diff -r d900f74456cc -r 8228e8dd96dd mercurial/localrepo.py
--- a/mercurial/localrepo.py	Tue Oct 10 10:03:09 2023 +0200
+++ b/mercurial/localrepo.py	Tue Oct 10 10:03:18 2023 +0200
@@ -1140,7 +1140,7 @@
     # experimental config: format.maxchainlen
     maxchainlen = ui.configint(b'format', b'maxchainlen', maxchainlen)
     if maxchainlen is not None:
-        options[b'maxchainlen'] = maxchainlen
+        delta_config.max_chain_len = maxchainlen
 
     for r in requirements:
         # we allow multiple compression engine requirement to co-exist because