diff --git a/src/applications/config/option/PhabricatorCoreConfigOptions.php b/src/applications/config/option/PhabricatorCoreConfigOptions.php
--- a/src/applications/config/option/PhabricatorCoreConfigOptions.php
+++ b/src/applications/config/option/PhabricatorCoreConfigOptions.php
@@ -37,6 +37,24 @@
     $proto_doc_name = pht('User Guide: Prototype Applications');
     $applications_app_href = '/applications/';
 
+    $silent_description = $this->deformat(pht(<<<EOREMARKUP
+This option allows you to stop Phabricator from sending data to most external
+services: it will disable email, SMS, repository mirroring, remote builds,
+Doorkeeper writes, and webhooks.
+
+This option is intended to allow a Phabricator instance to be exported, copied,
+imported, and run in a test environment without impacting users. For example,
+if you are migrating to new hardware, you could perform a test migration first
+with this flag set, make sure things work, and then do a production cutover
+later with higher confidence and less disruption.
+
+Without making use of this flag to silence the temporary test environment,
+users would receive duplicate email during the time the test instance and old
+production instance were both in operation.
+EOREMARKUP
+      ));
+
+
     return array(
       $this->newOption('phabricator.base-uri', 'string', null)
         ->setLocked(true)
@@ -232,21 +250,7 @@
             pht('Run Normally'),
           ))
         ->setSummary(pht('Stop Phabricator from sending any email, etc.'))
-        ->setDescription(
-          pht(
-            'This option allows you to stop Phabricator from sending '.
-            'any data to external services. Among other things, it will '.
-            'disable email, SMS, repository mirroring, and HTTP hooks.'.
-            "\n\n".
-            'This option is intended to allow a Phabricator instance to '.
-            'be exported, copied, imported, and run in a test environment '.
-            'without impacting users. For example, if you are migrating '.
-            'to new hardware, you could perform a test migration first, '.
-            'make sure things work, and then do a production cutover '.
-            'later with higher confidence and less disruption. Without '.
-            'this flag, users would receive duplicate email during the '.
-            'time the test instance and old production instance were '.
-            'both in operation.')),
+        ->setDescription($silent_description),
       );
 
   }
diff --git a/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php
--- a/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php
+++ b/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php
@@ -295,6 +295,18 @@
       ->append($body);
   }
 
+  protected function logSilencedCall(
+    HarbormasterBuild $build,
+    HarbormasterBuildTarget $build_target,
+    $label) {
+
+    $build_target
+      ->newLog($label, 'silenced')
+      ->append(
+        pht(
+          'Declining to make service call because `phabricator.silent` is '.
+          'enabled in configuration.'));
+  }
 
 
 /* -(  Automatic Targets  )-------------------------------------------------- */
diff --git a/src/applications/harbormaster/step/HarbormasterBuildkiteBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterBuildkiteBuildStepImplementation.php
--- a/src/applications/harbormaster/step/HarbormasterBuildkiteBuildStepImplementation.php
+++ b/src/applications/harbormaster/step/HarbormasterBuildkiteBuildStepImplementation.php
@@ -72,6 +72,11 @@
     HarbormasterBuildTarget $build_target) {
     $viewer = PhabricatorUser::getOmnipotentUser();
 
+    if (PhabricatorEnv::getEnvConfig('phabricator.silent')) {
+      $this->logSilencedCall($build, $build_target, pht('Buildkite'));
+      throw new HarbormasterBuildFailureException();
+    }
+
     $buildable = $build->getBuildable();
 
     $object = $buildable->getBuildableObject();
diff --git a/src/applications/harbormaster/step/HarbormasterCircleCIBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterCircleCIBuildStepImplementation.php
--- a/src/applications/harbormaster/step/HarbormasterCircleCIBuildStepImplementation.php
+++ b/src/applications/harbormaster/step/HarbormasterCircleCIBuildStepImplementation.php
@@ -84,6 +84,11 @@
     HarbormasterBuildTarget $build_target) {
     $viewer = PhabricatorUser::getOmnipotentUser();
 
+    if (PhabricatorEnv::getEnvConfig('phabricator.silent')) {
+      $this->logSilencedCall($build, $build_target, pht('CircleCI'));
+      throw new HarbormasterBuildFailureException();
+    }
+
     $buildable = $build->getBuildable();
 
     $object = $buildable->getBuildableObject();
diff --git a/src/applications/harbormaster/step/HarbormasterHTTPRequestBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterHTTPRequestBuildStepImplementation.php
--- a/src/applications/harbormaster/step/HarbormasterHTTPRequestBuildStepImplementation.php
+++ b/src/applications/harbormaster/step/HarbormasterHTTPRequestBuildStepImplementation.php
@@ -43,6 +43,12 @@
     HarbormasterBuildTarget $build_target) {
 
     $viewer = PhabricatorUser::getOmnipotentUser();
+
+    if (PhabricatorEnv::getEnvConfig('phabricator.silent')) {
+      $this->logSilencedCall($build, $build_target, pht('HTTP Request'));
+      throw new HarbormasterBuildFailureException();
+    }
+
     $settings = $this->getSettings();
     $variables = $build_target->getVariables();