diff -r c6b8eab5db19 -r b66ea3fc3a86 mercurial/configitems.py
--- a/mercurial/configitems.py	Fri Sep 07 11:18:45 2018 -0400
+++ b/mercurial/configitems.py	Fri Sep 07 11:18:45 2018 -0400
@@ -665,7 +665,7 @@
     default=None,
 )
 coreconfigitem('format', 'maxchainlen',
-    default=None,
+    default=dynamicdefault,
 )
 coreconfigitem('format', 'obsstore-version',
     default=None,