diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index 44e6d9d7d3..aea1adb006 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -505,6 +505,8 @@ phutil_register_library_map(array(
     'DiffusionMercurialFileContentQuery' => 'applications/diffusion/query/filecontent/DiffusionMercurialFileContentQuery.php',
     'DiffusionMercurialRawDiffQuery' => 'applications/diffusion/query/rawdiff/DiffusionMercurialRawDiffQuery.php',
     'DiffusionMercurialRequest' => 'applications/diffusion/request/DiffusionMercurialRequest.php',
+    'DiffusionMercurialResponse' => 'applications/diffusion/response/DiffusionMercurialResponse.php',
+    'DiffusionMercurialWireProtocol' => 'applications/diffusion/protocol/DiffusionMercurialWireProtocol.php',
     'DiffusionPathChange' => 'applications/diffusion/data/DiffusionPathChange.php',
     'DiffusionPathChangeQuery' => 'applications/diffusion/query/pathchange/DiffusionPathChangeQuery.php',
     'DiffusionPathCompleteController' => 'applications/diffusion/controller/DiffusionPathCompleteController.php',
@@ -2757,6 +2759,7 @@ phutil_register_library_map(array(
     'DiffusionMercurialFileContentQuery' => 'DiffusionFileContentQuery',
     'DiffusionMercurialRawDiffQuery' => 'DiffusionRawDiffQuery',
     'DiffusionMercurialRequest' => 'DiffusionRequest',
+    'DiffusionMercurialResponse' => 'AphrontResponse',
     'DiffusionPathCompleteController' => 'DiffusionController',
     'DiffusionPathQueryTestCase' => 'PhabricatorTestCase',
     'DiffusionPathValidateController' => 'DiffusionController',
diff --git a/src/applications/diffusion/controller/DiffusionServeController.php b/src/applications/diffusion/controller/DiffusionServeController.php
index bb35aac549..74b57b7594 100644
--- a/src/applications/diffusion/controller/DiffusionServeController.php
+++ b/src/applications/diffusion/controller/DiffusionServeController.php
@@ -176,6 +176,9 @@ final class DiffusionServeController extends DiffusionController {
       case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
         $result = $this->serveGitRequest($repository, $viewer);
         break;
+      case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
+        $result = $this->serveMercurialRequest($repository, $viewer);
+        break;
       default:
         $result = new PhabricatorVCSResponse(
           999,
@@ -224,13 +227,43 @@ final class DiffusionServeController extends DiffusionController {
         break;
       case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
         $cmd = $request->getStr('cmd');
-        switch ($cmd) {
-          case 'capabilities':
+        if ($cmd == 'batch') {
+          // 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.
+
+          $args = $this->getMercurialArguments();
+          $cmds = idx($args, 'cmds');
+          if ($cmds) {
+
+            // 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(' ', $sub_cmd, 2));
+              if (!DiffusionMercurialWireProtocol::isReadOnlyCommand($name)) {
+                return false;
+              }
+            }
             return true;
-          default:
-            return false;
+          }
         }
-        break;
+        return DiffusionMercurialWireProtocol::isReadOnlyCommand($cmd);
       case PhabricatorRepositoryType::REPOSITORY_TYPE_SUBVERSION:
         break;
     }
@@ -357,5 +390,127 @@ final class DiffusionServeController extends DiffusionController {
 
     return $user;
   }
+
+  private function serveMercurialRequest(PhabricatorRepository $repository) {
+    $request = $this->getRequest();
+
+    $bin = Filesystem::resolveBinary('hg');
+    if (!$bin) {
+      throw new Exception("Unable to find `hg` in PATH!");
+    }
+
+    $env = array();
+    $input = PhabricatorStartup::getRawInput();
+
+    $cmd = $request->getStr('cmd');
+
+    $args = $this->getMercurialArguments();
+    $args = $this->formatMercurialArguments($cmd, $args);
+
+    if (strlen($input)) {
+      $input = strlen($input)."\n".$input."0\n";
+    }
+
+    list($err, $stdout, $stderr) = id(new ExecFuture('%s serve --stdio', $bin))
+      ->setEnv($env, true)
+      ->setCWD($repository->getLocalPath())
+      ->write("{$cmd}\n{$args}{$input}")
+      ->resolve();
+
+    if ($err) {
+      return new PhabricatorVCSResponse(
+        500,
+        pht('Error %d: %s', $err, $stderr));
+    }
+
+    if ($cmd == 'getbundle' ||
+        $cmd == 'changegroup' ||
+        $cmd == 'changegroupsubset') {
+      // We're not completely sure that "changegroup" and "changegroupsubset"
+      // actually work, they're for very old Mercurial.
+      $body = gzcompress($stdout);
+    } else if ($cmd == 'unbundle') {
+      // This includes diagnostic information and anything echoed by commit
+      // hooks. We ignore `stdout` since it just has protocol garbage, and
+      // substitute `stderr`.
+      $body = strlen($stderr)."\n".$stderr;
+    } else {
+      list($length, $body) = explode("\n", $stdout, 2);
+    }
+
+    return id(new DiffusionMercurialResponse())->setContent($body);
+  }
+
+
+  private function getMercurialArguments() {
+    // Mercurial sends arguments in HTTP headers. "Why?", you might wonder,
+    // "Why would you do this?".
+
+    $args_raw = array();
+    for ($ii = 1; ; $ii++) {
+      $header = 'HTTP_X_HGARG_'.$ii;
+      if (!array_key_exists($header, $_SERVER)) {
+        break;
+      }
+      $args_raw[] = $_SERVER[$header];
+    }
+    $args_raw = implode('', $args_raw);
+
+    return id(new PhutilQueryStringParser())
+      ->parseQueryString($args_raw);
+  }
+
+  private function formatMercurialArguments($command, array $arguments) {
+    $spec = DiffusionMercurialWireProtocol::getCommandArgs($command);
+
+    $out = array();
+
+    // Mercurial takes normal arguments like this:
+    //
+    //   name <length(value)>
+    //   value
+
+    $has_star = false;
+    foreach ($spec as $arg_key) {
+      if ($arg_key == '*') {
+        $has_star = true;
+        continue;
+      }
+      if (isset($arguments[$arg_key])) {
+        $value = $arguments[$arg_key];
+        $size = strlen($value);
+        $out[] = "{$arg_key} {$size}\n{$value}";
+        unset($arguments[$arg_key]);
+      }
+    }
+
+    if ($has_star) {
+
+      // Mercurial takes arguments for variable argument lists roughly like
+      // this:
+      //
+      //   * <count(args)>
+      //   argname1 <length(argvalue1)>
+      //   argvalue1
+      //   argname2 <length(argvalue2)>
+      //   argvalue2
+
+      $count = count($arguments);
+
+      $out[] = "* {$count}\n";
+
+      foreach ($arguments as $key => $value) {
+        if (in_array($key, $spec)) {
+          // We already added this argument above, so skip it.
+          continue;
+        }
+        $size = strlen($value);
+        $out[] = "{$key} {$size}\n{$value}";
+      }
+    }
+
+    return implode('', $out);
+  }
+
 }
 
diff --git a/src/applications/diffusion/protocol/DiffusionMercurialWireProtocol.php b/src/applications/diffusion/protocol/DiffusionMercurialWireProtocol.php
new file mode 100644
index 0000000000..0f6a8ae74e
--- /dev/null
+++ b/src/applications/diffusion/protocol/DiffusionMercurialWireProtocol.php
@@ -0,0 +1,62 @@
+<?php
+
+final class DiffusionMercurialWireProtocol {
+
+  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?".
+
+    $commands = array(
+      'batch' => array('cmds', '*'),
+      'between' => array('pairs'),
+      'branchmap' => array(),
+      'branches' => array('nodes'),
+      'capabilities' => array(),
+      'changegroup' => array('roots'),
+      'changegroupsubset' => array('bases heads'),
+      'debugwireargs' => array('one two *'),
+      'getbundle' => array('*'),
+      'heads' => array(),
+      'hello' => array(),
+      'known' => array('nodes', '*'),
+      'listkeys' => array('namespace'),
+      'lookup' => array('key'),
+      'pushkey' => array('namespace', 'key', 'old', 'new'),
+      'stream_out' => array(''),
+      'unbundle' => array('heads'),
+    );
+
+    if (!isset($commands[$command])) {
+      throw new Exception("Unknown Mercurial command '{$command}!");
+    }
+
+    return $commands[$command];
+  }
+
+  public static function isReadOnlyCommand($command) {
+    $read_only = array(
+      'between' => true,
+      'branchmap' => true,
+      'branches' => true,
+      'capabilities' => true,
+      'changegroup' => true,
+      'changegroupsubset' => true,
+      'debugwireargs' => true,
+      'getbundle' => true,
+      'heads' => true,
+      'hello' => true,
+      'known' => true,
+      'listkeys' => true,
+      'lookup' => true,
+      'stream_out' => true,
+    );
+
+    // Notably, the write commands are "pushkey" and "unbundle". The
+    // "batch" command is theoretically read only, but we require explicit
+    // analysis of the actual commands.
+
+    return isset($read_only[$command]);
+  }
+
+}
diff --git a/src/applications/diffusion/response/DiffusionGitResponse.php b/src/applications/diffusion/response/DiffusionGitResponse.php
index edf8d4f051..96a7b5bd75 100644
--- a/src/applications/diffusion/response/DiffusionGitResponse.php
+++ b/src/applications/diffusion/response/DiffusionGitResponse.php
@@ -30,7 +30,7 @@ final class DiffusionGitResponse extends AphrontResponse {
   }
 
   public function getHeaders() {
-    return $this->headers;
+    return array_merge(parent::getHeaders(), $this->headers);
   }
 
   public function getCacheHeaders() {
diff --git a/src/applications/diffusion/response/DiffusionMercurialResponse.php b/src/applications/diffusion/response/DiffusionMercurialResponse.php
new file mode 100644
index 0000000000..9c00cd468f
--- /dev/null
+++ b/src/applications/diffusion/response/DiffusionMercurialResponse.php
@@ -0,0 +1,31 @@
+<?php
+
+final class DiffusionMercurialResponse extends AphrontResponse {
+
+  private $content;
+
+  public function setContent($content) {
+    $this->content = $content;
+    return $this;
+  }
+
+  public function buildResponseString() {
+    return $this->content;
+  }
+
+  public function getHeaders() {
+    $headers = array(
+      array('Content-Type', 'application/mercurial-0.1'),
+    );
+    return array_merge(parent::getHeaders(), $headers);
+  }
+
+  public function getCacheHeaders() {
+    return array();
+  }
+
+  public function getHTTPResponseCode() {
+    return 200;
+  }
+
+}