diff --git a/resources/sql/autopatches/20210715.harborcommand.01.xactions.php b/resources/sql/autopatches/20210715.harborcommand.01.xactions.php
new file mode 100644
--- /dev/null
+++ b/resources/sql/autopatches/20210715.harborcommand.01.xactions.php
@@ -0,0 +1,34 @@
+<?php
+
+// See T13072. Turn the old "process a command" transaction into modular
+// transactions that each handle one particular type of command.
+
+$xactions_table = new HarbormasterBuildTransaction();
+$xactions_conn = $xactions_table->establishConnection('w');
+$row_iterator = new LiskRawMigrationIterator(
+  $xactions_conn,
+  $xactions_table->getTableName());
+
+$map = array(
+  '"pause"' => 'message/pause',
+  '"abort"' => 'message/abort',
+  '"resume"' => 'message/resume',
+  '"restart"' => 'message/restart',
+);
+
+foreach ($row_iterator as $row) {
+  if ($row['transactionType'] !== 'harbormaster:build:command') {
+    continue;
+  }
+
+  $raw_value = $row['newValue'];
+
+  if (isset($map[$raw_value])) {
+    queryfx(
+      $xactions_conn,
+      'UPDATE %R SET transactionType = %s WHERE id = %d',
+      $xactions_table,
+      $map[$raw_value],
+      $row['id']);
+  }
+}
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
@@ -1408,7 +1408,11 @@
     'HarbormasterBuildLogView' => 'applications/harbormaster/view/HarbormasterBuildLogView.php',
     'HarbormasterBuildLogViewController' => 'applications/harbormaster/controller/HarbormasterBuildLogViewController.php',
     'HarbormasterBuildMessage' => 'applications/harbormaster/storage/HarbormasterBuildMessage.php',
+    'HarbormasterBuildMessageAbortTransaction' => 'applications/harbormaster/xaction/build/HarbormasterBuildMessageAbortTransaction.php',
+    'HarbormasterBuildMessagePauseTransaction' => 'applications/harbormaster/xaction/build/HarbormasterBuildMessagePauseTransaction.php',
     'HarbormasterBuildMessageQuery' => 'applications/harbormaster/query/HarbormasterBuildMessageQuery.php',
+    'HarbormasterBuildMessageRestartTransaction' => 'applications/harbormaster/xaction/build/HarbormasterBuildMessageRestartTransaction.php',
+    'HarbormasterBuildMessageResumeTransaction' => 'applications/harbormaster/xaction/build/HarbormasterBuildMessageResumeTransaction.php',
     'HarbormasterBuildMessageTransaction' => 'applications/harbormaster/xaction/build/HarbormasterBuildMessageTransaction.php',
     'HarbormasterBuildPHIDType' => 'applications/harbormaster/phid/HarbormasterBuildPHIDType.php',
     'HarbormasterBuildPlan' => 'applications/harbormaster/storage/configuration/HarbormasterBuildPlan.php',
@@ -7629,7 +7633,11 @@
       'PhabricatorPolicyInterface',
       'PhabricatorDestructibleInterface',
     ),
+    'HarbormasterBuildMessageAbortTransaction' => 'HarbormasterBuildMessageTransaction',
+    'HarbormasterBuildMessagePauseTransaction' => 'HarbormasterBuildMessageTransaction',
     'HarbormasterBuildMessageQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
+    'HarbormasterBuildMessageRestartTransaction' => 'HarbormasterBuildMessageTransaction',
+    'HarbormasterBuildMessageResumeTransaction' => 'HarbormasterBuildMessageTransaction',
     'HarbormasterBuildMessageTransaction' => 'HarbormasterBuildTransactionType',
     'HarbormasterBuildPHIDType' => 'PhabricatorPHIDType',
     'HarbormasterBuildPlan' => array(
diff --git a/src/applications/harbormaster/engine/HarbormasterBuildEngine.php b/src/applications/harbormaster/engine/HarbormasterBuildEngine.php
--- a/src/applications/harbormaster/engine/HarbormasterBuildEngine.php
+++ b/src/applications/harbormaster/engine/HarbormasterBuildEngine.php
@@ -124,12 +124,18 @@
 
     $xactions = array();
 
-    $message_xaction = HarbormasterBuildMessageTransaction::TRANSACTIONTYPE;
-
     $messages = $build->getUnprocessedMessagesForApply();
     foreach ($messages as $message) {
       $message_type = $message->getType();
 
+      $message_xaction =
+        HarbormasterBuildMessageTransaction::getTransactionTypeForMessageType(
+          $message_type);
+
+      if (!$message_xaction) {
+        continue;
+      }
+
       $xactions[] = $build->getApplicationTransactionTemplate()
         ->setAuthorPHID($message->getAuthorPHID())
         ->setTransactionType($message_xaction)
diff --git a/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageAbortTransaction.php b/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageAbortTransaction.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageAbortTransaction.php
@@ -0,0 +1,41 @@
+<?php
+
+final class HarbormasterBuildMessageAbortTransaction
+  extends HarbormasterBuildMessageTransaction {
+
+  const TRANSACTIONTYPE = 'message/abort';
+
+  public function getMessageType() {
+    return 'abort';
+  }
+
+  public function getTitle() {
+    return pht(
+      '%s aborted this build.',
+      $this->renderAuthor());
+  }
+
+  public function getIcon() {
+    return 'fa-exclamation-triangle';
+  }
+
+  public function getColor() {
+    return 'red';
+  }
+
+  public function applyInternalEffects($object, $value) {
+    $actor = $this->getActor();
+    $build = $object;
+
+    $build->setBuildStatus(HarbormasterBuildStatus::STATUS_ABORTED);
+  }
+
+  public function applyExternalEffects($object, $value) {
+    $actor = $this->getActor();
+    $build = $object;
+
+    $build->releaseAllArtifacts($actor);
+  }
+
+
+}
diff --git a/src/applications/harbormaster/xaction/build/HarbormasterBuildMessagePauseTransaction.php b/src/applications/harbormaster/xaction/build/HarbormasterBuildMessagePauseTransaction.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/xaction/build/HarbormasterBuildMessagePauseTransaction.php
@@ -0,0 +1,33 @@
+<?php
+
+final class HarbormasterBuildMessagePauseTransaction
+  extends HarbormasterBuildMessageTransaction {
+
+  const TRANSACTIONTYPE = 'message/pause';
+
+  public function getMessageType() {
+    return 'pause';
+  }
+
+  public function getTitle() {
+    return pht(
+      '%s paused this build.',
+      $this->renderAuthor());
+  }
+
+  public function getIcon() {
+    return 'fa-pause';
+  }
+
+  public function getColor() {
+    return 'red';
+  }
+
+  public function applyInternalEffects($object, $value) {
+    $actor = $this->getActor();
+    $build = $object;
+
+    $build->setBuildStatus(HarbormasterBuildStatus::STATUS_PAUSED);
+  }
+
+}
diff --git a/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageRestartTransaction.php b/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageRestartTransaction.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageRestartTransaction.php
@@ -0,0 +1,30 @@
+<?php
+
+final class HarbormasterBuildMessageRestartTransaction
+  extends HarbormasterBuildMessageTransaction {
+
+  const TRANSACTIONTYPE = 'message/restart';
+
+  public function getMessageType() {
+    return 'restart';
+  }
+
+  public function getTitle() {
+    return pht(
+      '%s restarted this build.',
+      $this->renderAuthor());
+  }
+
+  public function getIcon() {
+    return 'fa-backward';
+  }
+
+  public function applyInternalEffects($object, $value) {
+    $actor = $this->getActor();
+    $build = $object;
+
+    $build->restartBuild($actor);
+    $build->setBuildStatus(HarbormasterBuildStatus::STATUS_BUILDING);
+  }
+
+}
diff --git a/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageResumeTransaction.php b/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageResumeTransaction.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageResumeTransaction.php
@@ -0,0 +1,29 @@
+<?php
+
+final class HarbormasterBuildMessageResumeTransaction
+  extends HarbormasterBuildMessageTransaction {
+
+  const TRANSACTIONTYPE = 'message/resume';
+
+  public function getMessageType() {
+    return 'resume';
+  }
+
+  public function getTitle() {
+    return pht(
+      '%s resumed this build.',
+      $this->renderAuthor());
+  }
+
+  public function getIcon() {
+    return 'fa-play';
+  }
+
+  public function applyInternalEffects($object, $value) {
+    $actor = $this->getActor();
+    $build = $object;
+
+    $build->setBuildStatus(HarbormasterBuildStatus::STATUS_BUILDING);
+  }
+
+}
diff --git a/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageTransaction.php b/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageTransaction.php
--- a/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageTransaction.php
+++ b/src/applications/harbormaster/xaction/build/HarbormasterBuildMessageTransaction.php
@@ -1,80 +1,37 @@
 <?php
 
-final class HarbormasterBuildMessageTransaction
+abstract class HarbormasterBuildMessageTransaction
   extends HarbormasterBuildTransactionType {
 
-  const TRANSACTIONTYPE = 'harbormaster:build:command';
-
-  public function generateOldValue($object) {
+  final public function generateOldValue($object) {
     return null;
   }
 
-  public function getTitle() {
-    $new = $this->getNewValue();
-
-    switch ($new) {
-      case HarbormasterBuildCommand::COMMAND_RESTART:
-        return pht(
-          '%s restarted this build.',
-          $this->renderAuthor());
-      case HarbormasterBuildCommand::COMMAND_ABORT:
-        return pht(
-          '%s aborted this build.',
-          $this->renderAuthor());
-      case HarbormasterBuildCommand::COMMAND_RESUME:
-        return pht(
-          '%s resumed this build.',
-          $this->renderAuthor());
-      case HarbormasterBuildCommand::COMMAND_PAUSE:
-        return pht(
-          '%s paused this build.',
-          $this->renderAuthor());
-    }
-
-    return pht(
-      '%s issued an unknown command ("%s") to this build.',
-      $this->renderAuthor(),
-      $this->renderValue($new));
+  final public function getTransactionTypeForConduit($xaction) {
+    return 'message';
   }
 
-  public function getIcon() {
-    $new = $this->getNewValue();
-
-    switch ($new) {
-      case HarbormasterBuildCommand::COMMAND_RESTART:
-        return 'fa-backward';
-      case HarbormasterBuildCommand::COMMAND_RESUME:
-        return 'fa-play';
-      case HarbormasterBuildCommand::COMMAND_PAUSE:
-        return 'fa-pause';
-      case HarbormasterBuildCommand::COMMAND_ABORT:
-        return 'fa-exclamation-triangle';
-      default:
-        return 'fa-question';
-    }
+  final public function getFieldValuesForConduit($xaction, $data) {
+    return array(
+      'type' => $xaction->getNewValue(),
+    );
   }
 
-  public function getColor() {
-    $new = $this->getNewValue();
+  final public static function getTransactionTypeForMessageType($message_type) {
+    $message_xactions = id(new PhutilClassMapQuery())
+      ->setAncestorClass(__CLASS__)
+      ->execute();
 
-    switch ($new) {
-      case HarbormasterBuildCommand::COMMAND_PAUSE:
-      case HarbormasterBuildCommand::COMMAND_ABORT:
-        return 'red';
+    foreach ($message_xactions as $message_xaction) {
+      if ($message_xaction->getMessageType() === $message_type) {
+        return $message_xaction->getTransactionTypeConstant();
+      }
     }
 
-    return parent::getColor();
+    return null;
   }
 
-  public function getTransactionTypeForConduit($xaction) {
-    return 'message';
-  }
-
-  public function getFieldValuesForConduit($xaction, $data) {
-    return array(
-      'type' => $xaction->getNewValue(),
-    );
-  }
+  abstract public function getMessageType();
 
   public function validateTransactions($object, array $xactions) {
     $errors = array();
@@ -88,41 +45,4 @@
     return $errors;
   }
 
-  public function applyInternalEffects($object, $value) {
-    $actor = $this->getActor();
-    $build = $object;
-
-    $new = $this->getNewValue();
-
-    switch ($new) {
-      case HarbormasterBuildCommand::COMMAND_ABORT:
-        $build->setBuildStatus(HarbormasterBuildStatus::STATUS_ABORTED);
-        break;
-      case HarbormasterBuildCommand::COMMAND_RESTART:
-        $build->restartBuild($actor);
-        $build->setBuildStatus(HarbormasterBuildStatus::STATUS_BUILDING);
-        break;
-      case HarbormasterBuildCommand::COMMAND_RESUME:
-        $build->setBuildStatus(HarbormasterBuildStatus::STATUS_BUILDING);
-        break;
-      case HarbormasterBuildCommand::COMMAND_PAUSE:
-        $build->setBuildStatus(HarbormasterBuildStatus::STATUS_PAUSED);
-        break;
-    }
-  }
-
-  public function applyExternalEffects($object, $value) {
-    $actor = $this->getActor();
-    $build = $object;
-
-    $new = $this->getNewValue();
-
-    switch ($new) {
-      case HarbormasterBuildCommand::COMMAND_ABORT:
-        $build->releaseAllArtifacts($actor);
-        break;
-    }
-  }
-
-
 }