diff -r 40df727b6f4f -r 7eedbd5d4880 mercurial/bundle2.py
--- a/mercurial/bundle2.py	Wed Jan 17 14:13:46 2018 +0100
+++ b/mercurial/bundle2.py	Wed Jan 17 16:41:44 2018 +0100
@@ -1487,6 +1487,7 @@
                 'remote-changegroup': ('http', 'https'),
                 'hgtagsfnodes': (),
                 'phases': ('heads',),
+                'stream': ('v2',),
                }
 
 def getrepocaps(repo, allowpushback=False):
@@ -1507,6 +1508,8 @@
         caps['checkheads'] = ('related',)
     if 'phases' in repo.ui.configlist('devel', 'legacy.exchange'):
         caps.pop('phases')
+    if not repo.ui.configbool('experimental', 'bundle2.stream'):
+        caps.pop('stream')
     return caps
 
 def bundle2caps(remote):