Merge pull request #2628 from giorni/fix-helper-inheritance

Fix grape mounted helpers inheritance

Author: dblock

CHANGELOG.md

@@ -8,6 +8,7 @@
 
 #### Fixes
 
+* [#2628](https://github.com/ruby-grape/grape/pull/2628): Fix helpers inheritance - [@giorni](https://github.com/giorni).
 * Your contribution here.
 
 ### 3.0.0 (2025-11-15)

lib/grape/endpoint.rb

@@ -75,7 +75,6 @@ def initialize(new_settings, options = {}, &block)
       @stream = nil
       @body = nil
       @source = block
-      @helpers = build_helpers
     end
 
     # Update our settings from a given set of stackable parameters. Used when
@@ -258,6 +257,7 @@ def lazy_initialize!
         return true if @lazy_initialized
 
         @app = options[:app] || build_stack
+        @helpers = build_helpers
         @lazy_initialized = true
       end
     end

spec/grape/api/mounted_helpers_inheritance_spec.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+describe Grape::API do
+  context 'when mounting a child API that inherits helpers from parent API' do
+    let(:child_api) do
+      Class.new(Grape::API) do
+        get '/test' do
+          parent_helper
+        end
+      end
+    end
+
+    let(:parent_api) do
+      context = self
+      Class.new(Grape::API) do
+        helpers do
+          def parent_helper
+            'parent helper value'
+          end
+        end
+
+        mount context.child_api
+      end
+    end
+
+    def app
+      parent_api
+    end
+
+    it 'inherits helpers from parent API to mounted child API' do
+      get '/test'
+      expect(last_response.status).to eq(200)
+      expect(last_response.body).to eq('parent helper value')
+    end
+  end
+end