It seems that handler name has to be given in Symbol since Rails 7

test/string_template_test.rb

@@ -54,7 +54,7 @@ def test_locals
   end
 
   def test_render_partial
-    result = @view.render(template: 'main', handlers: 'string')
+    result = @view.render(template: 'main', handlers: :string)
     assert_equal 'hello, world!', result.strip
   end
 end