src/Tools/VSCode/src/server.scala

       }
       catch { case ERROR(msg) => reply(msg); None }
 
     for (session <- try_session) {
       session_.change(_ => Some(session))
+
+      session.commands_changed += prover_output
+      session.all_messages += syslog
+
+      dynamic_output.init()
 
       var session_phase: Session.Consumer[Session.Phase] = null
       session_phase =
         Session.Consumer(getClass.getName) {
           case Session.Ready =>

             session.phase_changed -= session_phase
             reply("Prover startup failed")
           case _ =>
         }
       session.phase_changed += session_phase
-
-      session.commands_changed += prover_output
-      session.all_messages += syslog
-
-      dynamic_output.init()
 
       Isabelle_Process.start(session, options,
         logic = session_name, dirs = session_dirs, modes = modes)
     }
   }