialization and filesystem writes rather than treating all failures as migrations. A new clean prefix can eventually be diagnostic, but only after confirming backup and recovery of personal game data. Keep the backup outside the Steam library and record the application identity to avoid restoring it into the wrong game.
Gather evidence before changing the system
Run one command at a time in the relevant host or game environment. Read its requirements and interpretation first. The website displays commands and never runs them.
Read-only observation
df -hRequirements: Run in the relevant host/session; compare with the actual game environment and redact private paths before sharing.
Check whether the filesystem holding compatdata can accept further writes.
Temporary diagnostic environment
PROTON_LOG=1 %command%Requirements: Run in the relevant host/session; compare with the actual game environment and redact private paths before sharing.
Steam launch option to capture one Proton run; not a repair by itself.
A controlled test with a way back
Record Proton version, back up compatdata and saves, then compare one supported compatibility version with the same game files.
Rollback: Restore the original tool selection; separately restore backed-up prefix only if necessary and after verifying its identity.
What this test cannot establish: Re-selecting an older Proton may not reverse prefix migrations; Steam Cloud is not a universal backup.
Sources and scope
This editorial review uses primary project and distribution references. It does not establish that a fix has been reproduced on your hardware. Installed versions, the game runtime and the selected graphics API can change the result.
- Valve Proton runtime and logging
Valve documents per-game Proton logging and runtime options.
- Steam Linux Runtime bug-report guidance
Valve separates pressure-vessel logging from game-level failures.