In-game name: renzy.1337
Time: N/A
Details: Unable to mention players with $ chat strings, for example , $sus, and $medic.
Procedure to replicate mention a player using chat string example : @$sus

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I think you don't need to add @ before writing anything like $cop or $mw. Try to write in chat '$cop' only not @ so it will mention the player I guess only.
The message 'Failed to mention player' maybe is appearing because you add @ before $sus and here @ isn't used as $sus automatically mentioned RedX.
Maybe I'm wrong but this is how I use $cop and other things... Try it
$ commands without @ just shows name
For mentioning u need to add @
It's a feature which was added with a purpose but its not working properly
That's why I reported it 😉😉
It’s because mentions are processed before quick strings. It should be as simple as swapping them around. I’ll fix it.
This isn't so easy, and I don't think it's worth 'fixing' to be honest. It'd mean completely rethinking how it works and I haven't the time nor energy.