Stop entity autodetection from picking up a top-level ::Entity constant

CHANGELOG.md

@@ -47,6 +47,7 @@
 * [#2817](https://github.com/ruby-grape/grape/pull/2817): Remove request-time mutable state from Array and custom-type coercers - [@ericproulx](https://github.com/ericproulx).
 * [#2819](https://github.com/ruby-grape/grape/pull/2819): Freeze coercers at construction, synchronize `Grape::Util::Cache` lookups, and assign `Route#regexp_capture_index` eagerly - [@ericproulx](https://github.com/ericproulx).
 * [#2824](https://github.com/ruby-grape/grape/pull/2824): Fix cascaded routes (`X-Cascade: pass`) leaking `route_info` and path captures into the next matched route's `route` and `params` - [@ericproulx](https://github.com/ericproulx).
+* [#2825](https://github.com/ruby-grape/grape/pull/2825): Stop `present` entity autodetection from picking up a top-level `::Entity` constant - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 3.3.4 (2026-07-25)

lib/grape/dsl/entity.rb

@@ -45,7 +45,8 @@ def present(*args, root: nil, with: nil, **options)
       # Attempt to locate the Entity class for a given object, if not given
       # explicitly. This is done by looking for the presence of Klass::Entity,
       # where Klass is the class of the `object` parameter, or one of its
-      # ancestors.
+      # ancestors. Object is excluded from the search: top-level constants
+      # live on it, so a global ::Entity class is not a representer.
       # @param object [Object] the object to locate the Entity class for
       # @return [Class] the located Entity class, or nil if none is found
       def entity_class_for_obj(object)
@@ -57,9 +58,10 @@ def entity_class_for_obj(object)
           return representations[potential] if potential && representations[potential]
         end
 
-        return unless klass.const_defined?(:Entity)
+        owner = klass.ancestors.detect { |ancestor| ancestor != Object && ancestor.const_defined?(:Entity, false) }
+        return unless owner
 
-        entity = klass.const_get(:Entity)
+        entity = owner.const_get(:Entity, false)
         entity if entity.respond_to?(:represent)
       end
 

spec/integration/grape_entity/entity_spec.rb

@@ -133,6 +133,19 @@ def first
       expect(last_response.body).to eq('Auto-detect!')
     end
 
+    it 'autodetection does not use a top-level ::Entity constant' do
+      some_model = Class.new
+      entity = Class.new(Grape::Entity)
+      stub_const('Entity', entity)
+
+      subject.get '/example' do
+        present some_model.new
+      end
+
+      expect(entity).not_to receive(:represent)
+      get '/example'
+    end
+
     it 'autodetection does not use Entity if it is not a presenter' do
       some_model = Class.new
       entity = Class.new