diff -r cb2b1a75ff59 -r 984c3dc46cc0 src/Pure/Thy/thy_resources.scala
--- a/src/Pure/Thy/thy_resources.scala	Sat Mar 24 20:47:54 2018 +0100
+++ b/src/Pure/Thy/thy_resources.scala	Sat Mar 24 20:51:42 2018 +0100
@@ -27,7 +27,7 @@
       session_base getOrElse
       Sessions.base_info(options, session_name, progress = progress, dirs = session_dirs).check_base
     val resources = new Thy_Resources(base, log = log)
-    val session = new Session(options, resources)
+    val session = new Session(session_name, options, resources)
 
     val session_error = Future.promise[String]
     var session_phase: Session.Consumer[Session.Phase] = null
@@ -73,6 +73,7 @@
   }
 
   class Session private[Thy_Resources](
+    session_name: String,
     session_options: Options,
     override val resources: Thy_Resources) extends isabelle.Session(session_options, resources)
   {
@@ -80,6 +81,8 @@
 
     val tmp_dir: JFile = Isabelle_System.tmp_dir("server_session")
 
+    override def toString: String = session_name
+
     override def stop(): Process_Result =
     {
       try { super.stop() }