Welcome to Calva! Calva’s debugger doesn’t fit the VS Code model of a debugger completely. It works a little bit differently. One such difference is that you don’t really start the debugger. Calva is attached to the running application and the debugger will stop at breakpoints. However, you don’t place breakpoints the VS Code way either (at least not yet). Instead you instrument functions for debugging. Instrumenting is easier than it sounds. Instead of evaluating a function the normal way, you use a special inststument command that will evaluate it in a way such that breakpoints are attached. See https://calva.io/debugger/ for how to use it. There you will also find the various limitation that the implementation has. One of which is that it does not work in ClojureScript, only in Clojure.