I picked Celeste as a pretty easy to run and well supported game. So I selected the new runtime, click Launch and…. we get an actually error dialog from Steam. “Compatibility tool failed”, not very helpful, but expected. 

Browsing through steam log files, we find this in `compat_log.txt`:

 ``` 
[2026-04-08 20:35:22] Tool 0 "proton-localbuild" has a dependency on tool 4185400. [2026-04-08 20:35:22] Tool 4185400 "" is unknown for appID 504230.
 ```
 
A quick look on steamdb.info is enough to figure out the name of our missing steam app: “Steam Linux Runtime 4.0 - Arm64”.
 🧵 4/7