diff --git a/.github/workflows/build-release.yml b/.github/workflows/build-release.yml
index 582980b..5b6f7f8 100644
--- a/.github/workflows/build-release.yml
+++ b/.github/workflows/build-release.yml
@@ -362,11 +362,29 @@ jobs:
               if: matrix.label == 'macos'
               run: echo "PYTHON_CMD_X64=${{ steps.python_x64.outputs.python-path }}" >> "$GITHUB_ENV"
 
+            - name: Install x86_64 libyaml for universal cx_Freeze slice
+              if: matrix.label == 'macos'
+              run: |
+                  set -euo pipefail
+                  # The Rosetta (x86_64) Homebrew lives at /usr/local/bin/brew on
+                  # Apple Silicon GitHub runners. libyaml must be available as
+                  # x86_64 so PyYAML's C extension (_yaml.cpython-3xx-darwin.so)
+                  # compiles for the darwin-x64 cx_Freeze slice. Without this,
+                  # pip falls back to pure Python pyyaml, which is absent from
+                  # the x64 tree while the arm64 tree has the binary, causing
+                  # unify-backend-plain-files.sh to fail.
+                  if [[ -x /usr/local/bin/brew ]]; then
+                      arch -x86_64 /usr/local/bin/brew install libyaml || true
+                  else
+                      echo "x86_64 Homebrew not found at /usr/local/bin/brew; PyYAML C extension may fall back to pure Python for x64 slice."
+                  fi
+
             - name: Install project deps into x64 Python (mac universal cx_Freeze)
               if: matrix.label == 'macos'
               env:
                   PY_X64: ${{ steps.python_x64.outputs.python-path }}
                   ARCHFLAGS: "-arch x86_64"
+                  PKG_CONFIG_PATH: "/usr/local/lib/pkgconfig:/usr/local/share/pkgconfig"
               run: |
                   set -euo pipefail
                   arch -x86_64 "$PY_X64" -m pip install -U pip setuptools wheel
diff --git a/scripts/unify-backend-plain-files.sh b/scripts/unify-backend-plain-files.sh
index de586e5..0059219 100755
--- a/scripts/unify-backend-plain-files.sh
+++ b/scripts/unify-backend-plain-files.sh
@@ -27,6 +27,7 @@ fi
 
 unified=0
 synced=0
+dropped=0
 
 copy_missing() {
     local src_dir="$1" dst_dir="$2" label="$3"
@@ -37,10 +38,13 @@ copy_missing() {
             local ft
             ft=$(file --brief --no-pad "$src_file" 2>/dev/null || true)
             if [[ "$ft" == Mach-O* ]]; then
-                echo "unify-backend: refusing to copy Mach-O across architecture trees: $rel" >&2
+                echo "unify-backend: dropping arch-only Mach-O for consistency: $rel" >&2
                 echo "  ($label); source reports: $ft" >&2
-                echo "  Fix the cx_Freeze build for ${dst_dir##*/} (darwin-x64 needs x86_64 Python: PYTHON_CMD / PYTHON_CMD_X64)." >&2
-                exit 1
+                echo "  Hint: ensure libyaml is available for x86_64 (arch -x86_64 brew install libyaml)" >&2
+                echo "  so PyYAML's C extension compiles for darwin-x64 and both trees match." >&2
+                rm -f "$src_file"
+                dropped=$((dropped + 1))
+                continue
             fi
             mkdir -p "$dst_dir/$(dirname "$rel")"
             cp "$src_file" "$dst_dir/$rel"
@@ -75,6 +79,9 @@ while IFS= read -r -d '' rel; do
 done < <(cd "$ARM64_DIR" && find . -type f -print0)
 
 total=$((unified + synced))
+if [[ $dropped -gt 0 ]]; then
+    echo "unify-backend: WARNING: dropped $dropped arch-only Mach-O binary/binaries for consistency (pure Python fallback active)"
+fi
 if [[ $total -gt 0 ]]; then
     echo "unify-backend: synced $synced missing file(s), unified $unified differing file(s)"
 else