diff --git a/test/unit/config_default_timing_test.rb b/test/unit/config_default_timing_test.rb
new file mode 100644
index 00000000..3bb376e0
--- /dev/null
+++ b/test/unit/config_default_timing_test.rb
@@ -0,0 +1,153 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "open3"
+
+# ADR-005 step 1 guard: pins WHEN each config default is evaluated, per
+# documented entry point, in fresh subprocesses (test_helper preloads the
+# whole gem, so only a subprocess can observe require-time behavior).
+#
+# Current behavior being pinned (the coming storage inversion must not
+# shift any of these):
+#
+# - mattr_accessor block defaults (fail_if_new from ENV["CI"], root from
+#   Rails.root/pwd) are evaluated ONCE, at class-body eval time, when
+#   config_legacy.rb is first required. Mutating ENV, cwd, or Rails.root
+#   after the require -- even before the first read -- must NOT change the
+#   value. A refactor that turns any of these into a lazy (read-time)
+#   default, memoized or not, goes red here.
+# - Diff.default_options[:wait] is the opposite: it reads
+#   Capybara.default_max_wait_time at CALL time, live, every call.
+#
+# Value snapshots are asserted through BOTH surfaces (SnapDiff.config.x
+# and the legacy mattr_accessor) so the inversion can't silently fork them.
+class ConfigDefaultTimingTest < ActiveSupport::TestCase
+  ENTRY_POINTS = %w[
+    capybara_screenshot_diff
+    capybara_screenshot_diff/minitest
+    snap_diff
+    capybara/screenshot/diff
+  ].freeze
+
+  def run_probe(script, env)
+    out, status = Open3.capture2e(env, RbConfig.ruby, "-Ilib", "-e", script)
+
+    assert status.success?, "probe failed:\n#{out}"
+  end
+
+  CHECK_HELPER = <<~'RUBY'
+    def check(name, expected, actual)
+      return if expected == actual
+      abort("#{name}: expected #{expected.inspect}, got #{actual.inspect}")
+    end
+
+    def check_both(name, expected, mod, mattr)
+      check("#{mod}.#{mattr}", expected, mod.public_send(mattr))
+      check("SnapDiff.config.#{name}", expected, SnapDiff.config.public_send(name))
+    end
+  RUBY
+
+  # Probe A: defaults snapshot without CI/Rails + require-time freezing of
+  # ENV- and pwd-derived defaults + call-time liveness of the Capybara-
+  # coupled wait. ENV/cwd are mutated after the require but BEFORE the
+  # first read, so both eager-at-require (current, expected) and any lazy
+  # read-time variant are distinguished.
+  SNAPSHOT_SCRIPT = CHECK_HELPER + <<~'RUBY'
+    require "pathname"
+    launch_pwd = Pathname(".").expand_path
+
+    require ENV.fetch("PROBE_ENTRY")
+
+    ENV["CI"] = "1"
+    require "tmpdir"
+    Dir.chdir(Dir.tmpdir)
+
+    # 1) Every mapped setting reads the same through both surfaces.
+    SnapDiff::Config::MAPPING.each do |name, (mod, mattr)|
+      check("SnapDiff.config.#{name} vs #{mod}.#{mattr}", mod.public_send(mattr), SnapDiff.config.public_send(name))
+    end
+
+    # 2) Expected default values (CI unset, no Rails, at require time).
+    #    fail_if_new false / root == launch pwd also pin require-time
+    #    evaluation: ENV["CI"] and cwd were changed above, pre-first-read.
+    screenshot = Capybara::Screenshot
+    diff = Capybara::Screenshot::Diff
+    {
+      add_driver_path: [nil, screenshot, :add_driver_path],
+      add_os_path: [nil, screenshot, :add_os_path],
+      blur_active_element: [true, screenshot, :blur_active_element],
+      screenshot_enabled: [nil, screenshot, :enabled],
+      hide_caret: [true, screenshot, :hide_caret],
+      disable_animations: [nil, screenshot, :disable_animations],
+      root: [launch_pwd, screenshot, :root],
+      stability_time_limit: [nil, screenshot, :stability_time_limit],
+      window_size: [nil, screenshot, :window_size],
+      save_path: ["doc/screenshots", screenshot, :save_path],
+      use_lfs: [nil, screenshot, :use_lfs],
+      screenshot_format: ["png", screenshot, :screenshot_format],
+      capybara_screenshot_options: [{}, screenshot, :capybara_screenshot_options],
+      delayed: [true, diff, :delayed],
+      area_size_limit: [nil, diff, :area_size_limit],
+      fail_if_new: [false, diff, :fail_if_new],
+      pending_if_new: [false, diff, :pending_if_new],
+      fail_on_difference: [true, diff, :fail_on_difference],
+      color_distance_limit: [nil, diff, :color_distance_limit],
+      enabled: [true, diff, :enabled],
+      shift_distance_limit: [nil, diff, :shift_distance_limit],
+      skip_area: [nil, diff, :skip_area],
+      driver: [:auto, diff, :driver],
+      tolerance: [nil, diff, :tolerance],
+      perceptual_threshold: [nil, diff, :perceptual_threshold],
+      screenshoter: [SnapDiff::Screenshoter, diff, :screenshoter],
+      manager: [SnapDiff::SnapManager, diff, :manager]
+    }.each do |name, (expected, mod, mattr)|
+      check_both(name, expected, mod, mattr)
+    end
+
+    # 3) Capybara-coupled wait is read at CALL time (live), not frozen.
+    Capybara.default_max_wait_time = 42.5
+    check("default_options[:wait] follows Capybara.default_max_wait_time set after require",
+      42.5, Capybara::Screenshot::Diff.default_options[:wait])
+  RUBY
+
+  # Probe B: ENV["CI"] present (non-empty) BEFORE the require flips the
+  # fail_if_new default on -- and unsetting it after the require does not
+  # flip it back (frozen at require time).
+  CI_SET_SCRIPT = CHECK_HELPER + <<~RUBY
+    require ENV.fetch("PROBE_ENTRY")
+    ENV.delete("CI")
+    check_both(:fail_if_new, true, Capybara::Screenshot::Diff, :fail_if_new)
+  RUBY
+
+  # Probe C: a Rails module with .root defined BEFORE the require wins over
+  # the pwd fallback -- and reassigning Rails.root after the require (pre
+  # first read) is not seen (frozen at require time).
+  RAILS_ROOT_SCRIPT = CHECK_HELPER + <<~RUBY
+    require "pathname"
+
+    module Rails
+      @root = Pathname("/fake-rails-root-at-require")
+      class << self
+        attr_accessor :root
+      end
+    end
+
+    require ENV.fetch("PROBE_ENTRY")
+    Rails.root = Pathname("/fake-rails-root-after-require")
+    check_both(:root, Pathname("/fake-rails-root-at-require"), Capybara::Screenshot, :root)
+  RUBY
+
+  ENTRY_POINTS.each do |entry|
+    test "#{entry}: defaults snapshot matches through both surfaces; ENV/pwd frozen at require, wait live" do
+      run_probe(SNAPSHOT_SCRIPT, {"PROBE_ENTRY" => entry, "CI" => nil})
+    end
+
+    test "#{entry}: CI=1 before require turns fail_if_new on; unset after require does not turn it off" do
+      run_probe(CI_SET_SCRIPT, {"PROBE_ENTRY" => entry, "CI" => "1"})
+    end
+
+    test "#{entry}: Rails.root defined before require wins; reassigning it after require is not seen" do
+      run_probe(RAILS_ROOT_SCRIPT, {"PROBE_ENTRY" => entry, "CI" => nil})
+    end
+  end
+end