diff --git a/contrib/chg/hgclient.c b/contrib/chg/hgclient.c
--- a/contrib/chg/hgclient.c
+++ b/contrib/chg/hgclient.c
@@ -237,10 +237,10 @@ static void handlesystemrequest(hgclient
 	ctx->data[ctx->datasize] = '\0';  /* terminate last string */
 
 	const char **args = unpackcmdargsnul(ctx);
-	if (!args[0] || !args[1])
-		abortmsg("missing command or cwd in system request");
-	debugmsg("run '%s' at '%s'", args[0], args[1]);
-	int32_t r = runshellcmd(args[0], args + 2, args[1]);
+	if (!args[0] || !args[1] || !args[2])
+		abortmsg("missing type or command or cwd in system request");
+	debugmsg("run '%s' at '%s'", args[1], args[2]);
+	int32_t r = runshellcmd(args[1], args + 3, args[2]);
 	free(args);
 
 	uint32_t r_n = htonl(r);
diff --git a/mercurial/chgserver.py b/mercurial/chgserver.py
--- a/mercurial/chgserver.py
+++ b/mercurial/chgserver.py
@@ -287,13 +287,14 @@ class channeledsystem(object):
     """Propagate ui.system() request in the following format:
 
     payload length (unsigned int),
+    type, '\0',
     cmd, '\0',
     cwd, '\0',
     envkey, '=', val, '\0',
     ...
     envkey, '=', val
 
-    and waits:
+    if type == 'system', waits for:
 
     exitcode length (unsigned int),
     exitcode (int)
@@ -303,8 +304,8 @@ class channeledsystem(object):
         self.out = out
         self.channel = channel
 
-    def __call__(self, cmd, environ, cwd):
-        args = [util.quotecommand(cmd), os.path.abspath(cwd or '.')]
+    def __call__(self, cmd, environ, cwd, type='system'):
+        args = [type, util.quotecommand(cmd), os.path.abspath(cwd or '.')]
         args.extend('%s=%s' % (k, v) for k, v in environ.iteritems())
         data = '\0'.join(args)
         self.out.write(struct.pack('>cI', self.channel, len(data)))