diff -r 3c9dbb743d20 -r 090b1a665901 mercurial/filemerge.py
--- a/mercurial/filemerge.py	Sun Feb 03 19:29:05 2008 -0600
+++ b/mercurial/filemerge.py	Sun Feb 03 19:29:05 2008 -0600
@@ -36,6 +36,8 @@
             ui.warn(_("tool %s can't handle symlinks\n") % tmsg)
         elif binary and not _toolbool(ui, tool, "binary"):
             ui.warn(_("tool %s can't handle binary\n") % tmsg)
+        elif not util.gui() and _toolbool(ui, tool, "gui"):
+            ui.warn(_("tool %s requires a GUI\n") % tmsg)
         else:
             return True
         return False