diff --git a/scripts/init/init-arcanist.php b/scripts/init/init-arcanist.php
--- a/scripts/init/init-arcanist.php
+++ b/scripts/init/init-arcanist.php
@@ -75,6 +75,17 @@
 
     $unconsumed_argv = $args->getUnconsumedArgumentVector();
 
+    if (!$unconsumed_argv) {
+      // TOOLSETS: This means the user just ran "arc" or some other top-level
+      // toolset without any workflow argument. We should give them a summary
+      // of the toolset, a list of workflows, and a pointer to "arc help" for
+      // more details.
+
+      // A possible exception is "arc --help", which should perhaps pass
+      // through and act like "arc help".
+      throw new PhutilArgumentUsageException(pht('Choose a workflow!'));
+    }
+
     $result = $this->resolveAliases($workflows, $unconsumed_argv, $config);
     if (is_int($result)) {
       return $result;
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
@@ -187,6 +187,7 @@
     'ArcanistHLintLinter' => 'lint/linter/ArcanistHLintLinter.php',
     'ArcanistHLintLinterTestCase' => 'lint/linter/__tests__/ArcanistHLintLinterTestCase.php',
     'ArcanistHardpointLoader' => 'loader/ArcanistHardpointLoader.php',
+    'ArcanistHelpWorkflow' => 'toolset/ArcanistHelpWorkflow.php',
     'ArcanistHexadecimalNumericScalarCasingXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistHexadecimalNumericScalarCasingXHPASTLinterRule.php',
     'ArcanistHexadecimalNumericScalarCasingXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistHexadecimalNumericScalarCasingXHPASTLinterRuleTestCase.php',
     'ArcanistHgClientChannel' => 'hgdaemon/ArcanistHgClientChannel.php',
@@ -639,6 +640,7 @@
     'ArcanistHLintLinter' => 'ArcanistExternalLinter',
     'ArcanistHLintLinterTestCase' => 'ArcanistExternalLinterTestCase',
     'ArcanistHardpointLoader' => 'Phobject',
+    'ArcanistHelpWorkflow' => 'ArcanistWorkflow',
     'ArcanistHexadecimalNumericScalarCasingXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistHexadecimalNumericScalarCasingXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistHgClientChannel' => 'PhutilProtocolChannel',
diff --git a/src/toolset/ArcanistAliasWorkflow.php b/src/toolset/ArcanistAliasWorkflow.php
--- a/src/toolset/ArcanistAliasWorkflow.php
+++ b/src/toolset/ArcanistAliasWorkflow.php
@@ -132,7 +132,7 @@
 
   public static function resolveAliases(
     $command,
-    ArcanistConfiguration $config,
+    ArcanistRuntime $config,
     array $argv,
     ArcanistConfigurationManager $configuration_manager) {
 
diff --git a/src/toolset/ArcanistHelpWorkflow.php b/src/toolset/ArcanistHelpWorkflow.php
new file mode 100644
--- /dev/null
+++ b/src/toolset/ArcanistHelpWorkflow.php
@@ -0,0 +1,13 @@
+<?php
+
+final class ArcanistHelpWorkflow extends ArcanistWorkflow {
+
+  public function getWorkflowName() {
+    return 'help';
+  }
+
+  public function newPhutilWorkflow() {
+    return new PhutilHelpArgumentWorkflow();
+  }
+
+}