Steam still will not run CachyOS's native Proton build, and the merged fix only logs an error
Steam's experimental container client will not launch a Proton build installed in /usr, and it refuses without writing a word to any log. The change that landed on October 16 fixes the silence rather than the launch: a pull request merged at 20:30 UTC that evening turns a quiet exit with status 0 into the line "reaper: execvp(/nonexistent) failed: No such file or directory" and an exit status of 1.
From the official Steam store gallery for Halo Infinite, developed by 343 Industries. Image: Xbox Game Studios
The developer who wrote it attached the caveat himself. Simon McVittie, an engineer at Collabora who works on Valve's Steam Runtime rather than for Valve, wrote on Valve's tracker eight minutes after opening the request that "someone from the Steam Client team will still need to integrate it by replacing the Steam Client's reaper with an updated build". Nothing in the thread says when that will happen, and until it does the build still will not run.
The failure leaves nothing to read
Report 13520 has been open since August 15 and has drawn seven comments. "The failure is completely silent", it says: "no stderr, nothing in console-linux.txt, no PROTON_LOG file, no Wine process. The game returns to 'Play' in under a second with no diagnosable trace."
By the reporter's account it is a regression rather than a limitation, because "the same tool works under the legacy ubuntu12_32 client". An owner reproduced it on October 16 on CachyOS, on Steam beta build 1792110651 with proton-cachyos 11.0-20261005's native build, and says it "exits in under a second" with "no error at all" after one line.
From the official Steam store gallery for Monster Hunter Wilds. A Windows game needs a compatibility tool of some kind to run on Linux at all. Image: Capcom Co., Ltd.
The path comes out of the container wrong
The thread's first guess was a missing interpreter, since the steamrt3c container image ships no python3 and Proton's launcher script asks for one. The owner who reproduced the fault pointed elsewhere: the path Steam hands to reaper "looks like it was resolved from inside the container (where the host's /usr shows up as /run/host/usr)", so the exec fails before python is involved.
McVittie calls that "a correct analysis" and suggests the containerised client "should special-case paths below /run/host/ that were discovered inside the container". He also says why Valve's own developers would not meet it, since Valve's official compatibility tools "don't get installed into /usr". Valve's kisak-valve account renamed the report at 15:03 UTC on October 16, to the title McVittie had proposed four hours earlier.
From the official Steam store gallery for Sons Of The Forest, developed by Endnight Games. Image: Endnight Games Ltd
The packager is happy to let his build go
The developer who makes the affected package is the least worried voice in the thread. Stelios Tsampas, who says he makes the Proton-CachyOS package, says he does "not particularly care" if that native build stops working under the steamrt3c client, because the version built inside the Proton SDK and installed under /usr/share/steam/compatibilitytools.d/ "runs fine with the SteamRT3 client so nothing is lost really".
He says most users were moved to the Steam Linux Runtime build before any of this, and that if his native build is the only thing a client change would rescue, "I am more than happy to deprecate it or hide it from Steam". That puts a limit on how many people the silent failure strands, which the report on its own does not.
Beyond that one rename, no Valve account has commented on 13520, so the merged commit is the whole of the answer. Steam's Linux client became a 64-bit application running inside a Steam Runtime container in a beta note of October 15, and the October 13 stable client made the compatibility tool picker always visible on Linux, which is the menu a third-party Proton build turns up in. Whenever a client ships with the updated reaper, a tool it cannot reach will fail with a line of text instead of in silence.
