diff --git a/test/test_router_namespace.rb b/test/test_router_namespace.rb
new file mode 100644
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+++ b/test/test_router_namespace.rb
@@ -0,0 +1,186 @@
+require 'minitest/autorun'
+require 'prism'
+require 'support'
+
+module Admin
+  class DashboardController < Prism::Controller
+    def index
+      html('admin dashboard')
+    end
+  end
+
+  class WidgetsController < Prism::Controller
+    def index
+      html('admin widgets index')
+    end
+
+    def show
+      html("admin widget #{params['id']}")
+    end
+  end
+
+  class GadgetsController < Prism::Controller
+    def index
+      html('admin gadgets under widget ' + params['widget_id'])
+    end
+  end
+
+  # Lives directly under Admin (not Admin::LegacyUi) - proves that
+  # `module: false` on an INNER namespace only skips its own module
+  # contribution, while the enclosing `admin` module still applies.
+  class ArchivedController < Prism::Controller
+    def index
+      html('admin archived (legacy_ui contributed no module)')
+    end
+  end
+
+  # Registered AFTER the `legacy_ui, module: false` block closes (see
+  # NamespacedApp below) - only resolves correctly if closing that block
+  # popped exactly its own frame off module_stack, leaving ["admin"]
+  # behind for this route instead of popping "admin" too.
+  class AuditsController < Prism::Controller
+    def index
+      html('admin audits after legacy_ui closed')
+    end
+  end
+end
+
+module Api
+  module V1
+    class ReportsController < Prism::Controller
+      def index
+        html('api v1 reports')
+      end
+    end
+  end
+end
+
+module Legacy
+  class DashboardController < Prism::Controller
+    def index
+      html('legacy ping')
+    end
+  end
+end
+
+# Deliberately NOT inside a `Settings` module - this is exactly what
+# `module: false` is for: a URL that looks namespaced without forcing the
+# controller into a matching Ruby module/directory.
+class ProfilesController < Prism::Controller
+  def show
+    html("moduleless profile #{params['id']}")
+  end
+end
+
+class NamespacedApp < Prism::Application
+  namespace :admin do
+    root to: 'dashboard#index'
+    resources :widgets do
+      resources :gadgets, only: [:index]
+    end
+
+    namespace :legacy_ui, module: false do
+      resources :archived, only: [:index]
+    end
+
+    resources :audits, only: [:index]
+  end
+
+  namespace :api do
+    namespace :v1 do
+      resources :reports, only: [:index]
+    end
+  end
+
+  namespace :legacy, path: '/old' do
+    get '/ping', to: 'dashboard#index', as: :ping
+  end
+
+  namespace :settings, module: false do
+    resources :profiles, only: [:show]
+  end
+end
+
+class RouterNamespaceTest < Minitest::Test
+  include TestSupport
+
+  def call(method, path, **opts)
+    NamespacedApp.call(env_for(method, path, **opts))
+  end
+
+  def test_namespaced_root_dispatches_to_nested_module_controller
+    status, _, body = call('GET', '/admin/')
+    assert_equal 200, status
+    assert_equal 'admin dashboard', body.join
+  end
+
+  def test_namespaced_resources_dispatch_to_nested_module_controller
+    status, _, body = call('GET', '/admin/widgets')
+    assert_equal 200, status
+    assert_equal 'admin widgets index', body.join
+
+    _, _, body = call('GET', '/admin/widgets/7')
+    assert_equal 'admin widget 7', body.join
+  end
+
+  def test_nested_resources_under_namespace_carry_the_parent_id
+    _, _, body = call('GET', '/admin/widgets/3/gadgets')
+    assert_equal 'admin gadgets under widget 3', body.join
+  end
+
+  def test_double_nested_namespace_dispatches_to_double_nested_module
+    status, _, body = call('GET', '/api/v1/reports')
+    assert_equal 200, status
+    assert_equal 'api v1 reports', body.join
+  end
+
+  def test_named_routes_are_prefixed_with_the_namespace_name
+    assert_equal '/admin/widgets', NamespacedApp.router.path_to(:admin_widgets)
+    assert_equal '/admin/widgets/7', NamespacedApp.router.path_to(:admin_widget, id: 7)
+    assert_equal '/admin/', NamespacedApp.router.path_to(:admin_root)
+  end
+
+  def test_nested_namespace_names_join_with_underscores
+    assert_equal '/api/v1/reports', NamespacedApp.router.path_to(:api_v1_reports)
+  end
+
+  def test_namespace_accepts_a_custom_path_independent_of_the_name
+    status, _, body = call('GET', '/old/ping')
+    assert_equal 200, status
+    assert_equal 'legacy ping', body.join
+    assert_equal '/old/ping', NamespacedApp.router.path_to(:legacy_ping)
+  end
+
+  def test_controller_path_keeps_the_full_module_for_view_resolution
+    assert_equal 'admin/widgets', Admin::WidgetsController.controller_path
+    assert_equal 'api/v1/reports', Api::V1::ReportsController.controller_path
+  end
+
+  def test_module_false_keeps_the_url_prefix_but_dispatches_to_a_top_level_controller
+    status, _, body = call('GET', '/settings/profiles/9')
+    assert_equal 200, status
+    assert_equal 'moduleless profile 9', body.join
+  end
+
+  def test_module_false_still_prefixes_the_route_name
+    assert_equal '/settings/profiles/9', NamespacedApp.router.path_to(:settings_profile, id: 9)
+  end
+
+  def test_nested_module_false_keeps_the_enclosing_module_but_skips_its_own
+    status, _, body = call('GET', '/admin/legacy_ui/archived')
+    assert_equal 200, status
+    assert_equal 'admin archived (legacy_ui contributed no module)', body.join
+    assert_equal '/admin/legacy_ui/archived', NamespacedApp.router.path_to(:admin_legacy_ui_archived)
+  end
+
+  def test_module_false_does_not_leak_into_a_route_declared_after_it_closes
+    # Regression guard for the `ensure` block: if `module_stack.pop` ran
+    # unconditionally (instead of only `if use_module`) it would pop one
+    # frame too many once `legacy_ui`'s block above closes, silently
+    # popping "admin" too - this route (declared right after) would then
+    # resolve to a top-level AuditsController instead of Admin::Audits.
+    status, _, body = call('GET', '/admin/audits')
+    assert_equal 200, status
+    assert_equal 'admin audits after legacy_ui closed', body.join
+  end
+end