is it possible to build a typesafe kotlin web api that works on client using kotlinjs & server with jvm? something kinda like servant in haskell where you only have to write one api for the client & server

If you mean what I think you do (not familiar with Haskell), Locations is as close as you can get with pure Kotlin, and my krosstalk library gets you most of the rest of the way: https://github.com/rnett/krosstalk. Kvision has something similar, too

wow this is cool

@rnett can krosstalk be used with JSON as well? or only CBor for now?

Json works fine, it's not picky about serialization. See https://github.com/rnett/krosstalk/blob/main/tests/client-test/src/jsMain/kotlin/com/rnett/krosstalk/client_test/Test.kt#L71

You can read how this idea works in KVision here: https://kvision.gitbook.io/kvision-guide/part-3-server-side-interface/overview

So, does

@KService

use krosstalk under the hood?

No