diff --git a/src/applications/differential/conduit/DifferentialParseCommitMessageConduitAPIMethod.php b/src/applications/differential/conduit/DifferentialParseCommitMessageConduitAPIMethod.php
index dc29ff07a9..1e82dc935b 100644
--- a/src/applications/differential/conduit/DifferentialParseCommitMessageConduitAPIMethod.php
+++ b/src/applications/differential/conduit/DifferentialParseCommitMessageConduitAPIMethod.php
@@ -1,105 +1,57 @@
 <?php
 
 final class DifferentialParseCommitMessageConduitAPIMethod
   extends DifferentialConduitAPIMethod {
 
-  private $errors;
-
   public function getAPIMethodName() {
     return 'differential.parsecommitmessage';
   }
 
   public function getMethodDescription() {
     return pht('Parse commit messages for Differential fields.');
   }
 
   protected function defineParamTypes() {
     return array(
       'corpus'  => 'required string',
       'partial' => 'optional bool',
     );
   }
 
   protected function defineReturnType() {
     return 'nonempty dict';
   }
 
   protected function execute(ConduitAPIRequest $request) {
-    $viewer = $request->getUser();
-    $corpus = $request->getValue('corpus');
-    $is_partial = $request->getValue('partial');
-
-    $field_list = PhabricatorCustomField::getObjectFields(
-      new DifferentialRevision(),
-      DifferentialCustomField::ROLE_COMMITMESSAGE);
-    $field_list->setViewer($viewer);
-    $field_map = mpull($field_list->getFields(), null, 'getFieldKeyForConduit');
-
-    $corpus_map = $this->parseCommitMessage($corpus);
-
-    $values = array();
-    foreach ($corpus_map as $field_key => $text_value) {
-      $field = idx($field_map, $field_key);
+    $viewer = $this->getViewer();
 
-      if (!$field) {
-        throw new Exception(
-          pht(
-            'Parser emitted text value for field key "%s", but no such '.
-            'field exists.',
-            $field_key));
-      }
+    $parser = DifferentialCommitMessageParser::newStandardParser($viewer);
 
-      try {
-        $values[$field_key] = $field->parseValueFromCommitMessage($text_value);
-      } catch (DifferentialFieldParseException $ex) {
-        $this->errors[] = pht(
-          'Error parsing field "%s": %s',
-          $field->renderCommitMessageLabel(),
-          $ex->getMessage());
-      }
+    $is_partial = $request->getValue('partial');
+    if ($is_partial) {
+      $parser->setRaiseMissingFieldErrors(false);
     }
 
-    if (!$is_partial) {
-      foreach ($field_map as $key => $field) {
-        try {
-          $field->validateCommitMessageValue(idx($values, $key));
-        } catch (DifferentialFieldValidationException $ex) {
-          $this->errors[] = pht(
-            'Invalid or missing field "%s": %s',
-            $field->renderCommitMessageLabel(),
-            $ex->getMessage());
-        }
-      }
-    }
+    $corpus = $request->getValue('corpus');
+    $field_map = $parser->parseFields($corpus);
+
+    $errors = $parser->getErrors();
 
     // grab some extra information about the Differential Revision: field...
     $revision_id_field = new DifferentialRevisionIDField();
     $revision_id_value = idx(
-      $corpus_map,
+      $field_map,
       $revision_id_field->getFieldKeyForConduit());
     $revision_id_valid_domain = PhabricatorEnv::getProductionURI('');
 
     return array(
-      'errors' => $this->errors,
-      'fields' => $values,
+      'errors' => $errors,
+      'fields' => $field_map,
       'revisionIDFieldInfo' => array(
         'value' => $revision_id_value,
         'validDomain' => $revision_id_valid_domain,
       ),
     );
   }
 
-  private function parseCommitMessage($corpus) {
-    $viewer = $this->getViewer();
-    $parser = DifferentialCommitMessageParser::newStandardParser($viewer);
-    $result = $parser->parseCorpus($corpus);
-
-    $this->errors = array();
-    foreach ($parser->getErrors() as $error) {
-      $this->errors[] = $error;
-    }
-
-    return $result;
-  }
-
 }
diff --git a/src/applications/differential/parser/DifferentialCommitMessageParser.php b/src/applications/differential/parser/DifferentialCommitMessageParser.php
index ff76b30e51..8cd508ef5f 100644
--- a/src/applications/differential/parser/DifferentialCommitMessageParser.php
+++ b/src/applications/differential/parser/DifferentialCommitMessageParser.php
@@ -1,255 +1,348 @@
 <?php
 
 /**
  * Parses commit messages (containing relatively freeform text with textual
  * field labels) into a dictionary of fields.
  *
  *   $parser = id(new DifferentialCommitMessageParser())
  *     ->setLabelMap($label_map)
  *     ->setTitleKey($key_title)
  *     ->setSummaryKey($key_summary);
  *
  *   $fields = $parser->parseCorpus($corpus);
  *   $errors = $parser->getErrors();
  *
  * This is used by Differential to parse messages entered from the command line.
  *
  * @task config   Configuring the Parser
  * @task parse    Parsing Messages
  * @task support  Support Methods
  * @task internal Internals
  */
 final class DifferentialCommitMessageParser extends Phobject {
 
+  private $viewer;
   private $labelMap;
   private $titleKey;
   private $summaryKey;
   private $errors;
-
+  private $raiseMissingFieldErrors = true;
 
   public static function newStandardParser(PhabricatorUser $viewer) {
 
     $key_title = id(new DifferentialTitleField())->getFieldKeyForConduit();
     $key_summary = id(new DifferentialSummaryField())->getFieldKeyForConduit();
 
     $field_list = PhabricatorCustomField::getObjectFields(
       new DifferentialRevision(),
       DifferentialCustomField::ROLE_COMMITMESSAGE);
     $field_list->setViewer($viewer);
 
     $label_map = array();
 
     foreach ($field_list->getFields() as $field) {
       $labels = $field->getCommitMessageLabels();
       $key = $field->getFieldKeyForConduit();
 
       foreach ($labels as $label) {
         $normal_label = self::normalizeFieldLabel(
           $label);
         if (!empty($label_map[$normal_label])) {
           throw new Exception(
             pht(
               'Field label "%s" is parsed by two custom fields: "%s" and '.
               '"%s". Each label must be parsed by only one field.',
               $label,
               $key,
               $label_map[$normal_label]));
         }
         $label_map[$normal_label] = $key;
       }
     }
 
     return id(new self())
+      ->setViewer($viewer)
       ->setLabelMap($label_map)
       ->setTitleKey($key_title)
       ->setSummaryKey($key_summary);
   }
 
 
 /* -(  Configuring the Parser  )--------------------------------------------- */
 
 
+  /**
+   * @task config
+   */
+  public function setViewer(PhabricatorUser $viewer) {
+    $this->viewer = $viewer;
+    return $this;
+  }
+
+
+  /**
+   * @task config
+   */
+  public function getViewer() {
+    return $this->viewer;
+  }
+
+
+  /**
+   * @task config
+   */
+  public function setRaiseMissingFieldErrors($raise) {
+    $this->raiseMissingFieldErrors = $raise;
+    return $this;
+  }
+
+
+  /**
+   * @task config
+   */
+  public function getRaiseMissingFieldErrors() {
+    return $this->raiseMissingFieldErrors;
+  }
+
+
   /**
    * @task config
    */
   public function setLabelMap(array $label_map) {
     $this->labelMap = $label_map;
     return $this;
   }
 
 
   /**
    * @task config
    */
   public function setTitleKey($title_key) {
     $this->titleKey = $title_key;
     return $this;
   }
 
 
   /**
    * @task config
    */
   public function setSummaryKey($summary_key) {
     $this->summaryKey = $summary_key;
     return $this;
   }
 
 
 /* -(  Parsing Messages  )--------------------------------------------------- */
 
 
   /**
    * @task parse
    */
   public function parseCorpus($corpus) {
     $this->errors = array();
 
     $label_map = $this->labelMap;
     $key_title = $this->titleKey;
     $key_summary = $this->summaryKey;
 
     if (!$key_title || !$key_summary || ($label_map === null)) {
       throw new Exception(
         pht(
           'Expected %s, %s and %s to be set before parsing a corpus.',
           'labelMap',
           'summaryKey',
           'titleKey'));
     }
 
     $label_regexp = $this->buildLabelRegexp($label_map);
 
     // NOTE: We're special casing things here to make the "Title:" label
     // optional in the message.
     $field = $key_title;
 
     $seen = array();
     $lines = explode("\n", trim($corpus));
     $field_map = array();
     foreach ($lines as $key => $line) {
       $match = null;
       if (preg_match($label_regexp, $line, $match)) {
         $lines[$key] = trim($match['text']);
         $field = $label_map[self::normalizeFieldLabel($match['field'])];
         if (!empty($seen[$field])) {
           $this->errors[] = pht(
             'Field "%s" occurs twice in commit message!',
             $field);
         }
         $seen[$field] = true;
       }
       $field_map[$key] = $field;
     }
 
     $fields = array();
     foreach ($lines as $key => $line) {
       $fields[$field_map[$key]][] = $line;
     }
 
     // This is a piece of special-cased magic which allows you to omit the
     // field labels for "title" and "summary". If the user enters a large block
     // of text at the beginning of the commit message with an empty line in it,
     // treat everything before the blank line as "title" and everything after
     // as "summary".
     if (isset($fields[$key_title]) && empty($fields[$key_summary])) {
       $lines = $fields[$key_title];
       for ($ii = 0; $ii < count($lines); $ii++) {
         if (strlen(trim($lines[$ii])) == 0) {
           break;
         }
       }
       if ($ii != count($lines)) {
         $fields[$key_title] = array_slice($lines, 0, $ii);
         $summary = array_slice($lines, $ii);
         if (strlen(trim(implode("\n", $summary)))) {
           $fields[$key_summary] = $summary;
         }
       }
     }
 
     // Implode all the lines back into chunks of text.
     foreach ($fields as $name => $lines) {
       $data = rtrim(implode("\n", $lines));
       $data = ltrim($data, "\n");
       $fields[$name] = $data;
     }
 
     // This is another piece of special-cased magic which allows you to
     // enter a ridiculously long title, or just type a big block of stream
     // of consciousness text, and have some sort of reasonable result conjured
     // from it.
     if (isset($fields[$key_title])) {
       $terminal = '...';
       $title = $fields[$key_title];
       $short = id(new PhutilUTF8StringTruncator())
         ->setMaximumBytes(250)
         ->setTerminator($terminal)
         ->truncateString($title);
 
       if ($short != $title) {
 
         // If we shortened the title, split the rest into the summary, so
         // we end up with a title like:
         //
         //    Title title tile title title...
         //
         // ...and a summary like:
         //
         //    ...title title title.
         //
         //    Summary summary summary summary.
 
         $summary = idx($fields, $key_summary, '');
         $offset = strlen($short) - strlen($terminal);
         $remainder = ltrim(substr($fields[$key_title], $offset));
         $summary = '...'.$remainder."\n\n".$summary;
         $summary = rtrim($summary, "\n");
 
         $fields[$key_title] = $short;
         $fields[$key_summary] = $summary;
       }
     }
 
     return $fields;
   }
 
 
+  /**
+   * @task parse
+   */
+  public function parseFields($corpus) {
+    $viewer = $this->getViewer();
+    $text_map = $this->parseCorpus($corpus);
+
+    $field_list = PhabricatorCustomField::getObjectFields(
+      new DifferentialRevision(),
+      DifferentialCustomField::ROLE_COMMITMESSAGE);
+    $field_list->setViewer($viewer);
+
+    $field_map = $field_list->getFields();
+    $field_map = mpull($field_map, null, 'getFieldKeyForConduit');
+
+    $result_map = array();
+    foreach ($text_map as $field_key => $text_value) {
+      $field = idx($field_map, $field_key);
+      if (!$field) {
+        // This is a strict error, since we only parse fields which we have
+        // been told are valid. The caller probably handed us an invalid label
+        // map.
+        throw new Exception(
+          pht(
+            'Parser emitted a field with key "%s", but no corresponding '.
+            'field definition exists.',
+            $field_key));
+      }
+
+      try {
+        $result = $field->parseValueFromCommitMessage($text_value);
+        $result_map[$field_key] = $result;
+      } catch (DifferentialFieldParseException $ex) {
+        $this->errors[] = pht(
+          'Error parsing field "%s": %s',
+          $field->renderCommitMessageLabel(),
+          $ex->getMessage());
+      }
+    }
+
+    if ($this->getRaiseMissingFieldErrors()) {
+      foreach ($field_map as $key => $field) {
+        try {
+          $field->validateCommitMessageValue(idx($result_map, $key));
+        } catch (DifferentialFieldValidationException $ex) {
+          $this->errors[] = pht(
+            'Invalid or missing field "%s": %s',
+            $field->renderCommitMessageLabel(),
+            $ex->getMessage());
+        }
+      }
+    }
+
+    return $result_map;
+  }
+
+
   /**
    * @task parse
    */
   public function getErrors() {
     return $this->errors;
   }
 
 
 /* -(  Support Methods  )---------------------------------------------------- */
 
 
   /**
    * @task support
    */
   public static function normalizeFieldLabel($label) {
     return phutil_utf8_strtolower($label);
   }
 
 
 /* -(  Internals  )---------------------------------------------------------- */
 
 
   /**
    * @task internal
    */
   private function buildLabelRegexp(array $label_map) {
     $field_labels = array_keys($label_map);
     foreach ($field_labels as $key => $label) {
       $field_labels[$key] = preg_quote($label, '/');
     }
     $field_labels = implode('|', $field_labels);
 
     $field_pattern = '/^(?P<field>'.$field_labels.'):(?P<text>.*)$/i';
 
     return $field_pattern;
   }
 
 }