diff --git a/mercurial/commandserver.py b/mercurial/commandserver.py
--- a/mercurial/commandserver.py
+++ b/mercurial/commandserver.py
@@ -205,8 +205,12 @@ class server(object):
                     'getencoding' : getencoding}
 
     def serve(self):
-        self.cout.write('capabilities: %s' % ' '.join(self.capabilities.keys()))
-        self.cout.write('encoding: %s' % encoding.encoding)
+        hellomsg = 'capabilities: ' + ' '.join(self.capabilities.keys())
+        hellomsg += '\n'
+        hellomsg += 'encoding: ' + encoding.encoding
+
+        # write the hello msg in -one- chunk
+        self.cout.write(hellomsg)
 
         try:
             while self.serveone():