Is the Kotlin kernel providing its own version of kotlin-jupyter-api jar and put this also first on the classpath? The reason I ask is that I wanted to try out the latest version of kotlin-jupyter-api (since it has a nice new feature) and my code compiles fine. I also see that version in the local maven cache, so all good. But as soon as I try to load this in a notebook it fails and the reason is that somehow an older version of kotlin-jupyter-api is loaded instead.

Of course, if you use older version of kernel (which doesn't support corresponding API methods), it will fail with libraries which are built with newer version of API. You need to update kernel too (via conda, pypi or from sources) to use library which uses the latest API version

Thanks, assumed (wrongly 😉 ) that the API could evolve independently from the kernel at least for the smaller changes (a.b.c-d to a.b.c-e). When I check the github page there are a few versions floating around: pypi: 0.10.0.260 conda: 0.10.0.249 and then maven 0.10.0.261 (what I include in my own lib) Anyway to tell compatibility between these versions?

For now, I don't care much about versions compatibility, but you're right, it should work this way in the future. It's the outgoings of simple branching scheme and fast releases. For now, I only increment minor version (it's b in a.b.c-d) for backward incompatible changes. This change is forward incompatible, and it would be correct to increment micro in this case (c in a.b.c-d). I didn't do it this time, though