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
@@ -913,6 +913,7 @@
     'FundInitiativeTransactionQuery' => 'applications/fund/query/FundInitiativeTransactionQuery.php',
     'FundInitiativeViewController' => 'applications/fund/controller/FundInitiativeViewController.php',
     'FundSchemaSpec' => 'applications/fund/storage/FundSchemaSpec.php',
+    'HarbormasterAppendLogConduitAPIMethod' => 'applications/harbormaster/conduit/HarbormasterAppendLogConduitAPIMethod.php',
     'HarbormasterArcLintBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterArcLintBuildStepImplementation.php',
     'HarbormasterArcUnitBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterArcUnitBuildStepImplementation.php',
     'HarbormasterAutotargetsTestCase' => 'applications/harbormaster/__tests__/HarbormasterAutotargetsTestCase.php',
@@ -982,6 +983,7 @@
     'HarbormasterController' => 'applications/harbormaster/controller/HarbormasterController.php',
     'HarbormasterDAO' => 'applications/harbormaster/storage/HarbormasterDAO.php',
     'HarbormasterExternalBuildStepGroup' => 'applications/harbormaster/stepgroup/HarbormasterExternalBuildStepGroup.php',
+    'HarbormasterFinalizeLogConduitAPIMethod' => 'applications/harbormaster/conduit/HarbormasterFinalizeLogConduitAPIMethod.php',
     'HarbormasterHTTPRequestBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterHTTPRequestBuildStepImplementation.php',
     'HarbormasterLeaseHostBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterLeaseHostBuildStepImplementation.php',
     'HarbormasterLintMessagesController' => 'applications/harbormaster/controller/HarbormasterLintMessagesController.php',
@@ -1010,6 +1012,7 @@
     'HarbormasterScratchTable' => 'applications/harbormaster/storage/HarbormasterScratchTable.php',
     'HarbormasterSendMessageConduitAPIMethod' => 'applications/harbormaster/conduit/HarbormasterSendMessageConduitAPIMethod.php',
     'HarbormasterSleepBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterSleepBuildStepImplementation.php',
+    'HarbormasterStartLogConduitAPIMethod' => 'applications/harbormaster/conduit/HarbormasterStartLogConduitAPIMethod.php',
     'HarbormasterStepAddController' => 'applications/harbormaster/controller/HarbormasterStepAddController.php',
     'HarbormasterStepDeleteController' => 'applications/harbormaster/controller/HarbormasterStepDeleteController.php',
     'HarbormasterStepEditController' => 'applications/harbormaster/controller/HarbormasterStepEditController.php',
@@ -4600,6 +4603,7 @@
     'FundInitiativeTransactionQuery' => 'PhabricatorApplicationTransactionQuery',
     'FundInitiativeViewController' => 'FundController',
     'FundSchemaSpec' => 'PhabricatorConfigSchemaSpec',
+    'HarbormasterAppendLogConduitAPIMethod' => 'HarbormasterConduitAPIMethod',
     'HarbormasterArcLintBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterArcUnitBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterAutotargetsTestCase' => 'PhabricatorTestCase',
@@ -4701,6 +4705,7 @@
     'HarbormasterController' => 'PhabricatorController',
     'HarbormasterDAO' => 'PhabricatorLiskDAO',
     'HarbormasterExternalBuildStepGroup' => 'HarbormasterBuildStepGroup',
+    'HarbormasterFinalizeLogConduitAPIMethod' => 'HarbormasterConduitAPIMethod',
     'HarbormasterHTTPRequestBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterLeaseHostBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterLintMessagesController' => 'HarbormasterController',
@@ -4729,6 +4734,7 @@
     'HarbormasterScratchTable' => 'HarbormasterDAO',
     'HarbormasterSendMessageConduitAPIMethod' => 'HarbormasterConduitAPIMethod',
     'HarbormasterSleepBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
+    'HarbormasterStartLogConduitAPIMethod' => 'HarbormasterConduitAPIMethod',
     'HarbormasterStepAddController' => 'HarbormasterController',
     'HarbormasterStepDeleteController' => 'HarbormasterController',
     'HarbormasterStepEditController' => 'HarbormasterController',
diff --git a/src/applications/harbormaster/conduit/HarbormasterAppendLogConduitAPIMethod.php b/src/applications/harbormaster/conduit/HarbormasterAppendLogConduitAPIMethod.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/conduit/HarbormasterAppendLogConduitAPIMethod.php
@@ -0,0 +1,46 @@
+<?php
+
+final class HarbormasterAppendLogConduitAPIMethod
+  extends HarbormasterConduitAPIMethod {
+
+  public function getAPIMethodName() {
+    return 'harbormaster.appendlog';
+  }
+
+  public function getMethodDescription() {
+    return pht(
+      'Appends data to a Harbormaster log that was started '.
+      'with harbormaster.startlog.');
+  }
+
+  protected function defineParamTypes() {
+    return array(
+      'buildLogPHID' => 'required phid',
+      'data' => 'required string',
+    );
+  }
+
+  protected function defineReturnType() {
+    return 'void';
+  }
+
+  protected function execute(ConduitAPIRequest $request) {
+    $viewer = $request->getUser();
+
+    $build_log_phid = $request->getValue('buildLogPHID');
+    $data = $request->getValue('data');
+
+    $build_log = id(new HarbormasterBuildLogQuery())
+      ->setViewer($viewer)
+      ->withPHIDs(array($build_log_phid))
+      ->executeOne();
+    if (!$build_log) {
+      throw new Exception(pht('No such build log!'));
+    }
+
+    $build_log->append($data);
+
+    return null;
+  }
+
+}
diff --git a/src/applications/harbormaster/conduit/HarbormasterFinalizeLogConduitAPIMethod.php b/src/applications/harbormaster/conduit/HarbormasterFinalizeLogConduitAPIMethod.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/conduit/HarbormasterFinalizeLogConduitAPIMethod.php
@@ -0,0 +1,46 @@
+<?php
+
+final class HarbormasterFinalizeLogConduitAPIMethod
+  extends HarbormasterConduitAPIMethod {
+
+  public function getAPIMethodName() {
+    return 'harbormaster.finalizelog';
+  }
+
+  public function getMethodDescription() {
+    return pht(
+      'Finalizes a Harbormaster log that was started '.
+      'with harbormaster.startlog.');
+  }
+
+  protected function defineParamTypes() {
+    return array(
+      'buildLogPHID' => 'required phid',
+      'startTimestamp' => 'required int',
+    );
+  }
+
+  protected function defineReturnType() {
+    return 'void';
+  }
+
+  protected function execute(ConduitAPIRequest $request) {
+    $viewer = $request->getUser();
+
+    $build_log_phid = $request->getValue('buildLogPHID');
+    $start = $request->getValue('startTimestamp');
+
+    $build_log = id(new HarbormasterBuildLogQuery())
+      ->setViewer($viewer)
+      ->withPHIDs(array($build_log_phid))
+      ->executeOne();
+    if (!$build_log) {
+      throw new Exception(pht('No such build log!'));
+    }
+
+    $build_log->finalize($start);
+
+    return null;
+  }
+
+}
diff --git a/src/applications/harbormaster/conduit/HarbormasterStartLogConduitAPIMethod.php b/src/applications/harbormaster/conduit/HarbormasterStartLogConduitAPIMethod.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/conduit/HarbormasterStartLogConduitAPIMethod.php
@@ -0,0 +1,54 @@
+<?php
+
+final class HarbormasterStartLogConduitAPIMethod
+  extends HarbormasterConduitAPIMethod {
+
+  public function getAPIMethodName() {
+    return 'harbormaster.startlog';
+  }
+
+  public function getMethodDescription() {
+    return pht(
+      'Creates a new log artifact against a Harbormaster build target '.
+      'and returns the log artifact PHID.');
+  }
+
+  protected function defineParamTypes() {
+    return array(
+      'buildTargetPHID' => 'required phid',
+      'logSource' => 'required string',
+      'logType' => 'required string',
+    );
+  }
+
+  protected function defineReturnType() {
+    return 'array';
+  }
+
+  protected function execute(ConduitAPIRequest $request) {
+    $viewer = $request->getUser();
+
+    $build_target_phid = $request->getValue('buildTargetPHID');
+    $log_source = $request->getValue('logSource');
+    $log_type = $request->getValue('logType');
+
+    $build_target = id(new HarbormasterBuildTargetQuery())
+      ->setViewer($viewer)
+      ->withPHIDs(array($build_target_phid))
+      ->executeOne();
+    if (!$build_target) {
+      throw new Exception(pht('No such build target!'));
+    }
+
+    $build = $build_target->getBuild();
+    $log = $build->createLog($build_target, $log_source, $log_type);
+
+    $start = $log->start();
+
+    return array(
+      'buildLogPHID' => $log->getPHID(),
+      'startTimestamp' => $start,
+    );
+  }
+
+}