diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 3f7cf0950d..b618b9c400 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -664,7 +664,6 @@ module ActionDispatch
           def define_generate_prefix(app, name)
             _route = @set.named_routes.get name
             _routes = @set
-            _url_helpers = @set.url_helpers
 
             script_namer = ->(options) do
               prefix_options = options.slice(*_route.segment_keys)
@@ -676,7 +675,7 @@ module ActionDispatch
 
               # We must actually delete prefix segment keys to avoid passing them to next url_for.
               _route.segment_keys.each { |k| options.delete(k) }
-              _url_helpers.send("#{name}_path", prefix_options)
+              @set.url_helpers.send("#{name}_path", prefix_options)
             end
 
             app.routes.define_mounted_helper(name, script_namer)
diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index acce8a7ef3..8f21722a6a 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -378,6 +378,7 @@ module ActionDispatch
         @disable_clear_and_finalize = false
         @finalized                  = false
         @env_key                    = "ROUTES_#{object_id}_SCRIPT_NAME".freeze
+        @url_helpers                = nil
 
         @set    = Journey::Routes.new
         @router = Journey::Router.new @set
@@ -437,6 +438,7 @@ module ActionDispatch
         return if @finalized
         @append.each { |blk| eval_block(blk) }
         @finalized = true
+        @url_helpers = build_url_helper_module true
       end
 
       def clear!
@@ -465,11 +467,10 @@ module ActionDispatch
         return if MountedHelpers.method_defined?(name)
 
         routes = self
-        helpers = routes.url_helpers
 
         MountedHelpers.class_eval do
           define_method "_#{name}" do
-            RoutesProxy.new(routes, _routes_context, helpers, script_namer)
+            RoutesProxy.new(routes, _routes_context, routes.url_helpers, script_namer)
           end
         end
 
@@ -480,7 +481,20 @@ module ActionDispatch
         RUBY
       end
 
+      class UnfinalizedRouteSet < StandardError
+      end
+
       def url_helpers(supports_path = true)
+        raise UnfinalizedRouteSet, "routes have not been finalized. Please call `finalize!` or use `draw(&block)`" unless @finalized
+
+        if supports_path
+          @url_helpers
+        else
+          build_url_helper_module false
+        end
+      end
+
+      def build_url_helper_module(supports_path)
         routes = self
 
         Module.new do
diff --git a/actionpack/lib/action_dispatch/testing/assertions/routing.rb b/actionpack/lib/action_dispatch/testing/assertions/routing.rb
index af41521c5c..0e8712f8d9 100644
--- a/actionpack/lib/action_dispatch/testing/assertions/routing.rb
+++ b/actionpack/lib/action_dispatch/testing/assertions/routing.rb
@@ -138,6 +138,20 @@ module ActionDispatch
         assert_generates(path.is_a?(Hash) ? path[:path] : path, generate_options, defaults, extras, message)
       end
 
+      # Provides a hook on `finalize!` so we can mutate a controller after the
+      # route set has been drawn.
+      class WithRouting < ActionDispatch::Routing::RouteSet # :nodoc:
+        def initialize(&block)
+          super()
+          @block = block
+        end
+
+        def finalize!
+          super
+          @block.call self
+        end
+      end
+
       # A helper to make it easier to test different route configurations.
       # This method temporarily replaces @routes with a new RouteSet instance.
       #
@@ -152,16 +166,19 @@ module ActionDispatch
       #   end
       #
       def with_routing
-        old_routes, @routes = @routes, ActionDispatch::Routing::RouteSet.new
-        if defined?(@controller) && @controller
-          old_controller, @controller = @controller, @controller.clone
-          _routes = @routes
+        old_routes = @routes
+        old_controller = nil
+        @routes = WithRouting.new do |_routes|
+          if defined?(@controller) && @controller
+            old_controller, @controller = @controller, @controller.clone
+            _routes = @routes
 
-          @controller.singleton_class.include(_routes.url_helpers)
+            @controller.singleton_class.include(_routes.url_helpers)
 
-          if @controller.respond_to? :view_context_class
-            @controller.view_context_class = Class.new(@controller.view_context_class) do
-              include _routes.url_helpers
+            if @controller.respond_to? :view_context_class
+              @controller.view_context_class = Class.new(@controller.view_context_class) do
+                include _routes.url_helpers
+              end
             end
           end
         end
diff --git a/actionpack/test/abstract_unit.rb b/actionpack/test/abstract_unit.rb
index 65dd28b3d7..7c9f15108d 100644
--- a/actionpack/test/abstract_unit.rb
+++ b/actionpack/test/abstract_unit.rb
@@ -100,7 +100,10 @@ end
 
 class ActionDispatch::IntegrationTest < ActiveSupport::TestCase
   def self.build_app(routes = nil)
-    RoutedRackApp.new(routes || ActionDispatch::Routing::RouteSet.new) do |middleware|
+    routes ||= ActionDispatch::Routing::RouteSet.new.tap { |rs|
+      rs.draw { }
+    }
+    RoutedRackApp.new(routes) do |middleware|
       middleware.use ActionDispatch::ShowExceptions, ActionDispatch::PublicExceptions.new("#{FIXTURE_LOAD_PATH}/public")
       middleware.use ActionDispatch::DebugExceptions
       middleware.use ActionDispatch::Callbacks
diff --git a/actionpack/test/controller/integration_test.rb b/actionpack/test/controller/integration_test.rb
index 39ede1442a..b078e8ad9f 100644
--- a/actionpack/test/controller/integration_test.rb
+++ b/actionpack/test/controller/integration_test.rb
@@ -542,9 +542,6 @@ class IntegrationProcessTest < ActionDispatch::IntegrationTest
     def with_test_route_set
       with_routing do |set|
         controller = ::IntegrationProcessTest::IntegrationController.clone
-        controller.class_eval do
-          include set.url_helpers
-        end
 
         set.draw do
           get "moved" => redirect("/method")
@@ -555,6 +552,10 @@ class IntegrationProcessTest < ActionDispatch::IntegrationTest
           end
         end
 
+        controller.class_eval do
+          include set.url_helpers
+        end
+
         singleton_class.include(set.url_helpers)
 
         yield
diff --git a/actionpack/test/dispatch/routing/ipv6_redirect_test.rb b/actionpack/test/dispatch/routing/ipv6_redirect_test.rb
index 31559bffc7..fb423d2951 100644
--- a/actionpack/test/dispatch/routing/ipv6_redirect_test.rb
+++ b/actionpack/test/dispatch/routing/ipv6_redirect_test.rb
@@ -4,6 +4,11 @@ require "abstract_unit"
 
 class IPv6IntegrationTest < ActionDispatch::IntegrationTest
   Routes = ActionDispatch::Routing::RouteSet.new
+  Routes.draw do
+    get "/",    to: "bad_route_request#index", as: :index
+    get "/foo", to: "bad_route_request#foo", as: :foo
+  end
+
   include Routes.url_helpers
 
   class ::BadRouteRequestController < ActionController::Base
@@ -17,11 +22,6 @@ class IPv6IntegrationTest < ActionDispatch::IntegrationTest
     end
   end
 
-  Routes.draw do
-    get "/",    to: "bad_route_request#index", as: :index
-    get "/foo", to: "bad_route_request#foo", as: :foo
-  end
-
   def _routes
     Routes
   end
diff --git a/actionpack/test/dispatch/url_generation_test.rb b/actionpack/test/dispatch/url_generation_test.rb
index aef9351de1..b0096d26be 100644
--- a/actionpack/test/dispatch/url_generation_test.rb
+++ b/actionpack/test/dispatch/url_generation_test.rb
@@ -4,16 +4,13 @@ require "abstract_unit"
 
 module TestUrlGeneration
   class WithMountPoint < ActionDispatch::IntegrationTest
-    Routes = ActionDispatch::Routing::RouteSet.new
-    include Routes.url_helpers
-
     class ::MyRouteGeneratingController < ActionController::Base
-      include Routes.url_helpers
       def index
         render plain: foo_path
       end
     end
 
+    Routes = ActionDispatch::Routing::RouteSet.new
     Routes.draw do
       get "/foo", to: "my_route_generating#index", as: :foo
 
@@ -22,6 +19,10 @@ module TestUrlGeneration
       mount MyRouteGeneratingController.action(:index), at: "/bar"
     end
 
+    class ::MyRouteGeneratingController
+      include Routes.url_helpers
+    end
+
     APP = build_app Routes
 
     def _routes