@All in chat group

[Linnni]

When sending message to chat groups, commands with '@' do not work

[voided]

There's likely some special token that the Steam client sends for highlighting users/groups. You'll want to investigate with NetHook.

[yaakov-h]

This seems like part of #561?

[m4dEngi]

[mention=all]@all[/mention]
That's the real message client sends when you type in @all in chat.
[mention=AccountID]@User[/mention]
For user mentions.

Chat format seem like a mess of bbcode binary flags and plain text commands handled on client side ( /commands )

[Linnni]

This seems like part of #561?

Yes, i use code from there