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Not on top of IRC. Just similar in that they are both chat like

> Slack is built on top of IRC

Source? First time I hear that, and it seems unlikely, since they have quite a few features that don't fit nicely in stock IRC.

I’m not sure there are any Slack features that don’t fit into stock IRC v3. Basically everything they do is already supported or a draft. (Now just everyone has to implement it)

Now, possibly. But unless I'm totally wrong on the timeline, IRCv3 (not even talking about implementations) wasn't even close to that level when Slack started and thus is unlikely to be the base of their product. And even now, I wouldn't include v3 yet when talking about "stock IRC" in this context.

Any source on that. I don't think it follows any protocols at all. Just implements it's own thing.

Slack is definitely not built on top of IRC. There is, however, an IRC interface to Slack.