Use :error_prefix instead.

lib/simple_form/form_builder.rb

@@ -212,13 +212,12 @@ module SimpleForm
     #    f.full_error :token #=> <span class="error">Token is invalid</span>
     #
     def full_error(attribute_name, options={})
-      prefix = options.delete(:prefix) || if object.class.respond_to?(:human_attribute_name)
+      options[:error_prefix] ||= if object.class.respond_to?(:human_attribute_name)
         object.class.human_attribute_name(attribute_name.to_s)
       else
         attribute_name.to_s.humanize
       end
 
-      options[:error_prefix] = prefix
       error(attribute_name, options)
     end
 

test/form_builder_test.rb

@@ -473,7 +473,7 @@ class FormBuilderTest < ActionView::TestCase
   end
 
   test 'builder should allow passing options to full error tag' do
-    with_full_error_for @user, :name, :id => 'name_error', :prefix => "Your name"
+    with_full_error_for @user, :name, :id => 'name_error', :error_prefix => "Your name"
     assert_select 'span.error#name_error', "Your name can't be blank"
   end