diff -r b2cdd24e83b6 -r 74dce5658d4c src/Pure/Tools/server_commands.scala
--- a/src/Pure/Tools/server_commands.scala	Thu Mar 22 17:00:48 2018 +0100
+++ b/src/Pure/Tools/server_commands.scala	Thu Mar 22 17:05:06 2018 +0100
@@ -137,7 +137,12 @@
 
       val id = UUID()
 
-      (JSON.Object("session_id" -> id.toString), id -> session)
+      val res =
+        JSON.Object(
+          "session_id" -> id.toString,
+          "tmp_dir" -> File.path(session.tmp_dir).implode)
+
+      (res, id -> session)
     }
   }