3 files changed, 19 insertions, 7 deletions
diff --git a/actionpack/lib/abstract_controller/rendering.rb b/actionpack/lib/abstract_controller/rendering.rb
index 8ba2b25552..280af50a44 100644
--- a/actionpack/lib/abstract_controller/rendering.rb
+++ b/actionpack/lib/abstract_controller/rendering.rb
@@ -28,6 +28,7 @@ module AbstractController
       else
         _set_rendered_content_type rendered_format
       end
+      _set_vary_header
       self.response_body = rendered_body
     end
 
@@ -109,6 +110,9 @@ module AbstractController
     def _set_html_content_type # :nodoc:
     end
 
+    def _set_vary_header # :nodoc:
+    end
+
     def _set_rendered_content_type(format) # :nodoc:
     end
 
diff --git a/actionpack/lib/action_controller/metal/rendering.rb b/actionpack/lib/action_controller/metal/rendering.rb
index efa5de313c..fd22c4fa64 100644
--- a/actionpack/lib/action_controller/metal/rendering.rb
+++ b/actionpack/lib/action_controller/metal/rendering.rb
@@ -77,6 +77,10 @@ module ActionController
         end
       end
 
+      def _set_vary_header
+        self.headers["Vary"] = "Accept" if request.should_apply_vary_header?
+      end
+
       # Normalize arguments by catching blocks and setting them on :update.
       def _normalize_args(action = nil, options = {}, &blk)
         options = super
diff --git a/actionpack/lib/action_dispatch/http/mime_negotiation.rb b/actionpack/lib/action_dispatch/http/mime_negotiation.rb
index a2cac49082..ac0ff133eb 100644
--- a/actionpack/lib/action_dispatch/http/mime_negotiation.rb
+++ b/actionpack/lib/action_dispatch/http/mime_negotiation.rb
@@ -62,13 +62,7 @@ module ActionDispatch
 
       def formats
         fetch_header("action_dispatch.request.formats") do |k|
-          params_readable = begin
-                              parameters[:format]
-                            rescue *RESCUABLE_MIME_FORMAT_ERRORS
-                              false
-                            end
-
-          v = if params_readable
+          v = if params_readable?
             Array(Mime[parameters[:format]])
           elsif use_accept_header && valid_accept_header
             accepts
@@ -153,9 +147,19 @@ module ActionDispatch
         order.include?(Mime::ALL) ? format : nil
       end
 
+      def should_apply_vary_header?
+        !params_readable? && use_accept_header && valid_accept_header
+      end
+
       private
         BROWSER_LIKE_ACCEPTS = /,\s*\*\/\*|\*\/\*\s*,/
 
+        def params_readable? # :doc:
+          parameters[:format]
+        rescue *RESCUABLE_MIME_FORMAT_ERRORS
+          false
+        end
+
         def valid_accept_header # :doc:
           (xhr? && (accept.present? || content_mime_type)) ||
             (accept.present? && accept !~ BROWSER_LIKE_ACCEPTS)