Your system information

• Steam client version (build number or date): 1774306123 (Steam Beta)
• Distribution (e.g. Ubuntu): EndeavourOS (Arch) - installed with package manager
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes
• Steam Logs: steam-logs.tar.gz
• GPU: AMD - XFX 9060 XT 8 GB OC

Please describe your issue in as much detail as possible:

I'm pretty sure that this may be intentional by design (on paper), but environment variables (examples: gamescope and MANGOHUD=1) are unable to be used in Steam RT3. Where environment variables are:

• a bin program, it will straight up refuse to launch the game because the command does not exist
• an actual environment variable, that env will be ignored entirely and will launch the game as normal

I do believe that since the client is now containerized/sandboxed under RT3, it's pretty much in the same position like a typical Flatpak application where access to most of your system (including commands and envs) cannot be ran without explicitly telling the container/sandbox to access that part of the system.

Demo without RT3 enabled:
https://github.com/user-attachments/assets/2e1cd601-9eeb-447b-9d41-98a69a9601a5

Demo with RT3 enabled:
https://github.com/user-attachments/assets/db50d4c8-6729-4b64-8503-524a6ff8f513

Steps for reproducing this issue:

1. Ensure RT3 is enabled
2. Input an environment variable into any game's launch options through its properties
3. Attempt to launch that same game with the environment variable