Hello @TehFreek, this isn't generally useful feedback for a Proton dev to ponder. Instead, please describe how the game behaves with a mainline Proton build and provide logs to go along with any quirks you're observing.
Fair enough. Here's the log for the standalone version (since I don't have access to the Steam version yet) under Proton 11.0-2. Up to 1678872.439 is the launcher, everything after is trying to start the game proper. The game crashes silently and drops back to the launcher.
Unhandled exception: unimplemented function ntoskrnl.exe.KeAcquireGuardedMutex called in 64-bit code (0x006ffffff9c200). in your Proton log hints towards, https://github.com/ValveSoftware/Proton/issues/9331, but there's also a lot of access violations (c0000005) in EndfieldBase.dll before that which might be more important.
Endfield does use AntiCheatExpert, so that definitely sounds relevant in some manner.
proton 11.0-2x1 2026-08endfieldbase.dllx1 2026-08
With Arknights: Endfield coming to Steam... eventually, I feel that it's time to take a look at dwproton and GE-Proton to see which changes allow it to run (currently standalone) and pull them into base Proton.