diff --git a/.github/workflows/rubygems.yml b/.github/workflows/rubygems.yml
index c7419c954fb8..19c1273d4ebe 100644
--- a/.github/workflows/rubygems.yml
+++ b/.github/workflows/rubygems.yml
@@ -60,6 +60,10 @@ jobs:
           - ruby: { name: no symlinks, value: 4.0.5 }
             os: { name: Windows, value: windows-2025 }
             symlink: off
+
+          - ruby: { name: head (RUBY_BOX=1), value: head }
+            os: { name: Ubuntu, value: ubuntu-24.04 }
+            ruby_box: true
     env:
       RUBYGEMS_USE_PSYCH: ${{ matrix.use_psych || 'false' }}
 
@@ -79,7 +83,10 @@ jobs:
         run: bin/rake setup
       - name: Run Test
         run: bin/rake test
-        if: matrix.ruby.name != 'truffleruby' && matrix.ruby.name != 'jruby' && matrix.symlink != 'off'
+        if: matrix.ruby.name != 'truffleruby' && matrix.ruby.name != 'jruby' && matrix.symlink != 'off' && !matrix.ruby_box
+      - name: Run Test (RUBY_BOX=1)
+        run: RUBY_BOX=1 bin/rake test
+        if: matrix.ruby_box
       - name: Run Test isolatedly
         run: bin/rake test:isolated
         if: matrix.ruby.name == '3.4' && matrix.os.name != 'Windows'
diff --git a/lib/bundler/gem_helper.rb b/lib/bundler/gem_helper.rb
index e3af1b957f35..ea02e7e7712b 100644
--- a/lib/bundler/gem_helper.rb
+++ b/lib/bundler/gem_helper.rb
@@ -216,7 +216,7 @@ def sh_with_status(cmd, &block)
       Bundler.ui.debug(cmd)
       SharedHelpers.chdir(base) do
         outbuf = IO.popen(cmd, err: [:child, :out], &:read)
-        status = $?
+        status = Process.last_status
         block&.call(outbuf) if status.success?
         [outbuf, status]
       end
diff --git a/lib/rubygems/ext/builder.rb b/lib/rubygems/ext/builder.rb
index b7e80d6b06bb..691bc2825da0 100644
--- a/lib/rubygems/ext/builder.rb
+++ b/lib/rubygems/ext/builder.rb
@@ -101,7 +101,9 @@ def self.run(command, results, command_name = nil, dir = Dir.pwd, env = {})
 
       require "open3"
       # Set $SOURCE_DATE_EPOCH for the subprocess.
-      # Under Ruby::Box mkmf makes RbConfig.expand recurse until SystemStackError.
+      # Under Ruby::Box defined?($gvar) does not see assignments made inside the
+      # box, so mkmf have_devel? never memoizes and recurses until SystemStackError
+      # (https://bugs.ruby-lang.org/issues/22283).
       # Drop $RUBY_BOX last so no caller can restore it.
       build_env = { "SOURCE_DATE_EPOCH" => Gem.source_date_epoch_string }.merge(env).merge("RUBY_BOX" => nil)
       # A single-element command would be parsed as a shell command line,
diff --git a/lib/rubygems/source/git.rb b/lib/rubygems/source/git.rb
index baf2f9dd4c09..0ceb67b382f5 100644
--- a/lib/rubygems/source/git.rb
+++ b/lib/rubygems/source/git.rb
@@ -188,9 +188,11 @@ def rev_parse # :nodoc:
       hash = Gem::Util.popen(git_command, "rev-parse", @reference).strip
     end
 
+    # Process.last_status instead of $?, which Ruby::Box leaves uninitialized
+    # (https://bugs.ruby-lang.org/issues/22280)
     raise Gem::Exception,
           "unable to find reference #{@reference} in #{@repository}" unless
-            $?.success?
+            Process.last_status.success?
 
     hash
   end
diff --git a/spec/bundler/shared_helpers_spec.rb b/spec/bundler/shared_helpers_spec.rb
index 1619b0a14a59..232da4143a24 100644
--- a/spec/bundler/shared_helpers_spec.rb
+++ b/spec/bundler/shared_helpers_spec.rb
@@ -387,7 +387,11 @@
 
       before do
         ENV["RUBYOPT"] = "-r#{install_path}/bundler/setup"
-        allow(File).to receive(:expand_path).and_return("#{install_path}/bundler/setup")
+        # Only fake the resolution of bundler/setup itself. A blanket stub
+        # breaks unrelated RubyGems path lookups triggered lazily inside the
+        # example, see #set_rubyopt.
+        allow(File).to receive(:expand_path).and_call_original
+        allow(File).to receive(:expand_path).with("setup", anything).and_return("#{install_path}/bundler/setup")
         allow(Gem).to  receive(:bin_path).and_return("#{install_path}/bundler/setup")
       end
 
@@ -403,7 +407,8 @@
       let(:install_path) { "/opt/ruby with space/lib" }
 
       before do
-        allow(File).to receive(:expand_path).and_return("#{install_path}/bundler/setup")
+        allow(File).to receive(:expand_path).and_call_original
+        allow(File).to receive(:expand_path).with("setup", anything).and_return("#{install_path}/bundler/setup")
         allow(Gem).to receive(:bin_path).and_return("#{install_path}/bundler/setup")
       end
 
diff --git a/spec/commands/install_spec.rb b/spec/commands/install_spec.rb
index d4b88902e21e..b5e4a9aa76c7 100644
--- a/spec/commands/install_spec.rb
+++ b/spec/commands/install_spec.rb
@@ -2065,6 +2065,10 @@ def gem_make_out
   end
 
   it "preserves bundled native extensions when BUNDLE_CLEAN removes another gem" do
+    # The command-line/RUBYOPT -r bypasses gem activation under RUBY_BOX=1
+    # (https://bugs.ruby-lang.org/issues/22295)
+    skip "-r cannot activate gems under Ruby::Box" if defined?(Ruby::Box) && Ruby::Box.enabled?
+
     build_repo4 do
       build_gem "native_child", "1.0", &:add_c_extension
       build_gem "native_parent", "1.0" do |s|
diff --git a/spec/support/command_execution.rb b/spec/support/command_execution.rb
index e2915b996d9d..14da54dca4da 100644
--- a/spec/support/command_execution.rb
+++ b/spec/support/command_execution.rb
@@ -2,6 +2,13 @@
 
 module Spec
   class CommandExecution
+    # Under RUBY_BOX, every spawned ruby prints an experimental warning to
+    # stderr, breaking specs that assert clean stderr.
+    RUBY_BOX_WARNING = Regexp.union(
+      /^[^\n]*: warning: Ruby::Box is experimental, and the behavior may change in the future!\n?/,
+      %r{^See https://docs\.ruby-lang\.org/\S+ for known issues, etc\.\n?}
+    )
+
     def initialize(command, timeout:)
       @command = command
       @timeout = timeout
@@ -72,7 +79,13 @@ def failure?
     attr_reader :failure_reason
 
     def normalize(string)
-      string.dup.force_encoding(Encoding::UTF_8).scrub.strip.gsub("\r\n", "\n")
+      string = string.dup.force_encoding(Encoding::UTF_8).scrub.gsub("\r\n", "\n")
+      string = string.gsub(RUBY_BOX_WARNING, "") if ruby_box_enabled?
+      string.strip
+    end
+
+    def ruby_box_enabled?
+      defined?(Ruby::Box) && Ruby::Box.enabled?
     end
   end
 end
diff --git a/test/rubygems/helper.rb b/test/rubygems/helper.rb
index 09b7f427ebbf..c478bb86d8ba 100644
--- a/test/rubygems/helper.rb
+++ b/test/rubygems/helper.rb
@@ -404,6 +404,10 @@ def setup
     ENV["BUNDLE_COOLDOWN"] = nil
     ENV["RUBYGEMS_PREVENT_UPDATE_SUGGESTION"] = "true"
 
+    # Child ruby processes inherit RUBY_BOX and print an experimental
+    # warning on startup, breaking assertions on subprocess stderr.
+    ENV["RUBYOPT"] = [ENV["RUBYOPT"], "-W:no-experimental"].compact.join(" ") if ruby_box_enabled?
+
     @current_dir = Dir.pwd
     @fetcher     = nil
 
@@ -1440,6 +1444,32 @@ def ruby_repo?
     !ENV["GEM_COMMAND"].nil?
   end
 
+  ##
+  # Is this test running under Ruby::Box (RUBY_BOX=1)?
+
+  def ruby_box_enabled?
+    defined?(Ruby::Box) && Ruby::Box.enabled?
+  end
+
+  ##
+  # Ruby::Box gives each box detached copies of the stdio globals, so
+  # reassigning $stdout/$stderr cannot capture output written by Kernel#warn,
+  # Kernel#puts or subprocesses. Pends until the ruby-core fix for
+  # https://bugs.ruby-lang.org/issues/21867 lands.
+
+  def pend_for_ruby_box_stdio_capture
+    pend "Ruby::Box breaks $stdout/$stderr capture (https://bugs.ruby-lang.org/issues/21867)" if ruby_box_enabled?
+  end
+
+  ##
+  # Under Ruby::Box, Marshal in the main box cannot resolve Gem:: (and other
+  # boxed) constants. Pends until the ruby-core fix for
+  # https://bugs.ruby-lang.org/issues/22090 lands.
+
+  def pend_for_ruby_box_marshal
+    pend "Marshal cannot resolve boxed constants under Ruby::Box (https://bugs.ruby-lang.org/issues/22090)" if ruby_box_enabled?
+  end
+
   ##
   # Returns the make command for the current platform. For versions of Ruby
   # built on MS Windows with VC++ or Borland it will return 'nmake'. On all
diff --git a/test/rubygems/test_deprecate.rb b/test/rubygems/test_deprecate.rb
index bb6a0b5ceaaf..5700d356e756 100644
--- a/test/rubygems/test_deprecate.rb
+++ b/test/rubygems/test_deprecate.rb
@@ -132,6 +132,7 @@ def test_deprecated_method_calls_the_old_method
   end
 
   def test_deprecated_method_outputs_a_warning
+    pend_for_ruby_box_stdio_capture
     out, err = capture_output do
       thing = Thing.new
       thing.foo
@@ -165,6 +166,7 @@ def execute
   end
 
   def test_deprecated_method_outputs_a_warning_old_way
+    pend_for_ruby_box_stdio_capture
     out, err = capture_output do
       thing = OtherThing.new
       thing.foo
@@ -180,6 +182,7 @@ def test_deprecated_method_outputs_a_warning_old_way
   end
 
   def test_deprecated_method_when_class_overrides_format
+    pend_for_ruby_box_stdio_capture
     out, err = capture_output do
       thing = ThingWithFormat.new
       thing.foo
diff --git a/test/rubygems/test_exit.rb b/test/rubygems/test_exit.rb
index 396837edadfa..c339c39a67af 100644
--- a/test/rubygems/test_exit.rb
+++ b/test/rubygems/test_exit.rb
@@ -6,7 +6,8 @@
 class TestGemExit < Gem::TestCase
   def test_exit
     system(*ruby_with_rubygems_in_load_path, "-e", "raise Gem::SystemExitException.new(2)")
-    assert_equal 2, $?.exitstatus
+    # Process.last_status instead of $?, which Ruby::Box leaves uninitialized
+    assert_equal 2, Process.last_status.exitstatus
   end
 
   def test_status
diff --git a/test/rubygems/test_gem.rb b/test/rubygems/test_gem.rb
index 88b461a2a0a7..36067549c863 100644
--- a/test/rubygems/test_gem.rb
+++ b/test/rubygems/test_gem.rb
@@ -1297,6 +1297,7 @@ def test_self_try_activate_missing_prerelease
   end
 
   def test_self_try_activate_missing_extensions
+    pend_for_ruby_box_stdio_capture
     spec = util_spec "ext", "1" do |s|
       s.extensions = %w[ext/extconf.rb]
       s.installed_by_version = v("2.2")
@@ -1352,6 +1353,7 @@ def test_setting_paths_does_not_mutate_parameter_object
   end
 
   def test_deprecated_paths=
+    pend_for_ruby_box_stdio_capture
     stdout, stderr = capture_output do
       Gem.paths = { "GEM_HOME" => Gem.paths.home,
                     "GEM_PATH" => [Gem.paths.home, "foo"] }
diff --git a/test/rubygems/test_gem_commands_build_command.rb b/test/rubygems/test_gem_commands_build_command.rb
index 771eb07dbc9c..5bbb7c3b3e35 100644
--- a/test/rubygems/test_gem_commands_build_command.rb
+++ b/test/rubygems/test_gem_commands_build_command.rb
@@ -383,6 +383,7 @@ def test_execute_strict_with_warnings
   end
 
   def test_execute_bad_spec
+    pend_for_ruby_box_stdio_capture
     @gem.date = "2010-11-08"
 
     gemspec_file = File.join(@tempdir, @gem.spec_name)
diff --git a/test/rubygems/test_gem_commands_open_command.rb b/test/rubygems/test_gem_commands_open_command.rb
index 3a774a9343c0..c30117a58ecc 100644
--- a/test/rubygems/test_gem_commands_open_command.rb
+++ b/test/rubygems/test_gem_commands_open_command.rb
@@ -21,6 +21,7 @@ def gem(name, version = "1.0")
   end
 
   def test_execute
+    pend_for_ruby_box_stdio_capture
     omit "JRuby on Windows spawns the editor with a different cwd" if Gem.win_platform? && Gem.java_platform?
 
     @cmd.options[:args] = %w[foo]
diff --git a/test/rubygems/test_gem_commands_specification_command.rb b/test/rubygems/test_gem_commands_specification_command.rb
index 454d6ea1c6f7..ee96eaa892a5 100644
--- a/test/rubygems/test_gem_commands_specification_command.rb
+++ b/test/rubygems/test_gem_commands_specification_command.rb
@@ -137,6 +137,7 @@ def test_execute_file
   end
 
   def test_execute_marshal
+    pend_for_ruby_box_marshal
     foo = util_spec "foo", "2"
 
     install_specs foo
diff --git a/test/rubygems/test_gem_config_file.rb b/test/rubygems/test_gem_config_file.rb
index 2c33192a4b3a..7120c49e327a 100644
--- a/test/rubygems/test_gem_config_file.rb
+++ b/test/rubygems/test_gem_config_file.rb
@@ -314,6 +314,7 @@ def test_handle_arguments_backtrace
   end
 
   def test_handle_arguments_debug
+    pend_for_ruby_box_stdio_capture
     assert_equal false, $DEBUG
 
     args = %w[--debug]
diff --git a/test/rubygems/test_gem_dependency_installer.rb b/test/rubygems/test_gem_dependency_installer.rb
index 95d455ad5f3d..bf496c189ca1 100644
--- a/test/rubygems/test_gem_dependency_installer.rb
+++ b/test/rubygems/test_gem_dependency_installer.rb
@@ -1126,6 +1126,7 @@ def test_install_version_default
   end
 
   def test_install_legacy_spec_with_nil_required_ruby_version
+    pend_for_ruby_box_marshal
     path = File.expand_path "data/null-required-ruby-version.gemspec.rz", __dir__
     spec = Marshal.load Gem.read_binary(path)
     def spec.validate(*args); end
@@ -1151,6 +1152,7 @@ def spec.validate(*args); end
   end
 
   def test_install_legacy_spec_with_nil_required_rubygems_version
+    pend_for_ruby_box_marshal
     path = File.expand_path "data/null-required-rubygems-version.gemspec.rz", __dir__
     spec = Marshal.load Gem.read_binary(path)
     def spec.validate(*args); end
diff --git a/test/rubygems/test_gem_doctor.rb b/test/rubygems/test_gem_doctor.rb
index 1554e7af128d..da625000ff90 100644
--- a/test/rubygems/test_gem_doctor.rb
+++ b/test/rubygems/test_gem_doctor.rb
@@ -240,6 +240,7 @@ def test_doctor_preserves_valid_abi_scoped_gemspec
   end
 
   def test_doctor_removes_corrupt_abi_scoped_gemspec
+    pend_for_ruby_box_stdio_capture
     install_specs util_spec "regular_gem"
 
     spec = util_ca_spec "ca_gem", "1", "aabbccdd",
@@ -286,6 +287,7 @@ def test_doctor_preserves_other_abi_dir
   end
 
   def test_doctor_does_not_recurse_into_abi_symlink
+    pend_for_ruby_box_stdio_capture
     pend "symlinks not supported" unless symlink_supported?
 
     install_specs util_spec "regular_gem"
diff --git a/test/rubygems/test_gem_ext_builder.rb b/test/rubygems/test_gem_ext_builder.rb
index 6b4eed2cf211..f5ad791ec1c9 100644
--- a/test/rubygems/test_gem_ext_builder.rb
+++ b/test/rubygems/test_gem_ext_builder.rb
@@ -700,7 +700,7 @@ def self.expand(val, config = CONFIG); val; end
       system(Gem.ruby, "-rmkmf", "-e", "exit MakeMakefile::RbConfig::CONFIG['host_os'] == 'fake_os'",
              "--", "--target-rbconfig=#{fake_rbconfig}")
     end
-    unless $?.success?
+    unless Process.last_status.success?
       assert_include(stderr, "uninitialized constant MakeMakefile::RbConfig")
       pend "This version of mkmf does not support --target-rbconfig"
     end
diff --git a/test/rubygems/test_gem_ext_cargo_builder.rb b/test/rubygems/test_gem_ext_cargo_builder.rb
index b970e442c250..bf442125f275 100644
--- a/test/rubygems/test_gem_ext_cargo_builder.rb
+++ b/test/rubygems/test_gem_ext_cargo_builder.rb
@@ -111,7 +111,9 @@ def test_full_integration
         Open3.capture2e(*gem, "build", "rust_ruby_example.gemspec", "--output", built_gem)
         Open3.capture2e(*gem, "install", "--verbose", "--local", built_gem, *ARGV)
 
-        stdout_and_stderr_str, status = Open3.capture2e(env_for_subprocess, *ruby_with_rubygems_in_load_path, "-rrust_ruby_example", "-e", "puts 'Result: ' + RustRubyExample.reverse('hello world')")
+        # Require inside -e because -r bypasses gem activation under RUBY_BOX=1
+        # (https://bugs.ruby-lang.org/issues/22295)
+        stdout_and_stderr_str, status = Open3.capture2e(env_for_subprocess, *ruby_with_rubygems_in_load_path, "-e", "require 'rust_ruby_example'; puts 'Result: ' + RustRubyExample.reverse('hello world')")
         assert status.success?, stdout_and_stderr_str
         assert_match "Result: #{"hello world".reverse}", stdout_and_stderr_str
       end
@@ -134,7 +136,7 @@ def test_custom_name
         Open3.capture2e(*gem, "install", "--verbose", "--local", built_gem, *ARGV)
       end
 
-      stdout_and_stderr_str, status = Open3.capture2e(env_for_subprocess, *ruby_with_rubygems_in_load_path, "-rcustom_name", "-e", "puts 'Result: ' + CustomName.say_hello")
+      stdout_and_stderr_str, status = Open3.capture2e(env_for_subprocess, *ruby_with_rubygems_in_load_path, "-e", "require 'custom_name'; puts 'Result: ' + CustomName.say_hello")
 
       assert status.success?, stdout_and_stderr_str
       assert_match "Result: Hello world!", stdout_and_stderr_str
@@ -199,7 +201,7 @@ def skip_unsupported_platforms!
     pend "jruby not supported" if Gem.java_platform?
     pend "truffleruby not supported (yet)" if RUBY_ENGINE == "truffleruby"
     system(@rust_envs, "cargo", "-V", out: IO::NULL, err: [:child, :out])
-    pend "cargo not present" unless $?.success?
+    pend "cargo not present" unless Process.last_status.success?
     pend "ruby.h is not provided by ruby repo" if ruby_repo?
     pend "rust toolchain of mingw is broken" if mingw_windows?
   end
diff --git a/test/rubygems/test_gem_package.rb b/test/rubygems/test_gem_package.rb
index b0935693d1c7..4e83b3a69b46 100644
--- a/test/rubygems/test_gem_package.rb
+++ b/test/rubygems/test_gem_package.rb
@@ -1538,6 +1538,7 @@ def test_verify_corrupt
   end
 
   def test_verify_corrupt_tar_metadata_entry
+    pend_for_ruby_box_stdio_capture
     gem = tar_file_header("metadata.gz", "", 0, 999, Time.now)
 
     File.open "corrupt.gem", "wb" do |io|
@@ -1574,6 +1575,7 @@ def test_verify_corrupt_tar_checksums_entry
   end
 
   def test_verify_corrupt_tar_data_entry
+    pend_for_ruby_box_stdio_capture
     gem = tar_file_header("data.tar.gz", "", 0, 100, Time.now)
 
     File.open "corrupt.gem", "wb" do |io|
diff --git a/test/rubygems/test_gem_package_tar_header_ractor.rb b/test/rubygems/test_gem_package_tar_header_ractor.rb
index 57140648052e..d2fc0f69b1e9 100644
--- a/test/rubygems/test_gem_package_tar_header_ractor.rb
+++ b/test/rubygems/test_gem_package_tar_header_ractor.rb
@@ -8,6 +8,11 @@
 end
 
 class TestGemPackageTarHeaderRactor < Gem::Package::TarTestCase
+  def setup
+    super
+    pend "Ruby::Box ignores $VERBOSE=, so assert_ractor cannot keep the Ractor experimental warning out of the child stderr (https://bugs.ruby-lang.org/issues/22282)" if ruby_box_enabled?
+  end
+
   SETUP = <<~RUBY
     header = {
       name: "x",
diff --git a/test/rubygems/test_gem_request_set.rb b/test/rubygems/test_gem_request_set.rb
index 8c8be04fb9f9..60ff8724aef4 100644
--- a/test/rubygems/test_gem_request_set.rb
+++ b/test/rubygems/test_gem_request_set.rb
@@ -71,6 +71,7 @@ def test_install_from_gemdeps
   end
 
   def test_install_from_gemdeps_explain
+    pend_for_ruby_box_stdio_capture
     spec_fetcher do |fetcher|
       fetcher.gem "a", 2
     end
@@ -94,6 +95,7 @@ def test_install_from_gemdeps_explain
   end
 
   def test_install_from_gemdeps_explain_verbose
+    pend_for_ruby_box_stdio_capture
     spec_fetcher do |fetcher|
       fetcher.gem "a", 2
     end
diff --git a/test/rubygems/test_gem_request_set_gem_dependency_api.rb b/test/rubygems/test_gem_request_set_gem_dependency_api.rb
index 4b5eaa38eda8..d8f4e7f6e92b 100644
--- a/test/rubygems/test_gem_request_set_gem_dependency_api.rb
+++ b/test/rubygems/test_gem_request_set_gem_dependency_api.rb
@@ -78,6 +78,7 @@ def test_gem
   end
 
   def test_gem_duplicate
+    pend_for_ruby_box_stdio_capture
     @gda.gem "a"
 
     _, err = capture_output do
@@ -128,6 +129,7 @@ def test_gem_bitbucket_expand_path
   end
 
   def test_gem_git_branch
+    pend_for_ruby_box_stdio_capture
     _, err = capture_output do
       @gda.gem "a", git: "git/a", branch: "other", tag: "v1"
     end
@@ -149,6 +151,7 @@ def test_gem_git_gist
   end
 
   def test_gem_git_ref
+    pend_for_ruby_box_stdio_capture
     _, err = capture_output do
       @gda.gem "a", git: "git/a", ref: "abcd123", branch: "other"
     end
diff --git a/test/rubygems/test_gem_requirement.rb b/test/rubygems/test_gem_requirement.rb
index 00634dc7f43f..bd3988b32300 100644
--- a/test/rubygems/test_gem_requirement.rb
+++ b/test/rubygems/test_gem_requirement.rb
@@ -431,6 +431,7 @@ def self.exploit(arg)
   end
 
   def test_marshal_load_attack
+    pend_for_ruby_box_marshal
     wa = Gem::Net::WriteAdapter.allocate
     wa.instance_variable_set(:@socket, self.class)
     wa.instance_variable_set(:@method_id, :exploit)
diff --git a/test/rubygems/test_gem_safe_marshal.rb b/test/rubygems/test_gem_safe_marshal.rb
index c34d8570c6ec..9a7007eebea3 100644
--- a/test/rubygems/test_gem_safe_marshal.rb
+++ b/test/rubygems/test_gem_safe_marshal.rb
@@ -317,10 +317,12 @@ def test_array_subclass
   end
 
   def test_frozen_object
+    pend_for_ruby_box_marshal
     assert_safe_load_as Gem::Version.new("1.abc").freeze
   end
 
   def test_date
+    pend_for_ruby_box_marshal
     assert_safe_load_as Date.new(1994, 12, 9)
   end
 
@@ -404,6 +406,7 @@ def test_gem_spec_unmarshall_license
   end
 
   def test_gem_spec_unmarshall_required_ruby_rubygems_version
+    pend_for_ruby_box_marshal
     spec = Gem::Specification.new do |s|
       s.name = "hi"
       s.version = "1.2.3"
@@ -535,7 +538,12 @@ def test_date_user_defined_rejected
 
   def assert_safe_load_marshal(dumped, additional_methods: [], permitted_ivars: nil, equality: true, marshal_dump_equality: true,
     inspect: true, to_s: true)
-    loaded = Marshal.load(dumped)
+    loaded = begin
+      Marshal.load(dumped)
+    rescue ArgumentError => e
+      pend_for_ruby_box_marshal if e.message.include?("undefined class/module")
+      raise
+    end
     safe_loaded =
       assert_nothing_raised("dumped: #{dumped.b.inspect} loaded: #{loaded.inspect}") do
         if permitted_ivars
diff --git a/test/rubygems/test_gem_specification.rb b/test/rubygems/test_gem_specification.rb
index dc32a6290786..fb6df06b9c70 100644
--- a/test/rubygems/test_gem_specification.rb
+++ b/test/rubygems/test_gem_specification.rb
@@ -710,6 +710,7 @@ def test_self_attribute_names
   end
 
   def test_self__load_future
+    pend_for_ruby_box_marshal
     spec = Gem::Specification.new
     spec.name = "a"
     spec.version = "1"
@@ -1064,6 +1065,7 @@ def test_self_stubs_returns_only_specified_named_specs
   end
 
   def test_handles_private_null_type
+    pend_for_ruby_box_marshal
     yaml_defined = Object.const_defined?("YAML")
 
     path = File.expand_path "data/pry-0.4.7.gemspec.rz", __dir__
@@ -1076,6 +1078,7 @@ def test_handles_private_null_type
   end
 
   def test_handles_dependencies_with_syck_requirements_bug
+    pend_for_ruby_box_marshal
     yaml_defined = Object.const_defined?("YAML")
 
     path = File.expand_path "data/excon-0.7.7.gemspec.rz", __dir__
@@ -1281,6 +1284,7 @@ def test_set_version_to_nil_after_setting_version
   end
 
   def test__dump
+    pend_for_ruby_box_marshal
     @a2.platform = Gem::Platform.local
     @a2.instance_variable_set :@original_platform, "old_platform"
 
@@ -1576,6 +1580,7 @@ def test_contains_requirable_file_eh
   end
 
   def test_contains_requirable_file_eh_extension
+    pend_for_ruby_box_stdio_capture
     ext_spec
 
     _, err = capture_output do
@@ -3386,6 +3391,7 @@ def test_validate_files
   end
 
   def test_unresolved_specs
+    pend_for_ruby_box_stdio_capture
     specification = Gem::Specification.clone
 
     set_orig specification
@@ -3412,6 +3418,7 @@ def test_unresolved_specs
   end
 
   def test_unresolved_specs_with_versions
+    pend_for_ruby_box_stdio_capture
     specification = Gem::Specification.clone
 
     set_orig specification
@@ -3444,6 +3451,7 @@ def test_unresolved_specs_with_versions
   end
 
   def test_unresolved_specs_with_duplicated_versions
+    pend_for_ruby_box_stdio_capture
     specification = Gem::Specification.clone
 
     set_orig specification
@@ -3497,6 +3505,7 @@ def test_unresolved_specs_with_unrestricted_deps_on_default_gems
   end
 
   def test_duplicate_runtime_dependency
+    pend_for_ruby_box_stdio_capture
     expected = "WARNING: duplicated b dependency [\"~> 3.0\", \"~> 3.0\"]\n"
     out, err = capture_output do
       @a1.add_dependency "b", "~> 3.0", "~> 3.0"
@@ -3995,6 +4004,7 @@ def test_version_change_reset_cache_file
   end
 
   def test__load_fixes_Date_objects
+    pend_for_ruby_box_marshal
     spec = util_spec "a", 1
     spec.instance_variable_set :@date, Date.today
 
diff --git a/test/rubygems/test_gem_stub_specification.rb b/test/rubygems/test_gem_stub_specification.rb
index 1aa3b6532436..66bdd3d3fbbb 100644
--- a/test/rubygems/test_gem_stub_specification.rb
+++ b/test/rubygems/test_gem_stub_specification.rb
@@ -94,6 +94,7 @@ def test_contains_requirable_file_eh
   end
 
   def test_contains_requirable_file_eh_extension
+    pend_for_ruby_box_stdio_capture
     stub_with_extension do |stub|
       _, err = capture_output do
         if RUBY_ENGINE == "jruby"
diff --git a/test/rubygems/test_require.rb b/test/rubygems/test_require.rb
index db86a3090565..ef1bb2e465a2 100644
--- a/test/rubygems/test_require.rb
+++ b/test/rubygems/test_require.rb
@@ -484,7 +484,7 @@ def test_realworld_default_gem
       puts Gem.loaded_specs["json"]
     RUBY
     output = Gem::Util.popen(*ruby_with_rubygems_in_load_path, "-e", cmd).strip
-    assert $?.success?
+    assert Process.last_status.success?
     refute_empty output
   end
 
@@ -508,7 +508,7 @@ def test_realworld_upgraded_default_gem
     assert_equal "999.99.9", output.lines[0].chomp
     # Make sure only files from the newer json gem are loaded, and no files from the default json gem
     assert_equal ["#{@gemhome}/gems/json-999.99.9/lib/json.rb"], output.lines.grep(%r{/gems/json-}).map(&:chomp)
-    assert $?.success?
+    assert Process.last_status.success?
   end
 
   def test_default_gem_and_normal_gem
@@ -718,6 +718,7 @@ def test_require_bundler
 
   ["", "Kernel."].each do |prefix|
     define_method "test_no_kernel_require_in_#{prefix.tr(".", "_")}warn_with_uplevel" do
+      pend_for_ruby_box_stdio_capture
       Dir.mktmpdir("warn_test") do |dir|
         File.write(dir + "/sub.rb", "#{prefix}warn 'uplevel', 'test', uplevel: 1\n")
         File.write(dir + "/main.rb", "require 'sub'\n")
@@ -733,6 +734,7 @@ def test_require_bundler
     end
 
     define_method "test_no_other_behavioral_changes_with_#{prefix.tr(".", "_")}warn" do
+      pend_for_ruby_box_stdio_capture
       Dir.mktmpdir("warn_test") do |dir|
         File.write(dir + "/main.rb", "#{prefix}warn({x:1}, {y:2}, [])\n")
         _, err = capture_subprocess_io do
@@ -748,6 +750,7 @@ def test_require_bundler
   end
 
   def test_no_crash_when_overriding_warn_with_warning_module
+    pend_for_ruby_box_stdio_capture
     Dir.mktmpdir("warn_test") do |dir|
       File.write(dir + "/main.rb", "module Warning; def warn(str); super; end; end; warn 'Foo Bar'")
       _, err = capture_subprocess_io do
@@ -762,6 +765,7 @@ def test_no_crash_when_overriding_warn_with_warning_module
   end
 
   def test_expected_backtrace_location_when_inheriting_from_basic_object_and_including_kernel
+    pend_for_ruby_box_stdio_capture
     Dir.mktmpdir("warn_test") do |dir|
       File.write(dir + "/main.rb", "\nrequire 'sub'\n")
       File.write(dir + "/sub.rb", <<-'RUBY')
diff --git a/test/rubygems/test_rubygems.rb b/test/rubygems/test_rubygems.rb
index 6566b5981e69..02393c57fcae 100644
--- a/test/rubygems/test_rubygems.rb
+++ b/test/rubygems/test_rubygems.rb
@@ -5,7 +5,7 @@
 class GemTest < Gem::TestCase
   def test_rubygems_normal_behaviour
     _ = Gem::Util.popen(*ruby_with_rubygems_in_load_path, "-e", "'require \"rubygems\"'", { err: [:child, :out] }).strip
-    assert $?.success?
+    assert Process.last_status.success?
   end
 
   def test_operating_system_other_exceptions
@@ -17,7 +17,7 @@ def test_operating_system_other_exceptions
     RUBY
 
     output = Gem::Util.popen(*ruby_with_rubygems_and_fake_operating_system_in_load_path(path), "-e", "'require \"rubygems\"'", { err: [:child, :out] }).strip
-    assert !$?.success?
+    assert !Process.last_status.success?
     assert_match(/undefined local variable or method [`']intentionally_not_implemented_method'/, output)
     assert_includes output, "Loading the #{operating_system_rb_at(path)} file caused an error. " \
     "This file is owned by your OS, not by rubygems upstream. " \
diff --git a/tool/release.rb b/tool/release.rb
index 7585136b6c4f..2324b2d0f95f 100644
--- a/tool/release.rb
+++ b/tool/release.rb
@@ -442,7 +442,7 @@ def add_commit_authors!(pulls)
       ids = batch.flat_map {|pull| ["-F", "ids[]=#{pull.node_id}"] }
 
       json = IO.popen(["gh", "api", "graphql", "-f", "query=#{COMMIT_AUTHORS_QUERY}", *ids], &:read)
-      raise "Failed to list the commits of #{batch.map(&:number).join(", ")}" unless $?.success?
+      raise "Failed to list the commits of #{batch.map(&:number).join(", ")}" unless Process.last_status.success?
 
       credit_commit_authors(batch, JSON.parse(json).dig("data", "nodes"))
     end
@@ -501,7 +501,7 @@ def git_quietly(*args)
 
   def pull_requests_merged_into(base, from, to)
     commits = git_quietly("rev-list", "#{from}..#{to}")
-    raise "Failed to list the commits in #{from}..#{to}" unless $?.success?
+    raise "Failed to list the commits in #{from}..#{to}" unless Process.last_status.success?
 
     reachable = Set.new(commits.split("\n"))
 
@@ -511,12 +511,12 @@ def pull_requests_merged_into(base, from, to)
   # The date bound is deliberately loose. It bounds the query, not the result.
   def merged_pull_requests(base, since_ref)
     committed_at = git_quietly("log", "-1", "--format=%cI", since_ref).strip
-    raise "Failed to resolve #{since_ref}" unless $?.success?
+    raise "Failed to resolve #{since_ref}" unless Process.last_status.success?
 
     since = (Time.iso8601(committed_at) - 86_400).utc.strftime("%Y-%m-%d")
 
     json = `gh pr list --repo ruby/rubygems --state merged --base #{base} --search 'merged:>=#{since}' --limit #{MERGED_PULL_REQUEST_LIMIT} --json number,id,title,labels,mergeCommit,mergedAt,author,url`
-    raise "Failed to list pull requests merged into #{base} since #{since}" unless $?.success?
+    raise "Failed to list pull requests merged into #{base} since #{since}" unless Process.last_status.success?
 
     pull_requests_from(json, "#{base} since #{since}")
   end