diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 989064ab..da99c0dc 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -38,12 +38,27 @@ jobs:
       - name: Test
         run: bundle exec rake test:unit
 
+      # Idempotent so a failed run can be re-dispatched (bit us at beta1):
+      # skip when the tag already exists at HEAD, fail loudly if it exists
+      # at a different commit -- never retag.
       - name: Create tag
         run: |
-          git config user.name "github-actions[bot]"
-          git config user.email "github-actions[bot]@users.noreply.github.com"
-          git tag -a "v${{ github.event.inputs.version }}" -m "v${{ github.event.inputs.version }}"
-          git push origin "v${{ github.event.inputs.version }}"
+          TAG="v${{ github.event.inputs.version }}"
+          # checkout doesn't fetch tags; pull this one if it exists remotely.
+          git fetch --force origin "refs/tags/$TAG:refs/tags/$TAG" 2>/dev/null || true
+          if EXISTING=$(git rev-parse -q --verify "refs/tags/$TAG^{commit}"); then
+            if [ "$EXISTING" = "$(git rev-parse HEAD)" ]; then
+              echo "Tag $TAG already exists at HEAD; skipping tag creation."
+            else
+              echo "::error::Tag $TAG already exists at $EXISTING, but HEAD is $(git rev-parse HEAD). Refusing to retag."
+              exit 1
+            fi
+          else
+            git config user.name "github-actions[bot]"
+            git config user.email "github-actions[bot]@users.noreply.github.com"
+            git tag -a "$TAG" -m "$TAG"
+            git push origin "$TAG"
+          fi
 
       - name: Publish to RubyGems
         uses: rubygems/release-gem@v1
diff --git a/README.md b/README.md
index de912834..5f4a0ba7 100644
--- a/README.md
+++ b/README.md
@@ -11,7 +11,7 @@ Stop shipping UI bugs. Take screenshots in your Capybara tests, commit baselines
 
 **Why this gem?** Baselines live in git — review UI changes in pull requests like you review code. Runs offline, works in CI, zero vendor lock-in. Unlike Percy/Chromatic (paid SaaS), nothing to sign up for. Unlike BackstopJS, no Node required.
 
-> **2.0 experiment (alpha):** the gem is moving to a `SnapDiff` canonical namespace. Opt in with `gem "capybara-screenshot-diff", "2.0.0.alpha1"` (prereleases are never installed by default — normal installs stay on 1.x). Legacy names keep working with a one-time deprecation warning, silenceable via `SnapDiff.silence_deprecations = true` or `SNAP_DIFF_SILENCE_DEPRECATIONS=1`. See the [upgrade guide](docs/UPGRADING.md) and share feedback on [#166](https://github.com/snap-diff/snap_diff-capybara/issues/166).
+> **2.0 experiment (alpha):** the gem is moving to a `SnapDiff` canonical namespace. Opt in with `gem "capybara-screenshot-diff", "2.0.0.beta1"` (or the latest 2.0.0 prerelease; prereleases are never installed by default — normal installs stay on 1.x). Legacy names keep working with a one-time deprecation warning, silenceable via `SnapDiff.silence_deprecations = true` or `SNAP_DIFF_SILENCE_DEPRECATIONS=1`. See the [upgrade guide](docs/UPGRADING.md) and share feedback on [#166](https://github.com/snap-diff/snap_diff-capybara/issues/166).
 >
 > Starting with the 2.0 prereleases the gem is also published as [`snap_diff-capybara`](https://rubygems.org/gems/snap_diff-capybara) — identical content and versions under the forward-looking name, matching this repository. Install either; don't install both.
 
diff --git a/docs/UPGRADING.md b/docs/UPGRADING.md
index faba7181..d543dcbb 100644
--- a/docs/UPGRADING.md
+++ b/docs/UPGRADING.md
@@ -6,7 +6,7 @@
 
 Version 2.0 introduces a new canonical namespace (`SnapDiff`) for cleaner, more discoverable code. The public DSL remains unchanged — your existing `screenshot` and `assert_matches_screenshot` calls work without modification. This guide covers the optional migration path for settings and the new namespace.
 
-**Status:** `2.0.0.alpha1` is an opt-in prerelease. RubyGems never installs prereleases by default resolution — normal `bundle update` keeps you on the 1.x line. The final 2.0.0 ships only after adopter feedback; please report anything surprising on [#166](https://github.com/snap-diff/snap_diff-capybara/issues/166).
+**Status:** `2.0.0.beta1` is an opt-in prerelease. RubyGems never installs prereleases by default resolution — normal `bundle update` keeps you on the 1.x line. The final 2.0.0 ships only after adopter feedback; please report anything surprising on [#166](https://github.com/snap-diff/snap_diff-capybara/issues/166).
 
 **Estimated upgrade time:** 5–15 minutes (most users need only the Gemfile pin)
 
@@ -18,7 +18,7 @@ Version 2.0 introduces a new canonical namespace (`SnapDiff`) for cleaner, more
 
 ```ruby
 # In your Gemfile — the exact prerelease version is required to opt in
-gem "capybara-screenshot-diff", "2.0.0.alpha1"
+gem "capybara-screenshot-diff", "2.0.0.beta1" # or the latest 2.0.0 prerelease
 ```
 
 ```bash
@@ -218,7 +218,7 @@ All settings and baselines are compatible with v1.x. Simply pin your Gemfile bac
 
 ### Summary Checklist
 
-- [ ] Pin `gem "capybara-screenshot-diff", "2.0.0.alpha1"` in your Gemfile
+- [ ] Pin `gem "capybara-screenshot-diff", "2.0.0.beta1"` (or the latest 2.0.0 prerelease) in your Gemfile
 - [ ] Run `bundle install`
 - [ ] Run your test suite to verify no regressions
 - [ ] (Optional) Migrate config to the `SnapDiff` namespace
diff --git a/lib/snap_diff.rb b/lib/snap_diff.rb
index ef18e6fd..0bd5f8da 100644
--- a/lib/snap_diff.rb
+++ b/lib/snap_diff.rb
@@ -1,5 +1,26 @@
 # frozen_string_literal: true
 
+# Dual-install guard: capybara-screenshot-diff and snap_diff-capybara ship
+# identical files. With BOTH activated, every require silently resolves
+# from whichever gem activated first, so version skew between the two is
+# undetectable. Refuse that setup at the entry point. Local dev from
+# source loads neither spec, so the guard fires only when both are
+# genuinely installed as gems.
+module SnapDiff
+  DualInstallError = Class.new(StandardError)
+
+  # @api private
+  def self.assert_single_gem!(loaded_specs = Gem.loaded_specs)
+    return unless loaded_specs.key?("capybara-screenshot-diff") && loaded_specs.key?("snap_diff-capybara")
+
+    raise DualInstallError,
+      "Both `capybara-screenshot-diff` and `snap_diff-capybara` gems are installed. " \
+      "They ship identical files, so files load from whichever gem activated first " \
+      "and versions can silently diverge. Remove one of them from your Gemfile."
+  end
+end
+SnapDiff.assert_single_gem!
+
 # None of these requires ever leads back to this file: config_legacy is a
 # leaf (see its own header comment), the image_compare forwarder pulls in
 # snap_diff/comparison plus the legacy const_missing shims, and
diff --git a/lib/snap_diff/deprecation.rb b/lib/snap_diff/deprecation.rb
index 7f149426..8e9e8f1f 100644
--- a/lib/snap_diff/deprecation.rb
+++ b/lib/snap_diff/deprecation.rb
@@ -5,12 +5,16 @@ module SnapDiff
   #
   # Internal until the v2 namespace transition; not a public contract.
   #
-  # Warn-once-per-subject deprecation helper. Dormant: nothing in the
-  # current codebase calls this yet -- it exists so the gated v2 shim layer
-  # (ADR-004's +const_missing+-based legacy constant shim and the
-  # mattr_accessor-to-Config method shims) has pre-tested warning
-  # machinery to call into once it lands.
+  # Warn-once-per-subject deprecation engine for the legacy-namespace
+  # shims: snap_diff/legacy_shims routes every +const_missing+ hit on an
+  # old +Capybara::Screenshot::Diff+ / +CapybaraScreenshotDiff+ constant
+  # through {.warn}, so each deprecated name warns exactly once per
+  # process (ADR-004's v2 namespace transition).
   module Deprecation
+    # Everything under lib/ is "the gem"; the first caller frame outside
+    # it is the user code that referenced the deprecated name (same
+    # filtering idea as BacktraceFilter in error_with_filtered_backtrace).
+    GEM_LIB_DIR = File.expand_path("..", __dir__) + File::SEPARATOR
     # Emission channel: Kernel#warn, not a direct +$stderr.puts+.
     #
     # Kernel#warn delegates to +Warning.warn+ (Ruby >= 2.4), so anything
@@ -40,7 +44,7 @@ def warn(subject, replacement, category:)
         end
         return unless first_time
 
-        Kernel.warn(message_for(subject, replacement, category))
+        Kernel.warn(message_for(subject, replacement, category, caller_locations(1)))
       end
 
       # @api private
@@ -55,9 +59,23 @@ def reset!
 
       private
 
-      def message_for(subject, replacement, category)
-        "[snap_diff deprecation] `#{subject}` is deprecated (#{category}); " \
+      def message_for(subject, replacement, category, locations)
+        message = "[snap_diff deprecation] `#{subject}` is deprecated (#{category}); " \
           "use `#{replacement}` instead."
+        origin = origin_for(locations)
+        origin ? "#{message} (called from #{origin})" : message
+      end
+
+      # First frame outside the gem's lib dir, formatted "file:line";
+      # nil when every frame is internal (or paths are unavailable).
+      def origin_for(locations)
+        (locations || []).each do |location|
+          path = location.absolute_path || location.path
+          next if path.nil? || path.start_with?(GEM_LIB_DIR)
+
+          return "#{path}:#{location.lineno}"
+        end
+        nil
       end
     end
   end
diff --git a/test/unit/snap_diff_deprecation_test.rb b/test/unit/snap_diff_deprecation_test.rb
index 08e133e5..04db89bc 100644
--- a/test/unit/snap_diff_deprecation_test.rb
+++ b/test/unit/snap_diff_deprecation_test.rb
@@ -49,6 +49,17 @@ def capture_warnings
     assert_match(/New::Thing/, lines.first)
   end
 
+  # Actionable attribution: the first caller frame OUTSIDE the gem's lib
+  # dir is named, so users can find the deprecated reference. This test
+  # file plays the part of "user code" -- the warning must point here.
+  test "warning names the caller's file and line" do
+    lines = capture_warnings do
+      SnapDiff::Deprecation.warn("Old::Where", "New::Where", category: :constant)
+    end
+
+    assert_match(/called from #{Regexp.escape(File.expand_path(__FILE__))}:\d+/, lines.first)
+  end
+
   test "warns separately for different subjects" do
     lines = capture_warnings do
       SnapDiff::Deprecation.warn("Old::A", "New::A", category: :constant)
diff --git a/test/unit/snap_diff_test.rb b/test/unit/snap_diff_test.rb
index 181fe59d..5f3bcf65 100644
--- a/test/unit/snap_diff_test.rb
+++ b/test/unit/snap_diff_test.rb
@@ -74,6 +74,47 @@ class SnapDiffTest < ActiveSupport::TestCase
     assert status.success?, "expected bare `require \"snap_diff\"` to annotate a difference, got:\n#{out}"
   end
 
+  # Acyclicity contract (the #208 deadlock-class fix): the lean
+  # `require "snap_diff"` entry must NEVER pull the umbrella
+  # capybara_screenshot_diff.rb back in. The old autoload wiring had
+  # snap_diff <-> capybara_screenshot_diff requiring each other, which
+  # produced load-order deadlocks/partially-initialized constants; #208
+  # broke the cycle, but until now only discipline guarded it -- a probe
+  # that reintroduced the cycle left the whole suite green. This asserts
+  # the contract as data: after a bare require, the umbrella file must be
+  # absent from $LOADED_FEATURES.
+  test "bare require \"snap_diff\" never loads the umbrella capybara_screenshot_diff" do
+    script = <<~RUBY
+      require "snap_diff"
+      umbrella = $LOADED_FEATURES.grep(%r{/lib/capybara_screenshot_diff\\.rb\\z})
+      abort("umbrella loaded via: \#{umbrella.join(", ")}") unless umbrella.empty?
+    RUBY
+
+    out, status = Open3.capture2e(RbConfig.ruby, "-Ilib", "-e", script)
+
+    assert status.success?, "expected bare `require \"snap_diff\"` to keep the umbrella unloaded, got:\n#{out}"
+  end
+
+  # Dual-install guard: both gem names ship identical files, so with BOTH
+  # activated every require silently resolves from whichever gem activated
+  # first -- version skew between them is undetectable. The entry point
+  # refuses that setup outright.
+  test "raises when both capybara-screenshot-diff and snap_diff-capybara are activated" do
+    specs = {"capybara-screenshot-diff" => :spec, "snap_diff-capybara" => :spec}
+
+    error = assert_raises(SnapDiff::DualInstallError) { SnapDiff.assert_single_gem!(specs) }
+
+    assert_match(/capybara-screenshot-diff/, error.message)
+    assert_match(/snap_diff-capybara/, error.message)
+    assert_match(/remove one/i, error.message)
+  end
+
+  test "dual-install guard passes single-gem installs and local dev from source" do
+    SnapDiff.assert_single_gem!({"capybara-screenshot-diff" => :spec})
+    SnapDiff.assert_single_gem!({"snap_diff-capybara" => :spec})
+    SnapDiff.assert_single_gem!({}) # local dev from source: neither spec loaded
+  end
+
   test ".start yields the same objects Diff.configure yields" do
     yielded = []
     Capybara::Screenshot::Diff.configure { |screenshot, diff| yielded << [screenshot, diff] }