diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -104,6 +104,9 @@ coreconfigitem('devel', 'serverrequirece
 coreconfigitem('devel', 'strip-obsmarkers',
     default=True,
 )
+coreconfigitem('format', 'aggressivemergedeltas',
+    default=False,
+)
 coreconfigitem('hostsecurity', 'ciphers',
     default=None,
 )
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -439,7 +439,7 @@ class localrepository(object):
             self.svfs.options['manifestcachesize'] = manifestcachesize
         # experimental config: format.aggressivemergedeltas
         aggressivemergedeltas = self.ui.configbool('format',
-            'aggressivemergedeltas', False)
+                                                   'aggressivemergedeltas')
         self.svfs.options['aggressivemergedeltas'] = aggressivemergedeltas
         self.svfs.options['lazydeltabase'] = not scmutil.gddeltaconfig(self.ui)
         chainspan = self.ui.configbytes('experimental', 'maxdeltachainspan', -1)