diff --git a/hgext/narrow/__init__.py b/hgext/narrow/__init__.py
--- a/hgext/narrow/__init__.py
+++ b/hgext/narrow/__init__.py
@@ -62,6 +62,7 @@
     if not repo.local():
         return
 
+    repo.ui.setconfig('experimental', 'narrow', True, 'narrow-ext')
     if repository.NARROW_REQUIREMENT in repo.requirements:
         narrowrepo.wraprepo(repo)
         narrowwirepeer.reposetup(repo)
diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -538,6 +538,9 @@
 coreconfigitem('experimental', 'mmapindexthreshold',
     default=None,
 )
+coreconfigitem('experimental', 'narrow',
+    default=False,
+)
 coreconfigitem('experimental', 'nonnormalparanoidcheck',
     default=False,
 )