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
@@ -1211,6 +1211,7 @@
     'HarbormasterBuildableTransactionQuery' => 'applications/harbormaster/query/HarbormasterBuildableTransactionQuery.php',
     'HarbormasterBuildableViewController' => 'applications/harbormaster/controller/HarbormasterBuildableViewController.php',
     'HarbormasterBuildkiteBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterBuildkiteBuildStepImplementation.php',
+    'HarbormasterBuildkiteBuildableInterface' => 'applications/harbormaster/interface/HarbormasterBuildkiteBuildableInterface.php',
     'HarbormasterBuildkiteHookController' => 'applications/harbormaster/controller/HarbormasterBuildkiteHookController.php',
     'HarbormasterBuiltinBuildStepGroup' => 'applications/harbormaster/stepgroup/HarbormasterBuiltinBuildStepGroup.php',
     'HarbormasterCircleCIBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterCircleCIBuildStepImplementation.php',
@@ -5091,6 +5092,7 @@
       'PhabricatorExtendedPolicyInterface',
       'HarbormasterBuildableInterface',
       'HarbormasterCircleCIBuildableInterface',
+      'HarbormasterBuildkiteBuildableInterface',
       'PhabricatorApplicationTransactionInterface',
       'PhabricatorDestructibleInterface',
     ),
@@ -8789,6 +8791,7 @@
       'PhabricatorMentionableInterface',
       'HarbormasterBuildableInterface',
       'HarbormasterCircleCIBuildableInterface',
+      'HarbormasterBuildkiteBuildableInterface',
       'PhabricatorCustomFieldInterface',
       'PhabricatorApplicationTransactionInterface',
       'PhabricatorFulltextInterface',
diff --git a/src/applications/differential/storage/DifferentialDiff.php b/src/applications/differential/storage/DifferentialDiff.php
--- a/src/applications/differential/storage/DifferentialDiff.php
+++ b/src/applications/differential/storage/DifferentialDiff.php
@@ -7,6 +7,7 @@
     PhabricatorExtendedPolicyInterface,
     HarbormasterBuildableInterface,
     HarbormasterCircleCIBuildableInterface,
+    HarbormasterBuildkiteBuildableInterface,
     PhabricatorApplicationTransactionInterface,
     PhabricatorDestructibleInterface {
 
@@ -621,6 +622,27 @@
     return $ref;
   }
 
+
+/* -(  HarbormasterBuildkiteBuildableInterface  )---------------------------- */
+
+  public function getBuildkiteBranch() {
+    $ref = $this->getStagingRef();
+
+    // NOTE: Circa late January 2017, Buildkite fails with the error message
+    // "Tags have been disabled for this project" if we pass the "refs/tags/"
+    // prefix via the API and the project doesn't have GitHub tag builds
+    // enabled, even if GitHub builds are disabled. The tag builds fine
+    // without this prefix.
+    $ref = preg_replace('(^refs/tags/)', '', $ref);
+
+    return $ref;
+  }
+
+  public function getBuildkiteCommit() {
+    return 'HEAD';
+  }
+
+
   public function getStagingRef() {
     // TODO: We're just hoping to get lucky. Instead, `arc` should store
     // where it sent changes and we should only provide staging details
diff --git a/src/applications/harbormaster/interface/HarbormasterBuildkiteBuildableInterface.php b/src/applications/harbormaster/interface/HarbormasterBuildkiteBuildableInterface.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/interface/HarbormasterBuildkiteBuildableInterface.php
@@ -0,0 +1,11 @@
+<?php
+
+/**
+ * Support for Buildkite.
+ */
+interface HarbormasterBuildkiteBuildableInterface {
+
+  public function getBuildkiteBranch();
+  public function getBuildkiteCommit();
+
+}
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
@@ -75,7 +75,7 @@
     $buildable = $build->getBuildable();
 
     $object = $buildable->getBuildableObject();
-    if (!($object instanceof HarbormasterCircleCIBuildableInterface)) {
+    if (!($object instanceof HarbormasterBuildkiteBuildableInterface)) {
       throw new Exception(
         pht('This object does not support builds with Buildkite.'));
     }
@@ -89,8 +89,8 @@
       $pipeline);
 
     $data_structure = array(
-      'commit' => $object->getCircleCIBuildIdentifier(),
-      'branch' => 'master',
+      'commit' => $object->getBuildkiteCommit(),
+      'branch' => $object->getBuildkiteBranch(),
       'message' => pht(
         'Harbormaster Build %s ("%s") for %s',
         $build->getID(),
diff --git a/src/applications/repository/storage/PhabricatorRepositoryCommit.php b/src/applications/repository/storage/PhabricatorRepositoryCommit.php
--- a/src/applications/repository/storage/PhabricatorRepositoryCommit.php
+++ b/src/applications/repository/storage/PhabricatorRepositoryCommit.php
@@ -11,6 +11,7 @@
     PhabricatorMentionableInterface,
     HarbormasterBuildableInterface,
     HarbormasterCircleCIBuildableInterface,
+    HarbormasterBuildkiteBuildableInterface,
     PhabricatorCustomFieldInterface,
     PhabricatorApplicationTransactionInterface,
     PhabricatorFulltextInterface,
@@ -590,6 +591,44 @@
   }
 
 
+/* -(  HarbormasterBuildkiteBuildableInterface  )---------------------------- */
+
+
+  public function getBuildkiteBranch() {
+    $viewer = PhabricatorUser::getOmnipotentUser();
+    $repository = $this->getRepository();
+
+    $branches = DiffusionQuery::callConduitWithDiffusionRequest(
+      $viewer,
+      DiffusionRequest::newFromDictionary(
+        array(
+          'repository' => $repository,
+          'user' => $viewer,
+        )),
+      'diffusion.branchquery',
+      array(
+        'contains' => $this->getCommitIdentifier(),
+        'repository' => $repository->getPHID(),
+      ));
+
+    if (!$branches) {
+      throw new Exception(
+        pht(
+          'Commit "%s" is not an ancestor of any branch head, so it can not '.
+          'be built with Buildkite.',
+          $this->getCommitIdentifier()));
+    }
+
+    $branch = head($branches);
+
+    return 'refs/heads/'.$branch['shortName'];
+  }
+
+  public function getBuildkiteCommit() {
+    return $this->getCommitIdentifier();
+  }
+
+
 /* -(  PhabricatorCustomFieldInterface  )------------------------------------ */