diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -17,6 +17,7 @@ demandload(globals(), "os revlog time ut
 class localrepository(repo.repository):
     capabilities = ('lookup', 'changegroupsubset')
     supported = ('revlogv1', 'store')
+    branchcache_features = ('unnamed',)
 
     def __del__(self):
         self.transhandle = None
@@ -376,6 +377,22 @@ class localrepository(repo.repository):
             f = self.opener("branches.cache")
             lines = f.read().split('\n')
             f.close()
+            features = lines.pop(0).strip()
+            if not features.startswith('features: '):
+                raise ValueError(_('branch cache: no features specified'))
+            features = features.split(' ', 1)[1].split()
+            missing_features = []
+            for feature in self.branchcache_features:
+                try:
+                    features.remove(feature)
+                except ValueError, inst:
+                    missing_features.append(feature)
+            if missing_features:
+                raise ValueError(_('branch cache: missing features: %s')
+                                 % ', '.join(missing_features))
+            if features:
+                raise ValueError(_('branch cache: unknown features: %s')
+                                 % ', '.join(features))
             last, lrev = lines.pop(0).split(" ", 1)
             last, lrev = bin(last), int(lrev)
             if not (lrev < self.changelog.count() and
@@ -397,6 +414,7 @@ class localrepository(repo.repository):
     def _writebranchcache(self, branches, tip, tiprev):
         try:
             f = self.opener("branches.cache", "w")
+            f.write(" features: %s\n" % ' '.join(self.branchcache_features))
             f.write("%s %s\n" % (hex(tip), tiprev))
             for label, node in branches.iteritems():
                 f.write("%s %s\n" % (hex(node), label))
diff --git a/tests/test-mq-caches.out b/tests/test-mq-caches.out
--- a/tests/test-mq-caches.out
+++ b/tests/test-mq-caches.out
@@ -7,6 +7,7 @@ No .hg/branches.cache
 # some regular revisions
 Patch queue now empty
 tip: 1
+ features: unnamed
 3f910abad313ff802d3a23a7529433872df9b3ae 1
 3f910abad313ff802d3a23a7529433872df9b3ae bar
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
@@ -15,10 +16,12 @@ 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff
 applying p1
 Now at: p1
 tip: 2
+ features: unnamed
 3f910abad313ff802d3a23a7529433872df9b3ae 1
 3f910abad313ff802d3a23a7529433872df9b3ae bar
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
 tip: 3
+ features: unnamed
 3f910abad313ff802d3a23a7529433872df9b3ae 1
 3f910abad313ff802d3a23a7529433872df9b3ae bar
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
@@ -27,6 +30,7 @@ branch bar: 2
 
 # removing the cache
 tip: 3
+ features: unnamed
 3f910abad313ff802d3a23a7529433872df9b3ae 1
 3f910abad313ff802d3a23a7529433872df9b3ae bar
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
@@ -35,6 +39,7 @@ branch bar: 2
 
 # importing rev 1 (the cache now ends in one of the patches)
 tip: 3
+ features: unnamed
 3f910abad313ff802d3a23a7529433872df9b3ae 1
 3f910abad313ff802d3a23a7529433872df9b3ae bar
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
@@ -49,5 +54,6 @@ applying p1
 applying p2
 Now at: p2
 tip: 3
+ features: unnamed
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff 0
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
diff --git a/tests/test-newbranch.out b/tests/test-newbranch.out
--- a/tests/test-newbranch.out
+++ b/tests/test-newbranch.out
@@ -74,6 +74,7 @@ modify a branch
 
 
 4:4909a3732169
+ features: unnamed
 4909a3732169c0c20011c4f4b8fdff4e3d89b23f 4
 bf1bc2f45e834c75404d0ddab57d53beab56e2f8 
 4909a3732169c0c20011c4f4b8fdff4e3d89b23f foo