diff --git a/test/test_helper.rb b/test/test_helper.rb
index a6baf4ee..7d43ce7c 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -16,6 +16,33 @@
 require "pathname"
 TEST_IMAGES_DIR = Pathname.new(File.expand_path("fixtures/images", __dir__))
 
+# The child environment for a subprocess probe that chdirs OUT of the project
+# to exercise a load path it built itself. Such a probe must not inherit this
+# repo's development bundle, and dropping these four is the whole of it:
+#
+# - BUNDLER_SETUP is the one that bit us. RubyGems ends rubygems.rb with
+#   `require ENV["BUNDLER_SETUP"] if ENV["BUNDLER_SETUP"] && !defined?(Bundler)`,
+#   so ANY child re-enters bundler/setup before it reaches `-e`. That resolves
+#   BUNDLE_GEMFILE -- which .github/workflows/test.yml sets to the RELATIVE
+#   `gemfiles/<gemfile>` -- against the child's cwd, i.e. the tmpdir, and the
+#   child dies with Bundler::GemfileNotFound before running a line of the probe.
+#   Bundler >= 2.6 rewrites BUNDLE_GEMFILE to an absolute path during setup and
+#   the 2.5 that ships with ruby 3.3 does not, so this was green everywhere but
+#   the 3.3 cells -- and invisible under `bundle exec`, which expands it too.
+# - RUBYOPT carries `-r<abs>/bundler/setup`, the same door.
+# - RUBYLIB and BUNDLE_GEMFILE inject the development load path directly.
+#
+# Scrubbing is not sufficient on its own -- the probe must ALSO chdir out of
+# the project, or RubyGems finds gems.rb and puts all of it back. Probes that
+# stay inside the project root (`chdir: PROJECT_ROOT`) genuinely want the
+# bundle and must NOT use this.
+PROBE_ENV = {
+  "BUNDLER_SETUP" => nil,
+  "BUNDLE_GEMFILE" => nil,
+  "RUBYLIB" => nil,
+  "RUBYOPT" => nil
+}.freeze
+
 require "support/setup_rails_app"
 require "minitest/autorun"
 
diff --git a/test/unit/gem_name_entry_point_test.rb b/test/unit/gem_name_entry_point_test.rb
index 45ae35dc..7709cda4 100644
--- a/test/unit/gem_name_entry_point_test.rb
+++ b/test/unit/gem_name_entry_point_test.rb
@@ -24,6 +24,13 @@
 class GemNameEntryPointTest < ActiveSupport::TestCase
   LIB = File.expand_path("../../lib", __dir__)
 
+  # Opens every probe, ahead of the minitest gate below: the point of this
+  # probe is a CONSUMER's bundle, so inheriting ours proves nothing about
+  # either. See PROBE_ENV in test_helper.rb for what leaks and how.
+  NO_BUNDLE_GATE = <<~RUBY
+    abort("GATE: the probe inherited the development bundle, so it proves nothing") if defined?(Bundler)
+  RUBY
+
   # Opens every probe: a probe that quietly tested the wrong environment
   # would "prove" whatever we hoped it would.
   GATE = {
@@ -54,8 +61,12 @@ class GemNameEntryPointTest < ActiveSupport::TestCase
   # scratch Gemfile reproduces the same thing and was used to confirm this
   # one, but it needs the network and has no place in the unit suite.)
   #
+  # The cwd is half of that; PROBE_ENV is the other half, and the two only
+  # work together -- see its comment in test/test_helper.rb. +env+ is a seam
+  # for the gate test below, which restores a leak on purpose.
+  #
   # @return [Array(String, String, Process::Status)] stdout, stderr, status
-  def self.probe(script, minitest: true)
+  def self.probe(script, minitest: true, env: PROBE_ENV)
     Dir.mktmpdir do |dir|
       load_paths = ["-I#{LIB}"]
 
@@ -68,7 +79,7 @@ def self.probe(script, minitest: true)
         load_paths.unshift("-I#{shim}")
       end
 
-      Open3.capture3(RbConfig.ruby, *load_paths, "-e", GATE.fetch(minitest) + script, chdir: dir)
+      Open3.capture3(env, RbConfig.ruby, *load_paths, "-e", NO_BUNDLE_GATE + GATE.fetch(minitest) + script, chdir: dir)
     end
   end
 
@@ -98,6 +109,25 @@ def self.probe(script, minitest: true)
     assert_match(/require: false/, err, "the message must name the way to silence it")
   end
 
+  # The gate line has to be able to FAIL, or it is a comment with an `if`
+  # around it. Restore the ONE variable the scrub exists to drop -- RubyGems
+  # turns BUNDLER_SETUP back into a `require "bundler/setup"` in any child --
+  # and the same probe must refuse to run. Restoring it alone (BUNDLE_GEMFILE
+  # still scrubbed) is what makes this deterministic on every ruby: the child
+  # boots far enough to reach the gate instead of dying at Bundler's own
+  # GemfileNotFound, which is what CI saw and which only reproduces on the
+  # bundler that leaves a relative BUNDLE_GEMFILE alone.
+  test "the no-bundle gate rejects a probe that inherited the development bundle" do
+    bundler_setup = ENV["BUNDLER_SETUP"]
+    assert bundler_setup, "this test only means something when the suite itself runs under bundler"
+
+    out, err, status = self.class.probe(%(puts "RAN"), env: PROBE_ENV.merge("BUNDLER_SETUP" => bundler_setup))
+
+    refute_predicate status, :success?, "the gate let an inherited bundle through:\n#{out}\n#{err}"
+    assert_match(/GATE: the probe inherited the development bundle/, err)
+    refute_includes out, "RAN", "the gate must abort BEFORE the probe body runs"
+  end
+
   # The other direction: the documented zero-require Rails path.
   test "the gem-name entry point still auto-activates the Minitest assertions when minitest is present" do
     out, err, status = self.class.probe(<<~'RUBY')
diff --git a/test/unit/legacy_deletion_test.rb b/test/unit/legacy_deletion_test.rb
index a11b7166..25db93b8 100644
--- a/test/unit/legacy_deletion_test.rb
+++ b/test/unit/legacy_deletion_test.rb
@@ -194,9 +194,9 @@ def in_deleted_tree
 
   # A fresh process with ONLY +tree+ on the load path.
   #
-  # `chdir: tree` is the load-bearing half, and NOT a detail. Scrubbing
-  # RUBYOPT/BUNDLE_GEMFILE is not sufficient on its own: with the cwd still
-  # inside the project, RubyGems auto-discovers gems.rb, puts
+  # `chdir: tree` is the load-bearing half, and NOT a detail. PROBE_ENV is not
+  # sufficient on its own: with the cwd still inside the project, RubyGems
+  # auto-discovers gems.rb, puts
   # `-rbundler/setup` BACK into RUBYOPT, and the gemspec unshifts the real
   # lib/ ahead of the -I dir -- measured, this exact scrub with cwd at the
   # project root loads 24 files from the intact tree. Running from the
@@ -208,7 +208,7 @@ def probe(tree, script)
       $LOAD_PATH.unshift(#{tree.inspect})
       #{SupportLoadProbeTest::CUCUMBER_RUNTIME_STUB}
     RUBY
-    env = {"DELETED_TREE" => tree, "RUBYOPT" => nil, "BUNDLE_GEMFILE" => nil, "RUBYLIB" => nil}
+    env = PROBE_ENV.merge("DELETED_TREE" => tree)
     out, status = Open3.capture2e(env, RbConfig.ruby, "-e", preamble + script, chdir: tree)
 
     "#{script.lines.first.strip} -> #{out}" unless status.success?