diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -934,6 +934,7 @@
     'HeraldCommitAdapter' => 'applications/herald/adapter/HeraldCommitAdapter.php',
     'HeraldCondition' => 'applications/herald/storage/HeraldCondition.php',
     'HeraldConditionTranscript' => 'applications/herald/storage/transcript/HeraldConditionTranscript.php',
+    'HeraldContentSourceField' => 'applications/herald/field/HeraldContentSourceField.php',
     'HeraldController' => 'applications/herald/controller/HeraldController.php',
     'HeraldCustomAction' => 'applications/herald/extension/HeraldCustomAction.php',
     'HeraldDAO' => 'applications/herald/storage/HeraldDAO.php',
@@ -943,6 +944,7 @@
     'HeraldDisableController' => 'applications/herald/controller/HeraldDisableController.php',
     'HeraldEffect' => 'applications/herald/engine/HeraldEffect.php',
     'HeraldEngine' => 'applications/herald/engine/HeraldEngine.php',
+    'HeraldField' => 'applications/herald/field/HeraldField.php',
     'HeraldInvalidActionException' => 'applications/herald/engine/exception/HeraldInvalidActionException.php',
     'HeraldInvalidConditionException' => 'applications/herald/engine/exception/HeraldInvalidConditionException.php',
     'HeraldManageGlobalRulesCapability' => 'applications/herald/capability/HeraldManageGlobalRulesCapability.php',
@@ -4429,6 +4431,7 @@
     'HeraldCommitAdapter' => 'HeraldAdapter',
     'HeraldCondition' => 'HeraldDAO',
     'HeraldConditionTranscript' => 'Phobject',
+    'HeraldContentSourceField' => 'HeraldField',
     'HeraldController' => 'PhabricatorController',
     'HeraldCustomAction' => 'Phobject',
     'HeraldDAO' => 'PhabricatorLiskDAO',
@@ -4438,6 +4441,7 @@
     'HeraldDisableController' => 'HeraldController',
     'HeraldEffect' => 'Phobject',
     'HeraldEngine' => 'Phobject',
+    'HeraldField' => 'Phobject',
     'HeraldInvalidActionException' => 'Exception',
     'HeraldInvalidConditionException' => 'Exception',
     'HeraldManageGlobalRulesCapability' => 'PhabricatorPolicyCapability',
diff --git a/src/applications/herald/adapter/HeraldAdapter.php b/src/applications/herald/adapter/HeraldAdapter.php
--- a/src/applications/herald/adapter/HeraldAdapter.php
+++ b/src/applications/herald/adapter/HeraldAdapter.php
@@ -24,7 +24,6 @@
   const FIELD_RULE                   = 'rule';
   const FIELD_AFFECTED_PACKAGE       = 'affected-package';
   const FIELD_AFFECTED_PACKAGE_OWNER = 'affected-package-owner';
-  const FIELD_CONTENT_SOURCE         = 'contentsource';
   const FIELD_ALWAYS                 = 'always';
   const FIELD_AUTHOR_PROJECTS        = 'authorprojects';
   const FIELD_PROJECTS               = 'projects';
@@ -113,6 +112,7 @@
   private $emailPHIDs = array();
   private $forcedEmailPHIDs = array();
   private $unsubscribedPHIDs;
+  private $fieldMap;
 
   public function getEmailPHIDs() {
     return array_values($this->emailPHIDs);
@@ -190,11 +190,14 @@
   abstract public function getHeraldName();
 
   public function getHeraldField($field_name) {
+    $impl = $this->getFieldImplementation($field_name);
+    if ($impl) {
+      return $impl->getHeraldFieldValue($this->getObject());
+    }
+
     switch ($field_name) {
       case self::FIELD_RULE:
         return null;
-      case self::FIELD_CONTENT_SOURCE:
-        return $this->getContentSource()->getSource();
       case self::FIELD_ALWAYS:
         return true;
       case self::FIELD_IS_NEW_OBJECT:
@@ -354,9 +357,52 @@
 
 /* -(  Fields  )------------------------------------------------------------- */
 
+  private function getFieldImplementationMap() {
+    if ($this->fieldMap === null) {
+      // We can't use PhutilClassMapQuery here because field expansion
+      // depends on the adapter and object.
+
+      $object = $this->getObject();
+
+      $map = array();
+      $all = HeraldField::getAllFields();
+      foreach ($all as $key => $field) {
+        if (!$field->supportsObject($object)) {
+          continue;
+        }
+        $subfields = $field->getFieldsForObject($object);
+        foreach ($subfields as $subkey => $subfield) {
+          if (isset($map[$subkey])) {
+            throw new Exception(
+              pht(
+                'Two HeraldFields (of classes "%s" and "%s") have the same '.
+                'field key ("%s") after expansion for an object of class '.
+                '"%s" inside adapter "%s". Each field must have a unique '.
+                'field key.',
+                get_class($subfield),
+                get_class($map[$subkey]),
+                $subkey,
+                get_class($object),
+                get_class($this)));
+          }
+
+          $subfield = id(clone $subfield)->setAdapter($this);
+
+          $map[$subkey] = $subfield;
+        }
+      }
+      $this->fieldMap = $map;
+    }
+
+    return $this->fieldMap;
+  }
+
+  private function getFieldImplementation($key) {
+    return idx($this->getFieldImplementationMap(), $key);
+  }
 
   public function getFields() {
-    $fields = array();
+    $fields = array_keys($this->getFieldImplementationMap());
 
     $fields[] = self::FIELD_ALWAYS;
     $fields[] = self::FIELD_RULE;
@@ -373,7 +419,9 @@
   }
 
   public function getFieldNameMap() {
-    return array(
+    $map = mpull($this->getFieldImplementationMap(), 'getHeraldFieldName');
+
+    return $map + array(
       self::FIELD_TITLE => pht('Title'),
       self::FIELD_BODY => pht('Body'),
       self::FIELD_AUTHOR => pht('Author'),
@@ -394,7 +442,6 @@
       self::FIELD_AFFECTED_PACKAGE => pht('Any affected package'),
       self::FIELD_AFFECTED_PACKAGE_OWNER =>
         pht("Any affected package's owner"),
-      self::FIELD_CONTENT_SOURCE => pht('Content Source'),
       self::FIELD_ALWAYS => pht('Always'),
       self::FIELD_AUTHOR_PROJECTS => pht("Author's projects"),
       self::FIELD_PROJECTS => pht('Projects'),
@@ -452,6 +499,11 @@
   }
 
   public function getConditionsForField($field) {
+    $impl = $this->getFieldImplementation($field);
+    if ($impl) {
+      return $impl->getHeraldFieldConditions();
+    }
+
     switch ($field) {
       case self::FIELD_TITLE:
       case self::FIELD_BODY:
@@ -526,11 +578,6 @@
           self::CONDITION_RULE,
           self::CONDITION_NOT_RULE,
         );
-      case self::FIELD_CONTENT_SOURCE:
-        return array(
-          self::CONDITION_IS,
-          self::CONDITION_IS_NOT,
-        );
       case self::FIELD_ALWAYS:
         return array(
           self::CONDITION_UNCONDITIONALLY,
@@ -940,6 +987,10 @@
 
 
   public function getValueTypeForFieldAndCondition($field, $condition) {
+    $impl = $this->getFieldImplementation($field);
+    if ($impl) {
+      return $impl->getHeraldFieldValueType($condition);
+    }
 
     if ($this->isHeraldCustomKey($field)) {
       $value_type = $this->getCustomFieldValueTypeForFieldAndCondition(
@@ -958,13 +1009,7 @@
         return self::VALUE_TEXT;
       case self::CONDITION_IS:
       case self::CONDITION_IS_NOT:
-        switch ($field) {
-          case self::FIELD_CONTENT_SOURCE:
-            return self::VALUE_CONTENT_SOURCE;
-          default:
-            return self::VALUE_TEXT;
-        }
-        break;
+        return self::VALUE_TEXT;
       case self::CONDITION_IS_ANY:
       case self::CONDITION_IS_NOT_ANY:
         switch ($field) {
diff --git a/src/applications/herald/adapter/HeraldManiphestTaskAdapter.php b/src/applications/herald/adapter/HeraldManiphestTaskAdapter.php
--- a/src/applications/herald/adapter/HeraldManiphestTaskAdapter.php
+++ b/src/applications/herald/adapter/HeraldManiphestTaskAdapter.php
@@ -67,7 +67,6 @@
         self::FIELD_AUTHOR,
         self::FIELD_ASSIGNEE,
         self::FIELD_CC,
-        self::FIELD_CONTENT_SOURCE,
         self::FIELD_PROJECTS,
         self::FIELD_TASK_PRIORITY,
         self::FIELD_TASK_STATUS,
diff --git a/src/applications/herald/field/HeraldContentSourceField.php b/src/applications/herald/field/HeraldContentSourceField.php
new file mode 100644
--- /dev/null
+++ b/src/applications/herald/field/HeraldContentSourceField.php
@@ -0,0 +1,30 @@
+<?php
+
+final class HeraldContentSourceField extends HeraldField {
+
+  const FIELDCONST = 'contentsource';
+
+  public function getHeraldFieldName() {
+    return pht('Content source');
+  }
+
+  public function getHeraldFieldValue($object) {
+    return $this->getAdapter()->getContentSource()->getSource();
+  }
+
+  public function getHeraldFieldConditions() {
+    return array(
+      HeraldAdapter::CONDITION_IS,
+      HeraldAdapter::CONDITION_IS_NOT,
+    );
+  }
+
+  public function getHeraldFieldValueType($condition) {
+    return HeraldAdapter::VALUE_CONTENT_SOURCE;
+  }
+
+  public function supportsObject($object) {
+    return true;
+  }
+
+}
diff --git a/src/applications/herald/field/HeraldField.php b/src/applications/herald/field/HeraldField.php
new file mode 100644
--- /dev/null
+++ b/src/applications/herald/field/HeraldField.php
@@ -0,0 +1,60 @@
+<?php
+
+abstract class HeraldField extends Phobject {
+
+  private $adapter;
+
+  abstract public function getHeraldFieldName();
+  abstract public function getHeraldFieldValue($object);
+  abstract public function getHeraldFieldConditions();
+  abstract public function getHeraldFieldValueType($condition);
+
+  abstract public function supportsObject($object);
+
+  public function getFieldsForObject($object) {
+    return array($this->getFieldConstant() => $this);
+  }
+
+  final public function setAdapter(HeraldAdapter $adapter) {
+    $this->adapter = $adapter;
+    return $this;
+  }
+
+  final public function getAdapter() {
+    return $this->adapter;
+  }
+
+  final public function getFieldConstant() {
+    $class = new ReflectionClass($this);
+
+    $const = $class->getConstant('FIELDCONST');
+    if ($const === false) {
+      throw new Exception(
+        pht(
+          '"%s" class "%s" must define a "%s" property.',
+          __CLASS__,
+          get_class($this),
+          'FIELDCONST'));
+    }
+
+    if (!is_string($const) || (strlen($const) > 32)) {
+      throw new Exception(
+        pht(
+          '"%s" class "%s" has an invalid "%s" property. Field constants '.
+          'must be strings and no more than 32 bytes in length.',
+          __CLASS__,
+          get_class($this),
+          'FIELDCONST'));
+    }
+
+    return $const;
+  }
+
+  final public static function getAllFields() {
+    return id(new PhutilClassMapQuery())
+      ->setAncestorClass(__CLASS__)
+      ->setUniqueMethod('getFieldConstant')
+      ->execute();
+  }
+
+}