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Games crash on startup with assertion failure in lsteamclient on Proton Experimental / 10 / 9

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ValveSoftware/Proton#9475 · opened 2026-02-08 by yar85 · updated 2026-03-09 · 4 comments · github
Yyar85 2026-02-08 github

All games fail to start for me when using Proton Experimental, 10.0-4, or 9.0-4.
The crash occurs immediately on launch and results in a Wine C++ Runtime Library assertion failure related to lsteamclient.

When using Proton 8.0-5, games start and run successfully. Logs still contain the message err:steamclient:load_steamclient unable to load ReleaseThreadLocalMemory method, but no assertion failure occurs. This makes me think the issue could be related to Proton rather than Steam Runtime.

Proton log excerpt

536823.756:0020:0024:trace:loaddll:build_module Loaded L"C:\\windows\\system32\\lsteamclient.dll" at 00006FFFFDB80000: builtin
536823.757:0020:0024:err:steamclient:steamclient_init unable to load Steam_ReleaseThreadLocalMemory
536823.757:0020:0024:err:msvcrt:_wassert (L"!status",L"../src-lsteamclient/steamclient_main.c",375)

System information

EndeavourOS (Arch-based), normal kernel 6.18.7-arch1-1, KDE Plasma (X11), ext4 unencrypted
Steam ver. 1769025840 (stable branch) from official multilib repo

Troubleshooting performed

  • Verified Proton files
  • Reinstalled Steam
  • Reinstalled Proton versions
  • Removed and recreated Wine prefixes

(none of the above resolved the issue)

Logs and diagnostics

EEreilli 2026-03-01 github

I have the exact same problem currently, proton 8.0-5 works for me aswell

I'm using KDE Plasma 6.6.1, ArchLinux 6.18.13 and also from multilib repo steam

Yyar85 2026-03-01 github

@Ereilli try copying the steamclient.so file from ~/.steam/sdk64/linux64/ (actual location is ~/.local/share/Steam/linux64/steamclient.so) to your ~/.steam/sdk64/ directory. You can create a symlink instead of copying.
This helped me, although it’s not a real solution to the problem, but rather a workaround…

It looks like Steam is using a 32-bit library from ~/.local/share/Steam/linux32/ and Proton is having trouble with it as it is throwing an assertion error 🤷 I still don't know why this happens.

SSolarWings 2026-03-04 github

@yar85 @Ereilli

Hi

Can you try installing vcrun6 through protontricks and check if that solves the issue?

Launch protontricks -> select game -> select default wine prefix -> Install a Windows DLL or component -> vcrun6

Yyar85 2026-03-09 github

Hi!

Can you try installing vcrun6 through protontricks and check if that solves the issue?

I tried this (with Escape from Duckov and Proton Experimental).
Nothing changed; the logs show the same error.

Proton versions

DLLs