diff --git a/src/applications/conpherence/editor/ConpherenceEditor.php b/src/applications/conpherence/editor/ConpherenceEditor.php
--- a/src/applications/conpherence/editor/ConpherenceEditor.php
+++ b/src/applications/conpherence/editor/ConpherenceEditor.php
@@ -397,7 +397,7 @@
     return $body;
   }
 
-  protected function getMailSubjectPrefix() {
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     return PhabricatorEnv::getEnvConfig('metamta.conpherence.subject-prefix');
   }
 
diff --git a/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php b/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
--- a/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
+++ b/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
@@ -243,7 +243,11 @@
         'metamta.differential.subject-prefix',
         'string',
         '[Differential]')
-        ->setDescription(pht('Subject prefix for Differential mail.')),
+        ->setSummary(pht('Subject prefix template for Differential mail.'))
+        ->setDescription(pht('Available keys: id, title, repository, branch'))
+        ->addExample(
+          '[${repository}/${branch}] [Differential]',
+          pht('Use repository and branch')),
       $this->newOption(
         'metamta.differential.attach-patches',
         'bool',
diff --git a/src/applications/differential/editor/DifferentialTransactionEditor.php b/src/applications/differential/editor/DifferentialTransactionEditor.php
--- a/src/applications/differential/editor/DifferentialTransactionEditor.php
+++ b/src/applications/differential/editor/DifferentialTransactionEditor.php
@@ -1080,8 +1080,17 @@
     return $action;
   }
 
-  protected function getMailSubjectPrefix() {
-    return PhabricatorEnv::getEnvConfig('metamta.differential.subject-prefix');
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
+    $branch = nonempty($object->getActiveDiff()->getBranch(), '-');
+    $repository = $object->getRepository() ?
+      $object->getRepository()->getName() : '-';
+
+    return $this->mergeVariables(
+      PhabricatorEnv::getEnvConfig('metamta.differential.subject-prefix'),
+      array(
+        'branch' => $branch,
+        'repository' => $repository,
+      ));
   }
 
   protected function getMailThreadID(PhabricatorLiskDAO $object) {
@@ -1109,6 +1118,42 @@
       ->addHeader('Thread-Topic', $thread_topic);
   }
 
+  /**
+   * Convert a user-provided string with variables in it, like:
+   *
+   *   ${project}/${branch}
+   *
+   * ...into a string with variables merged into it safely:
+   *
+   *   myproject/mybranch
+   *
+   * Maybe move to libphutil?
+   * @param string User-provided pattern string containing `${variables}`.
+   * @param dict   List of available replacement variables.
+   * @return string String with variables replaced safely into it.
+   *
+   */
+  protected function mergeVariables($pattern, array $variables) {
+    $regexp = '/\\$\\{(?P<name>[a-z\\.]+)\\}/';
+
+    $matches = null;
+    preg_match_all($regexp, $pattern, $matches);
+
+    $argv = array();
+    foreach ($matches['name'] as $name) {
+      if (!array_key_exists($name, $variables)) {
+        throw new Exception(pht("No such variable '%s'!", $name));
+      }
+      $argv[] = $variables[$name];
+    }
+
+    $pattern = str_replace('%', '%%', $pattern);
+    $pattern = preg_replace($regexp, '%s', $pattern);
+
+    return vsprintf($pattern, $argv);
+  }
+
+
   protected function buildMailBody(
     PhabricatorLiskDAO $object,
     array $xactions) {
diff --git a/src/applications/files/editor/PhabricatorFileEditor.php b/src/applications/files/editor/PhabricatorFileEditor.php
--- a/src/applications/files/editor/PhabricatorFileEditor.php
+++ b/src/applications/files/editor/PhabricatorFileEditor.php
@@ -42,7 +42,7 @@
     return true;
   }
 
-  protected function getMailSubjectPrefix() {
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     return PhabricatorEnv::getEnvConfig('metamta.files.subject-prefix');
   }
 
diff --git a/src/applications/legalpad/editor/LegalpadDocumentEditor.php b/src/applications/legalpad/editor/LegalpadDocumentEditor.php
--- a/src/applications/legalpad/editor/LegalpadDocumentEditor.php
+++ b/src/applications/legalpad/editor/LegalpadDocumentEditor.php
@@ -178,7 +178,7 @@
     return $body;
   }
 
-  protected function getMailSubjectPrefix() {
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     return PhabricatorEnv::getEnvConfig('metamta.legalpad.subject-prefix');
   }
 
diff --git a/src/applications/macro/editor/PhabricatorMacroEditor.php b/src/applications/macro/editor/PhabricatorMacroEditor.php
--- a/src/applications/macro/editor/PhabricatorMacroEditor.php
+++ b/src/applications/macro/editor/PhabricatorMacroEditor.php
@@ -152,7 +152,7 @@
     return $body;
   }
 
-  protected function getMailSubjectPrefix() {
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     return PhabricatorEnv::getEnvConfig('metamta.macro.subject-prefix');
   }
 
diff --git a/src/applications/maniphest/editor/ManiphestTransactionEditor.php b/src/applications/maniphest/editor/ManiphestTransactionEditor.php
--- a/src/applications/maniphest/editor/ManiphestTransactionEditor.php
+++ b/src/applications/maniphest/editor/ManiphestTransactionEditor.php
@@ -311,7 +311,7 @@
     return $xactions;
   }
 
-  protected function getMailSubjectPrefix() {
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     return PhabricatorEnv::getEnvConfig('metamta.maniphest.subject-prefix');
   }
 
diff --git a/src/applications/paste/editor/PhabricatorPasteEditor.php b/src/applications/paste/editor/PhabricatorPasteEditor.php
--- a/src/applications/paste/editor/PhabricatorPasteEditor.php
+++ b/src/applications/paste/editor/PhabricatorPasteEditor.php
@@ -126,7 +126,7 @@
     return true;
   }
 
-  protected function getMailSubjectPrefix() {
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     return PhabricatorEnv::getEnvConfig('metamta.paste.subject-prefix');
   }
 
diff --git a/src/applications/pholio/editor/PholioMockEditor.php b/src/applications/pholio/editor/PholioMockEditor.php
--- a/src/applications/pholio/editor/PholioMockEditor.php
+++ b/src/applications/pholio/editor/PholioMockEditor.php
@@ -406,7 +406,7 @@
     return $body;
   }
 
-  protected function getMailSubjectPrefix() {
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     return PhabricatorEnv::getEnvConfig('metamta.pholio.subject-prefix');
   }
 
diff --git a/src/applications/phriction/editor/PhrictionDocumentEditor.php b/src/applications/phriction/editor/PhrictionDocumentEditor.php
--- a/src/applications/phriction/editor/PhrictionDocumentEditor.php
+++ b/src/applications/phriction/editor/PhrictionDocumentEditor.php
@@ -352,7 +352,7 @@
 
     $body = implode("\n\n", $body);
 
-    $subject_prefix = $this->getMailSubjectPrefix();
+    $subject_prefix = $this->getMailSubjectPrefix(null);
 
     $mail = new PhabricatorMetaMTAMail();
     $mail->setSubject($name)
@@ -370,7 +370,7 @@
 
   /* --( For less copy-pasting when switching to ApplicationTransactions )--- */
 
-  protected function getMailSubjectPrefix() {
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     return PhabricatorEnv::getEnvConfig('metamta.phriction.subject-prefix');
   }
 
diff --git a/src/applications/ponder/editor/PonderEditor.php b/src/applications/ponder/editor/PonderEditor.php
--- a/src/applications/ponder/editor/PonderEditor.php
+++ b/src/applications/ponder/editor/PonderEditor.php
@@ -27,7 +27,7 @@
     );
   }
 
-   protected function getMailSubjectPrefix() {
+   protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     return '[Ponder]';
   }
 
diff --git a/src/applications/releeph/editor/ReleephRequestTransactionalEditor.php b/src/applications/releeph/editor/ReleephRequestTransactionalEditor.php
--- a/src/applications/releeph/editor/ReleephRequestTransactionalEditor.php
+++ b/src/applications/releeph/editor/ReleephRequestTransactionalEditor.php
@@ -188,7 +188,7 @@
       ->setMailReceiver($object);
   }
 
-  protected function getMailSubjectPrefix() {
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     return '[Releeph]';
   }
 
diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -1768,7 +1768,7 @@
 
     $template
       ->setFrom($this->requireActor()->getPHID())
-      ->setSubjectPrefix($this->getMailSubjectPrefix())
+      ->setSubjectPrefix($this->getMailSubjectPrefix($object))
       ->setVarySubjectPrefix('['.$action.']')
       ->setThreadID($this->getMailThreadID($object), $this->getIsNewObject())
       ->setRelatedPHID($object->getPHID())
@@ -1841,7 +1841,7 @@
   /**
    * @task mail
    */
-  protected function getMailSubjectPrefix() {
+  protected function getMailSubjectPrefix(PhabricatorLiskDAO $object) {
     throw new Exception("Capability not supported.");
   }