src/Pure/Tools/build.scala

                 env = env, sessions_structure = Some(deps.sessions_structure), store = Some(store),
                 cleanup = () => args_file.delete)
             }
 
           process.result(
+            progress_stderr = Output.writeln(_),
             progress_stdout = (line: String) =>
               Library.try_unprefix("\floading_theory = ", line) match {
                 case Some(theory) =>
                   progress.theory(Progress.Theory(theory, session = name))
                 case None =>