diff --git a/middleman-core/lib/middleman-core/cli/build.rb b/middleman-core/lib/middleman-core/cli/build.rb
index 2d9e0ded..9f7f6024 100644
--- a/middleman-core/lib/middleman-core/cli/build.rb
+++ b/middleman-core/lib/middleman-core/cli/build.rb
@@ -99,10 +99,7 @@ module Middleman::Cli
       #
       # @return [Rack::Test::Session]
       def shared_rack
-        @_shared_rack ||= begin
-          mock = ::Rack::MockSession.new(shared_server.to_rack_app)
-          ::Rack::Test::Session.new(mock)
-        end
+        @_shared_rack ||= ::Rack::Test::Session.new(shared_server.to_rack_app)
       end
 
       # Set the root path to the Middleman::Application's root
diff --git a/middleman-core/lib/middleman-core/core_extensions/request.rb b/middleman-core/lib/middleman-core/core_extensions/request.rb
index 9f750e16..237eeb2e 100644
--- a/middleman-core/lib/middleman-core/core_extensions/request.rb
+++ b/middleman-core/lib/middleman-core/core_extensions/request.rb
@@ -37,7 +37,6 @@ module Middleman
         # @private
         def reset!
           @app = nil
-          @prototype = nil
         end
 
         # The shared Rack instance being build
@@ -94,7 +93,8 @@ module Middleman
         # @private
         # @return [Rack::Builder]
         def prototype
-          @prototype ||= to_rack_app
+          reset!
+          to_rack_app
         end
 
         # Call prototype, use in config.ru
diff --git a/middleman-more/features/asset_hash.feature b/middleman-more/features/asset_hash.feature
index b5945ba5..622e09ba 100644
--- a/middleman-more/features/asset_hash.feature
+++ b/middleman-more/features/asset_hash.feature
@@ -99,3 +99,8 @@ Feature: Assets get a file hash appended to their and references to them are upd
     Given the Server is running at "asset-hash-app"
     When I go to "/"
     Then I should see 'href="stylesheets/site-5770af52.css'
+    When I go to "stylesheets/site-5770af52.css"
+    Then I should see 'background-image'
+    Then I should see 'Added by Rack filter'
+    When I go to "stylesheets/site-50eaa978.css"
+    Then I should see 'Not Found'
diff --git a/middleman-more/lib/middleman-more/extensions/asset_hash.rb b/middleman-more/lib/middleman-more/extensions/asset_hash.rb
index 7d03535c..6605c754 100644
--- a/middleman-more/lib/middleman-more/extensions/asset_hash.rb
+++ b/middleman-more/lib/middleman-more/extensions/asset_hash.rb
@@ -36,7 +36,12 @@ module Middleman
             next if @ignore.any? { |ignore| Middleman::Util.path_match(ignore, resource.destination_path) }
 
             if resource.template? # if it's a template, render it out
-              digest = Digest::SHA1.hexdigest(resource.render)[0..7]
+              # Render through the Rack interface so middleware and mounted apps get a shot
+              rack_client = ::Rack::Test::Session.new(@app.class)
+              response = rack_client.get(URI.escape(resource.destination_path), {}, { "bypass_asset_hash" => true })
+              raise "#{resource.path} should be in the sitemap!" unless response.status == 200
+
+              digest = Digest::SHA1.hexdigest(response.body)[0..7]
             else # if it's a static file, just hash it
               digest = Digest::SHA1.file(resource.source_file).hexdigest[0..7]
             end
@@ -60,6 +65,9 @@ module Middleman
         def call(env)
           status, headers, response = @rack_app.call(env)
 
+          # We don't want to use this middleware when rendering files to figure out their hash!
+          return [status, headers, response] if env["bypass_asset_hash"]
+
           path = @middleman_app.full_path(env["PATH_INFO"])
           dirpath = Pathname.new(File.dirname(path))