diff --git a/class.c b/class.c
index da22fd3583006a..f3ea869573a6ca 100644
--- a/class.c
+++ b/class.c
@@ -1843,6 +1843,10 @@ ensure_origin(VALUE klass)
 {
     VALUE origin = RCLASS_ORIGIN(klass);
     if (origin == klass) {
+        /* Create the box-local classext before reading m_tbl, so that the
+         * origin shares the m_tbl with the box-local iclasses of klass,
+         * as rb_prepend_module relies on that identity. */
+        rb_class_ensure_writable(klass);
         origin = class_alloc(T_ICLASS, klass);
         RCLASS_SET_M_TBL(origin, RCLASS_M_TBL(klass));
         rb_class_set_super(origin, RCLASS_SUPER(klass));
diff --git a/lib/timeout.gemspec b/lib/timeout.gemspec
index 5449494e071b2f..03e71ee430afef 100644
--- a/lib/timeout.gemspec
+++ b/lib/timeout.gemspec
@@ -26,7 +26,7 @@ Gem::Specification.new do |spec|
 
   spec.files = Dir.chdir(__dir__) do
     `git ls-files -z`.split("\x0").reject do |f|
-      (f == __FILE__) || f.match(%r{\A(?:(?:bin|test|spec|features|rakelib)/|\.(?:git|travis|circleci)|appveyor|Rakefile)})
+      (f == File.basename(__FILE__)) || f.match(%r{\A(?:(?:bin|test|spec|features|rakelib)/|\.(?:git|travis|circleci)|appveyor|Rakefile)})
     end
   end
   spec.require_paths = ["lib"]
diff --git a/test/ruby/test_box.rb b/test/ruby/test_box.rb
index 9ee932b442a632..43f13494de3782 100644
--- a/test/ruby/test_box.rb
+++ b/test/ruby/test_box.rb
@@ -203,6 +203,58 @@ def test_autoload_in_box
     assert_raise(NameError) { BOX_B }
   end
 
+  def test_autoload_dispatches_prepended_require
+    # Autoload must go through the `Kernel#require` decorations (Zeitwerk, RubyGems, etc.)
+    # of the box that registered the autoload, not `Ruby::Box#require` directly.
+    assert_separately([ENV_ENABLE_BOX], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      FEATURE = "/nonexistent/virtual_feature"
+      module Decor
+        def require(path)
+          if path == FEATURE
+            Object.const_set(:AutoloadedFromDecorator, Module.new)
+            return true
+          end
+          super
+        end
+      end
+      Kernel.prepend(Decor)
+      Object.autoload(:AutoloadedFromDecorator, FEATURE)
+      assert_kind_of Module, AutoloadedFromDecorator
+    end;
+  end
+
+  def test_autoload_dispatches_prepended_require_of_the_registered_box
+    # Even when the autoload is triggered from outside, it must be dispatched to the
+    # (decorated) `Kernel#require` of the box that registered the autoload.
+    # --enable=gems because Kernel.prepend in a box without RubyGems has a separate
+    # ancestry ordering problem, and assert_separately runs with --disable=gems.
+    assert_in_out_err([ENV_ENABLE_BOX, "--enable=gems"], "#{<<-"begin;"}\n#{<<-'end;'}") do |output, error|
+      begin;
+        box = Ruby::Box.new
+        box.eval(<<~RUBY)
+          FEATURE = "/nonexistent/box_virtual_feature"
+          module BoxDecor
+            def require(path)
+              if path == FEATURE
+                Holder.const_set(:Virtual, "decorated in \#{Ruby::Box.current.inspect}")
+                return true
+              end
+              super
+            end
+          end
+          Kernel.prepend(BoxDecor)
+          module Holder
+            autoload :Virtual, FEATURE
+          end
+        RUBY
+        puts box::Holder::Virtual
+      end;
+      assert_equal 1, output.size
+      assert_match(/\Adecorated in #<Ruby::Box:\d+,user/, output.first)
+    end
+  end
+
   def test_continuous_top_level_method_in_a_box
     setup_box
 
@@ -372,6 +424,21 @@ def test_descendants_follow_ancestors_of_the_current_box
     assert_include @box::BoxedString.ancestors, String
     assert_include String.descendants, @box::BoxedString
   end
+
+  def test_prepend_to_builtin_module_in_box
+    # Use --disable-gems to keep Kernel untouched in the new box, so that
+    # the prepend below is the first copy-on-write of Kernel's classext
+    assert_separately([ENV_ENABLE_BOX, '--disable-gems'], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      box = Ruby::Box.new
+      box.eval('module BoxDecor; def itself; :decorated; end; end; Kernel.prepend(BoxDecor)')
+      ancestors = box.eval('Object.ancestors')
+      assert_operator ancestors.index(box::BoxDecor), :<, ancestors.index(Kernel)
+      assert_equal :decorated, box.eval('Object.new.itself')
+      assert_not_include Object.ancestors, box::BoxDecor
+      assert_equal 42, 42.itself
+    end;
+  end
 end
 
 class TestBoxDescendantsMain
diff --git a/test/rubygems/test_gem_rdoc.rb b/test/rubygems/test_gem_rdoc.rb
index 9ecbb7d8c39b38..da061c74486765 100644
--- a/test/rubygems/test_gem_rdoc.rb
+++ b/test/rubygems/test_gem_rdoc.rb
@@ -4,6 +4,11 @@
 require_relative "helper"
 require "rubygems/rdoc"
 
+# RDoc resolves its own dependencies lazily, on require. Load them here, while
+# the real gem paths are still in effect, because Gem::TestCase#setup points
+# GEM_HOME at an empty temporary directory where they cannot be found.
+Gem::RDoc.load_rdoc if defined?(Gem::RDoc)
+
 class TestGemRDoc < Gem::TestCase
   def setup
     super
diff --git a/variable.c b/variable.c
index aef6405d8e91f2..564bcda62fe576 100644
--- a/variable.c
+++ b/variable.c
@@ -3164,10 +3164,16 @@ autoload_apply_constants(VALUE _arguments)
 }
 
 static VALUE
-autoload_feature_require(VALUE _arguments)
+autoload_feature_require_in_box(VALUE receiver, VALUE feature)
 {
-    VALUE receiver = rb_vm_top_self();
+    rb_vm_frame_flag_set_box_require(GET_EC());
+
+    return rb_funcall(receiver, rb_intern("require"), 1, feature);
+}
 
+static VALUE
+autoload_feature_require(VALUE _arguments)
+{
     struct autoload_load_arguments *arguments = (struct autoload_load_arguments*)_arguments;
 
     struct autoload_const *autoload_const = arguments->autoload_const;
@@ -3176,9 +3182,6 @@ autoload_feature_require(VALUE _arguments)
     // We save this for later use in autoload_apply_constants:
     arguments->autoload_data = rb_check_typeddata(autoload_const->autoload_data_value, &autoload_data_type);
 
-    if (rb_box_available() && BOX_OBJ_P(autoload_box_value))
-        receiver = autoload_box_value;
-
     /*
      * Clear the global cc cache table because the require method can be different from the current
      * box's one and it may cause inconsistent cc-cme states.
@@ -3187,7 +3190,25 @@ autoload_feature_require(VALUE _arguments)
      */
     rb_gccct_clear_table();
 
-    VALUE result = rb_funcall(receiver, rb_intern("require"), 1, arguments->autoload_data->feature);
+    VALUE feature = arguments->autoload_data->feature;
+    rb_box_t *box = NULL;
+    if (rb_box_available() && BOX_OBJ_P(autoload_box_value)) {
+        box = rb_get_box_t(autoload_box_value);
+    }
+
+    VALUE result;
+    if (box && box->top_self) {
+        /*
+         * Call `require` on the top self of the box that registered the autoload, in a frame
+         * running in that box, so that `Kernel#require` decorations in the box (RubyGems,
+         * Zeitwerk, etc.) are dispatched and the feature is loaded into that box.
+         */
+        result = rb_vm_call_cfunc_in_box(box->top_self, autoload_feature_require_in_box,
+                                         box->top_self, feature, feature, box);
+    }
+    else {
+        result = rb_funcall(rb_vm_top_self(), rb_intern("require"), 1, feature);
+    }
 
     if (RTEST(result)) {
         return rb_mutex_synchronize(autoload_mutex, autoload_apply_constants, _arguments);