With Calva you do not start a debug session. Rather the REPL is sort of a debug session and will stop at breakpoints. Further you do not place breakpoints with the VS Code tooling for it. Instead you place #break or #dbg reader tags in the code and then evaluate form. The difference between #break and #dbg is that the former will place one breakpoint, and the second one will instrument the form with breakpoints. There is also the command: Calva Debug: Instrument Current Top Level Form for Debugging. This will instrument the top level form like the #dbg tag, but without you having to edit it. See: https://calva.io/debugger/ for the debugger docs, and about the limitations.