Your system information

• Steam client version (build number or date): 1778281814 (May 8 2026)
• Distribution (e.g. Ubuntu): CachyOS (Arch-based), kernel 7.0.9-1-cachyos (also tested 6.18.32-1-cachyos-lts)
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes — system is fully updated
• Steam Logs: See key excerpts below (Flatpak install — logs at ~/.var/app/com.valvesoftware.Steam/.local/share/Steam/logs/)
• GPU: Nvidia — GeForce RTX 5060 Ti (Blackwell, GB206), driver 595.71.05 (nvidia-open / open-source kernel module)

Additional info:

• Desktop: Hyprland 0.55.1 (Wayland)
• Steam installed via: Flatpak (com.valvesoftware.Steam, stable, v1.0.0.85)
• Also tested with native pacman Steam — identical crash

Please describe your issue in as much detail as possible:

Expected: Steam launches and shows the login window.

Actual: steamwebhelper (the CEF-based UI renderer) enters an infinite crash loop immediately after launch. The Steam UI never appears. Each instance crashes within seconds and Steam restarts it indefinitely (startcount=0, startcount=1, startcount=2... repeating).

The crash is a SIGTRAP (signal 5) caused by a NOTREACHED() assertion failure inside libcef.so (Chromium 126.0.6478.183, bundled with Steam, dated January 2026). The crash happens inside cef_execute_process — before any GPU initialization, windowing, or rendering takes place.

Key log from webhelper-linux.txt:

[ERROR:check.cc(376)] Check failed: false. NOTREACHED log messages are omitted in official builds. Sorry!

This repeats 3–4 times per launch cycle, then Steam restarts the webhelper.

Key log from cef_log.txt:

GPU process launch failed: error_code=1002
GPU process isn't usable. Goodbye.

Stack trace (from coredumpctl info):

Signal: 5 (TRAP)
Executable: steamwebhelper (ubuntu12_64/steamwebhelper)
libcef.so version: Chromium 126.0.6478.183

Stack trace of thread (zygote process):
#0  libcef.so + 0x61f589c
#1  libcef.so + 0x61efd3e
#2  libcef.so + 0x61f0979
#3  libcef.so + 0x61f1715
#4  libcef.so + 0x61ef8d0
#5  libcef.so + 0x293039c
#6  libcef.so + 0x2919107
#7  cef_execute_process (libcef.so + 0x2887b11)

What I have tried (none of these change the crash):

• Fresh Flatpak reinstall with --delete-data
• Both LTS (6.18.x) and bleeding-edge (7.0.x) kernels
• Gamescope wrap (gamescope -W 1920 -H 1080 -- flatpak run com.valvesoftware.Steam)
• Clearing GPU crash cache (~/.var/app/com.valvesoftware.Steam/.local/share/Steam/config/htmlcache/)
• --no-sandbox already passed by Steam by default
• LIBGL_ALWAYS_SOFTWARE=1 env var
• Removing WAYLAND_DISPLAY to force X11-only mode (made it worse)
• Native Steam (pacman) — same identical crash in same location

Steps for reproducing this issue:

1. Install Flatpak Steam on CachyOS (Arch-based) with NVIDIA RTX 5060 Ti (Blackwell) and nvidia-open driver 595.71.05, in a Hyprland/Wayland session
2. Run flatpak run com.valvesoftware.Steam
3. Observe: Steam bootstrap completes, then steamwebhelper starts crashing in a loop — Check failed: false. NOTREACHED in libcef.so — UI never appears

The crash appears specific to the NVIDIA Blackwell architecture (RTX 5060 Ti, GB206) with the open-source nvidia-open kernel module. The libcef.so bundled with Steam (Chromium 126, January 2026) triggers a NOTREACHED() assertion in cef_execute_process on this hardware/driver combination.