I don’t have an idea, really. But I’ve seen it too, at times. Doesn’t seem to cause any problems when I have seen it, though. Is it something you are trying to do that causes the error for you?

sometimes it's just warning but sometimes it just stops the Jack-In and doesn't go any further.

@U0ETXRFEW Nothing specific, general calva Jack-In shows this both in Cljs and Clj projects.

Please see if there’s an issue reported about it and add any info you think is lacking there. And if it’s not reported, please report it.

This could potentially be a result of an older nrepl or cider-nrepl version being loaded rather than what Calva injects at jack-in. In some situations another version can override what Calva injects, IIRC. Check out your classpath in the project by running the appropriate clj or lein command (whichever you are using) and see if you see an older nrepl or cider-nrepl version than what Calva is using in the jack-in command. You can also check your clj/lein profile(s) and project dependencies for other versions of those libraries.