diff --git a/src/applications/diffusion/protocol/DiffusionMercurialWireProtocol.php b/src/applications/diffusion/protocol/DiffusionMercurialWireProtocol.php
index 251935a6dc..8a9deb5d84 100644
--- a/src/applications/diffusion/protocol/DiffusionMercurialWireProtocol.php
+++ b/src/applications/diffusion/protocol/DiffusionMercurialWireProtocol.php
@@ -1,132 +1,137 @@
 <?php
 
 final class DiffusionMercurialWireProtocol extends Phobject {
 
   public static function getCommandArgs($command) {
     // We need to enumerate all of the Mercurial wire commands because the
     // argument encoding varies based on the command. "Why?", you might ask,
     // "Why would you do this?".
 
     $commands = array(
       'batch' => array('cmds', '*'),
       'between' => array('pairs'),
       'branchmap' => array(),
       'branches' => array('nodes'),
       'capabilities' => array(),
       'changegroup' => array('roots'),
       'changegroupsubset' => array('bases heads'),
       'debugwireargs' => array('one two *'),
       'getbundle' => array('*'),
       'heads' => array(),
       'hello' => array(),
       'known' => array('nodes', '*'),
       'listkeys' => array('namespace'),
       'lookup' => array('key'),
       'pushkey' => array('namespace', 'key', 'old', 'new'),
+      'protocaps' => array('caps'),
       'stream_out' => array(''),
       'unbundle' => array('heads'),
     );
 
     if (!isset($commands[$command])) {
-      throw new Exception(pht("Unknown Mercurial command '%s!", $command));
+      throw new Exception(
+        pht(
+          'Unknown Mercurial command "%s"!',
+          $command));
     }
 
     return $commands[$command];
   }
 
   public static function isReadOnlyCommand($command) {
     $read_only = array(
       'between' => true,
       'branchmap' => true,
       'branches' => true,
       'capabilities' => true,
       'changegroup' => true,
       'changegroupsubset' => true,
       'debugwireargs' => true,
       'getbundle' => true,
       'heads' => true,
       'hello' => true,
       'known' => true,
       'listkeys' => true,
       'lookup' => true,
+      'protocaps' => true,
       'stream_out' => true,
     );
 
     // Notably, the write commands are "pushkey" and "unbundle". The
     // "batch" command is theoretically read only, but we require explicit
     // analysis of the actual commands.
 
     return isset($read_only[$command]);
   }
 
   public static function isReadOnlyBatchCommand($cmds) {
     if (!strlen($cmds)) {
       // We expect a "batch" command to always have a "cmds" string, so err
       // on the side of caution and throw if we don't get any data here. This
       // either indicates a mangled command from the client or a programming
       // error in our code.
       throw new Exception(pht("Expected nonempty '%s' specification!", 'cmds'));
     }
 
     // For "batch" we get a "cmds" argument like:
     //
     //   heads ;known nodes=
     //
     // We need to examine the commands (here, "heads" and "known") to make sure
     // they're all read-only.
 
     // NOTE: Mercurial has some code to escape semicolons, but it does not
     // actually function for command separation. For example, these two batch
     // commands will produce completely different results (the former will run
     // the lookup; the latter will fail with a parser error):
     //
     //  lookup key=a:xb;lookup key=z* 0
     //  lookup key=a:;b;lookup key=z* 0
     //               ^
     //               |
     //               +-- Note semicolon.
     //
     // So just split unconditionally.
 
     $cmds = explode(';', $cmds);
     foreach ($cmds as $sub_cmd) {
       $name = head(explode(' ', $sub_cmd, 2));
       if (!self::isReadOnlyCommand($name)) {
         return false;
       }
     }
 
     return true;
   }
 
   /** If the server version is running 3.4+ it will respond
    * with 'bundle2' capability in the format of "bundle2=(url-encoding)".
    * Until we manage to properly package up bundles to send back we
    * disallow the client from knowing we speak bundle2 by removing it
    * from the capabilities listing.
    *
    * The format of the capabilities string is: "a space separated list
    * of strings representing what commands the server supports"
    * @link https://www.mercurial-scm.org/wiki/CommandServer#Protocol
    *
    * @param string  $capabilities - The string of capabilities to
    *   strip the bundle2 capability from. This is expected to be
    *   the space-separated list of strings resulting from the
    *   querying the 'capabilities' command.
    *
    * @return string  The resulting space-separated list of capabilities
    *   which no longer contains the 'bundle2' capability. This is meant
    *   to replace the original $body to send back to client.
    */
   public static function filterBundle2Capability($capabilities) {
     $parts = explode(' ', $capabilities);
     foreach ($parts as $key => $part) {
       if (preg_match('/^bundle2=/', $part)) {
         unset($parts[$key]);
         break;
       }
     }
     return implode(' ', $parts);
   }
 
 }