In Charcoal HQ I am unable to use
[tag:random-string] in a chat message, while this works fine in other rooms. I was told informally that this is because the room doesn't have a parent site, and thus the tags cannot be linked to a search for questions with that tag on the site in question.
However, tags in chat don't have to correspond to an existing tag on the parent site, and are a useful feature in chat regardless. For example, in chat rooms which do have a parent site, I can use
[tag:gobbledygook] in a message, and I will then be able to search for other chat messages with the same tag. This is used extensively e.g. in the SOCVR room, but also broadly in other chat rooms for various diverse purposes.
This functionality is useful even when there is no parent site to link to.
Here's a better example of a working search, which works because working tags from other sites have been quoted in messages: https://chat.stackexchange.com/search?q=tagged%2Fcv-pls&user=&room=11540
(My pet project Sloshy the Thawman also attempts to use tags in the generated messages, and it's a bit disappointing to find that this won't work in some rooms.)