diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -819,23 +819,7 @@
         caps.append('bundle2=' + urlreq.quote(capsblob))
     caps.append('unbundle=%s' % ','.join(bundle2.bundlepriority))
 
-    if proto.name == 'http-v1':
-        caps.append('httpheader=%d' %
-                    repo.ui.configint('server', 'maxhttpheaderlen'))
-        if repo.ui.configbool('experimental', 'httppostargs'):
-            caps.append('httppostargs')
-
-        # FUTURE advertise 0.2rx once support is implemented
-        # FUTURE advertise minrx and mintx after consulting config option
-        caps.append('httpmediatype=0.1rx,0.1tx,0.2tx')
-
-        compengines = supportedcompengines(repo.ui, util.SERVERROLE)
-        if compengines:
-            comptypes = ','.join(urlreq.quote(e.wireprotosupport().name)
-                                 for e in compengines)
-            caps.append('compression=%s' % comptypes)
-
-    return caps
+    return proto.addcapabilities(repo, caps)
 
 # If you are writing an extension and consider wrapping this function. Wrap
 # `_capabilities` instead.
diff --git a/mercurial/wireprotoserver.py b/mercurial/wireprotoserver.py
--- a/mercurial/wireprotoserver.py
+++ b/mercurial/wireprotoserver.py
@@ -121,6 +121,24 @@
             urlreq.quote(self._req.env.get('REMOTE_HOST', '')),
             urlreq.quote(self._req.env.get('REMOTE_USER', '')))
 
+    def addcapabilities(self, repo, caps):
+        caps.append('httpheader=%d' %
+                    repo.ui.configint('server', 'maxhttpheaderlen'))
+        if repo.ui.configbool('experimental', 'httppostargs'):
+            caps.append('httppostargs')
+
+        # FUTURE advertise 0.2rx once support is implemented
+        # FUTURE advertise minrx and mintx after consulting config option
+        caps.append('httpmediatype=0.1rx,0.1tx,0.2tx')
+
+        compengines = wireproto.supportedcompengines(repo.ui, util.SERVERROLE)
+        if compengines:
+            comptypes = ','.join(urlreq.quote(e.wireprotosupport().name)
+                                 for e in compengines)
+            caps.append('compression=%s' % comptypes)
+
+        return caps
+
 # This method exists mostly so that extensions like remotefilelog can
 # disable a kludgey legacy method only over http. As of early 2018,
 # there are no other known users, so with any luck we can discard this
@@ -368,6 +386,9 @@
         client = encoding.environ.get('SSH_CLIENT', '').split(' ', 1)[0]
         return 'remote:ssh:' + client
 
+    def addcapabilities(self, repo, caps):
+        return caps
+
 class sshv2protocolhandler(sshv1protocolhandler):
     """Protocol handler for version 2 of the SSH protocol."""
 
diff --git a/mercurial/wireprototypes.py b/mercurial/wireprototypes.py
--- a/mercurial/wireprototypes.py
+++ b/mercurial/wireprototypes.py
@@ -137,3 +137,12 @@
     @abc.abstractmethod
     def client(self):
         """Returns a string representation of this client (as bytes)."""
+
+    @abc.abstractmethod
+    def addcapabilities(self, repo, caps):
+        """Adds advertised capabilities specific to this protocol.
+
+        Receives the list of capabilities collected so far.
+
+        Returns a list of capabilities. The passed in argument can be returned.
+        """