Fixes #24239

actionpack/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Adds support for including ActionController::Cookies in API controllers.
+    Previously, including the module would raise when trying to define
+    a `cookies` helper method. Skip calling #helper_method if it is not
+    defined -- if we don't have helpers, we needn't define one.
+
+    Fixes #24304
+
+    *Ryan T. Hosford*
+
 *   ETags: Introduce `Response#strong_etag=` and `#weak_etag=` and analogous
     options for `fresh_when` and `stale?`. `Response#etag=` sets a weak ETag.
 

actionpack/lib/action_controller/metal/cookies.rb

@@ -3,7 +3,7 @@ module Cookies
     extend ActiveSupport::Concern
 
     included do
-      helper_method :cookies
+      helper_method :cookies if defined?(helper_method)
     end
 
     private

actionpack/test/controller/api/with_cookies_test.rb

@@ -0,0 +1,21 @@
+require 'abstract_unit'
+
+class WithCookiesController < ActionController::API
+  include ActionController::Cookies
+
+  def with_cookies
+    render plain: cookies[:foobar]
+  end
+end
+
+class WithCookiesTest < ActionController::TestCase
+  tests WithCookiesController
+
+  def test_with_cookies
+    request.cookies[:foobar] = 'bazbang'
+
+    get :with_cookies
+
+    assert_equal 'bazbang', response.body
+  end
+end