diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -139,7 +139,13 @@ class wirerepository(repo.repository):
         remote server as a bundle. Return an integer indicating the
         result of the push (see localrepository.addchangegroup()).'''
 
-        ret, output = self._callpush("unbundle", cg, heads=encodelist(heads))
+        if self.capable('unbundlehash'):
+            heads = encodelist(['hashed',
+                                util.sha1(''.join(sorted(heads))).digest()])
+        else:
+            heads = encodelist(heads)
+
+        ret, output = self._callpush("unbundle", cg, heads=heads)
         if ret == "":
             raise error.ResponseError(
                 _('push failed:'), output)
@@ -216,7 +222,8 @@ def branches(repo, proto, nodes):
     return "".join(r)
 
 def capabilities(repo, proto):
-    caps = 'lookup changegroupsubset branchmap pushkey known getbundle'.split()
+    caps = ('lookup changegroupsubset branchmap pushkey known getbundle '
+            'unbundlehash').split()
     if _allowstream(repo.ui):
         requiredformats = repo.requirements & repo.supportedformats
         # if our local revlogs are just revlogv1, add 'stream' cap
@@ -353,7 +360,9 @@ def unbundle(repo, proto, heads):
 
     def check_heads():
         heads = repo.heads()
-        return their_heads == ['force'] or their_heads == heads
+        heads_hash = util.sha1(''.join(sorted(heads))).digest()
+        return (their_heads == ['force'] or their_heads == heads or
+                their_heads == ['hashed', heads_hash])
 
     proto.redirect()
 
diff --git a/tests/test-hgweb-commands.t b/tests/test-hgweb-commands.t
--- a/tests/test-hgweb-commands.t
+++ b/tests/test-hgweb-commands.t
@@ -943,7 +943,7 @@ capabilities
   $ "$TESTDIR/get-with-headers.py" 127.0.0.1:$HGPORT '?cmd=capabilities'; echo
   200 Script output follows
   
-  lookup changegroupsubset branchmap pushkey known getbundle unbundle=HG10GZ,HG10BZ,HG10UN
+  lookup changegroupsubset branchmap pushkey known getbundle unbundlehash unbundle=HG10GZ,HG10BZ,HG10UN
 
 heads
 
diff --git a/tests/test-unbundlehash.t b/tests/test-unbundlehash.t
new file mode 100644
--- /dev/null
+++ b/tests/test-unbundlehash.t
@@ -0,0 +1,31 @@
+
+Test wire protocol unbundle with hashed heads (capability: unbundlehash)
+
+Create a remote repository.
+
+  $ hg init remote
+  $ hg serve -R remote --config web.push_ssl=False --config web.allow_push=* -p $HGPORT -d --pid-file=hg1.pid -E error.log -A access.log
+  $ cat hg1.pid >> $DAEMON_PIDS
+
+Clone the repository and push a change.
+
+  $ hg clone http://localhost:$HGPORT/ local
+  no changes found
+  updating to branch default
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ touch local/README
+  $ hg ci -R local -A -m hoge
+  adding README
+  $ hg push -R local
+  pushing to http://localhost:$HGPORT/
+  searching for changes
+  remote: adding changesets
+  remote: adding manifests
+  remote: adding file changes
+  remote: added 1 changesets with 1 changes to 1 files
+
+Ensure hashed heads format is used.
+The hash here is always the same since the remote repository only has the null head.
+
+  $ cat access.log | grep unbundle
+  * - - [*] "POST /?cmd=unbundle&heads=686173686564+6768033e216468247bd031a0a2d9876d79818f8f HTTP/1.1" 200 - (glob)