diff -r 5f388e514ab8 -r 77ef8bef0593 src/Pure/System/isabelle_tool.scala
--- a/src/Pure/System/isabelle_tool.scala	Thu Mar 04 19:55:52 2021 +0100
+++ b/src/Pure/System/isabelle_tool.scala	Thu Mar 04 21:04:27 2021 +0100
@@ -157,7 +157,7 @@
 
   Start Isabelle TOOL with ARGS; pass "-?" for tool-specific help.
 
-Available tools:""" + tool_descriptions.mkString("\n  ", "\n  ", "\n")).usage
+Available tools:""" + tool_descriptions.mkString("\n  ", "\n  ", "\n")).usage()
         case tool_name :: tool_args =>
           find_external(tool_name) orElse find_internal(tool_name) match {
             case Some(tool) => tool(tool_args)