[ObsoleteNerd]

Couldn't it be done in the UI by referencing the line/message ID, kinda like how forums "quote" the message you clicked Reply on, or 4ch references a link back to the one you're quoting?

Let a user click the timestamp to the left of the messages (or a "#" or other symbol), which inserts something like this into the reply box:

> Replying to username:

Then your message you send ends up looking like:

> Reply to username: I agree!

Clicking on "Reply to username" or just the username could scroll back the chat and highlight the message you replied to.

[wild_preference]

One problem with IRC is that everyone is using a different client. It's like how you can set it up so you receive messages when you're offline... but nobody else does that, so it's not a real solution.

Of course, the context here is a client that does indeed implement extra features, and you're right that a text format per message would be an interesting way to implement this: the client could simply hide those messages from the normal view and slot them into the thread system. People using other clients would just see some extraneous line noise like "Replying to <username>|<timestamp>: hello world".

[ObsoleteNerd]

Yeah exactly, it's still "useful" to people without the client just by showing them it's a reply to someone else, even without the embedded link and extra functionality the client adds. I think it's a good compromise.