diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -991,6 +991,9 @@
 coreconfigitem('server', 'streamunbundle',
     default=False,
 )
+coreconfigitem('server', 'stream-narrow-clones',
+    default=False,
+)
 coreconfigitem('server', 'uncompressed',
     default=True,
 )
diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
--- a/mercurial/help/config.txt
+++ b/mercurial/help/config.txt
@@ -1839,6 +1839,10 @@
     Older Mercurial clients only support zlib compression and this setting
     has no effect for legacy clients.
 
+``stream-narrow-clones``
+    Whether the server supports streaming narrow clones or not.
+    (default: False)
+
 ``uncompressed``
     Whether to allow clients to clone a repository using the
     uncompressed streaming protocol. This transfers about 40% more
diff --git a/mercurial/wireprotov1server.py b/mercurial/wireprotov1server.py
--- a/mercurial/wireprotov1server.py
+++ b/mercurial/wireprotov1server.py
@@ -286,6 +286,9 @@
         caps.append('bundle2=' + urlreq.quote(capsblob))
     caps.append('unbundle=%s' % ','.join(bundle2.bundlepriority))
 
+    if repo.ui.configbool('server', 'stream-narrow-clones'):
+        caps.append('narrowstream')
+
     return proto.addcapabilities(repo, caps)
 
 # If you are writing an extension and consider wrapping this function. Wrap
@@ -535,12 +538,17 @@
     output = output.getvalue() if output else ''
     return wireprototypes.bytesresponse('%d\n%s' % (int(r), output))
 
-@wireprotocommand('stream_out', permission='pull')
-def stream(repo, proto):
+@wireprotocommand('stream_out', '*', permission='pull')
+def stream(repo, proto, args):
     '''If the server supports streaming clone, it advertises the "stream"
     capability with a value representing the version and flags of the repo
     it is serving. Client checks to see if it understands the format.
     '''
+    includes = None
+    excludes = None
+    if 'narrowstream' in _capabilities(repo, proto):
+        includes = args.get('includes')
+        excludes = args.get('excludes')
     return wireprototypes.streamreslegacy(
         streamclone.generatev1wireproto(repo))
 
diff --git a/tests/test-http-bundle1.t b/tests/test-http-bundle1.t
--- a/tests/test-http-bundle1.t
+++ b/tests/test-http-bundle1.t
@@ -297,7 +297,7 @@
   "GET /?cmd=capabilities HTTP/1.1" 401 - (no-reposimplestore !)
   "GET /?cmd=capabilities HTTP/1.1" 200 - (no-reposimplestore !)
   "GET /?cmd=branchmap HTTP/1.1" 200 - x-hgproto-1:0.1 0.2 comp=$USUAL_COMPRESSIONS$ partial-pull (no-reposimplestore !)
-  "GET /?cmd=stream_out HTTP/1.1" 200 - x-hgproto-1:0.1 0.2 comp=$USUAL_COMPRESSIONS$ partial-pull (no-reposimplestore !)
+  "GET /?cmd=stream_out HTTP/1.1" 200 - x-hgarg-1:excludes=&includes= x-hgproto-1:0.1 0.2 comp=$USUAL_COMPRESSIONS$ partial-pull
   "GET /?cmd=listkeys HTTP/1.1" 200 - x-hgarg-1:namespace=bookmarks x-hgproto-1:0.1 0.2 comp=$USUAL_COMPRESSIONS$ partial-pull (no-reposimplestore !)
   "GET /?cmd=batch HTTP/1.1" 200 - x-hgarg-1:cmds=heads+%3Bknown+nodes%3D5fed3813f7f5e1824344fdc9cf8f63bb662c292d x-hgproto-1:0.1 0.2 comp=$USUAL_COMPRESSIONS$ partial-pull (no-reposimplestore !)
   "GET /?cmd=listkeys HTTP/1.1" 200 - x-hgarg-1:namespace=phases x-hgproto-1:0.1 0.2 comp=$USUAL_COMPRESSIONS$ partial-pull (no-reposimplestore !)