diff --git a/src/applications/conduit/controller/api/PhabricatorConduitAPIController.php b/src/applications/conduit/controller/api/PhabricatorConduitAPIController.php
index 04105e3fb6..6ab7948f3d 100644
--- a/src/applications/conduit/controller/api/PhabricatorConduitAPIController.php
+++ b/src/applications/conduit/controller/api/PhabricatorConduitAPIController.php
@@ -140,7 +140,11 @@ class PhabricatorConduitAPIController
         } catch (ConduitException $ex) {
           $result = null;
           $error_code = $ex->getMessage();
-          $error_info = $method_handler->getErrorDescription($error_code);
+          if ($ex->getErrorDescription()) {
+            $error_info = $ex->getErrorDescription();
+          } else {
+            $error_info = $method_handler->getErrorDescription($error_code);
+          }
         }
         if ($allow_unguarded_writes) {
           unset($unguarded);
diff --git a/src/applications/conduit/method/conduit/connect/ConduitAPI_conduit_connect_Method.php b/src/applications/conduit/method/conduit/connect/ConduitAPI_conduit_connect_Method.php
index f9cf7c6be3..8a966ed5bc 100644
--- a/src/applications/conduit/method/conduit/connect/ConduitAPI_conduit_connect_Method.php
+++ b/src/applications/conduit/method/conduit/connect/ConduitAPI_conduit_connect_Method.php
@@ -55,12 +55,6 @@ class ConduitAPI_conduit_connect_Method extends ConduitAPIMethod {
         "Client/server version mismatch. Update your client.",
       "ERR-UNKNOWN-CLIENT" =>
         "Client is unknown.",
-      "ERR-UPDATE-ARC" =>
-        "Arcanist is now open source! Update your scripts/aliases to use ".
-        "'/home/engshare/devtools/arcanist/bin/arc' if you're running from ".
-        "a Facebook host, or see ".
-        "<http://www.intern.facebook.com/intern/wiki/index.php/Arcanist> for ".
-        "laptop instructions.",
       "ERR-INVALID-USER" =>
         "The username you are attempting to authenticate with is not valid.",
       "ERR-INVALID-CERTIFICATE" =>
@@ -92,14 +86,23 @@ class ConduitAPI_conduit_connect_Method extends ConduitAPIMethod {
 
     switch ($client) {
       case 'arc':
-        $server_version = 2;
+        $server_version = 3;
         switch ($client_version) {
-          case 1:
-            throw new ConduitException('ERR-UPDATE-ARC');
           case $server_version:
             break;
           default:
-            throw new ConduitException('ERR-BAD-VERSION');
+            $ex = new ConduitException('ERR-BAD-VERSION');
+
+            if ($server_version < $client_version) {
+              $upgrade = "Upgrade your Phabricator install.";
+            } else {
+              $upgrade = "Upgrade your 'arc' client.";
+            }
+
+            $ex->setErrorDescription(
+              "Your 'arc' client version is '{$client_version}', but this ".
+              "server expects version '{$server_version}'. {$upgrade}");
+            throw $ex;
         }
         break;
       default:
diff --git a/src/applications/conduit/protocol/exception/ConduitException.php b/src/applications/conduit/protocol/exception/ConduitException.php
index a168a77ad4..6d59702c5e 100644
--- a/src/applications/conduit/protocol/exception/ConduitException.php
+++ b/src/applications/conduit/protocol/exception/ConduitException.php
@@ -20,4 +20,29 @@
  * @group conduit
  */
 class ConduitException extends Exception {
+
+  private $errorDescription;
+
+  /**
+   * Set a detailed error description. If omitted, the generic error description
+   * will be used instead. This is useful to provide specific information about
+   * an exception (e.g., which values were wrong in an invalid request).
+   *
+   * @param string Detailed error description.
+   * @return this
+   */
+  public function setErrorDescription($error_description) {
+    $this->errorDescription = $error_description;
+    return $this;
+  }
+
+  /**
+   * Get a detailed error description, if available.
+   *
+   * @return string|null Error description, if one is available.
+   */
+  public function getErrorDescription() {
+    return $this->errorDescription;
+  }
+
 }