In the past I used to do that using Steam Achievement Manager (SAM), but since then I have switched to using Linux. I couldn't get SAM to work on Linux, so I looked inside to see how it works and decided to make my own small tool to do what I needed.

I first tried some things with C, C++, and Go but while I achieved a little progress I couldn't get the whole thing right. In the end with some help from Claude I was able to make this tiny C application which does exactly what I need.

It loads the libsteam_api.so library and executes some calls, pretending to be the actual game from the Steam perspective. Funny thing is that your friends will even see that you are in-game while the application is running. While there does not seem to be any security or checks around this "hack", and I've used Steam Achievement Manager for many years now, there are claims and warnings online that you might get VAC (Valve Anti-Cheat) banned when you use tools like this with on-line games.

While possible, I currently don't have any intention to build a complete Steam Achievement Manager clone for Linux.

Any feedback is welcome.