Hello @kamikazeghost, we're tracking issues with games run with Proton in the Proton issue tracker. This issue has already been resolved (https://github.com/ValveSoftware/Proton/issues/124#issuecomment-4687424688).
Thank you for that information. Than a patch is not far away. Have a nice day. :)
Your system information
Please describe your issue in as much detail as possible:
If I start Metal Gear Solid: Phantom Pain (App-ID 287700), i got an error: "Failed to load SteamAPI." and game exits.
First I thought, it was an error of my nvidia-driver update and tried different versions without any change.
I tried to add some gl and glx libaries without any result.
I validated files of the game, the different proton and steam runtimes even without any change.
I tried wiping compatdata, caches, temps and then finally reinstalled game and steam, stable and beta... no change.
Without hope I stupidly copied the steam_api64.dll from another game.. Voila! The game runs...
If I validated the gamefiles again, the error come back instantly.
I am sure, that the steam_api64.dll of App-ID 287700 seems broken by default delivery. Maybe it's system specific, but you should spend a look.
If you need more information, I am here.