diff --git a/lib/middleman-s3_sync/commands.rb b/lib/middleman-s3_sync/commands.rb
index 7921e1b..8004636 100644
--- a/lib/middleman-s3_sync/commands.rb
+++ b/lib/middleman-s3_sync/commands.rb
@@ -70,8 +70,10 @@ def s3_sync
         verbose = options[:verbose] ? 0 : 1
         instrument = options[:instrument]
 
+        mode = options[:build] ? :build : :config
+
         ::Middleman::S3Sync.app = ::Middleman::Application.new do
-          config[:mode] = :build
+          config[:mode] = mode
           config[:environment] = env
           ::Middleman::Logger.singleton(verbose, instrument)
         end