diff --git a/scripts/unify-backend-plain-files.sh b/scripts/unify-backend-plain-files.sh
index 8f0a18a..b541442 100755
--- a/scripts/unify-backend-plain-files.sh
+++ b/scripts/unify-backend-plain-files.sh
@@ -1,11 +1,17 @@
 #!/usr/bin/env bash
-# Ensure non-binary files in the two per-arch cx_Freeze outputs are
-# byte-identical so @electron/universal's SHA check passes.
+# Make the two per-arch cx_Freeze backend trees compatible with
+# @electron/universal's merge requirements:
+#
+#   1. Every file must exist in BOTH trees (no unique-to-one-arch files).
+#   2. Every non-Mach-O file must be byte-identical across trees.
 #
 # Python bytecode (.pyc inside library.zip) is architecture-independent;
-# only timestamps and zip metadata differ between the arm64 and x64
-# builds.  Native extensions (.dylib/.so) are Mach-O binaries and
-# handled separately by lipo, so they are excluded here.
+# only timestamps and zip metadata cause SHA differences.
+#
+# Native extensions (.so/.dylib) that exist in only one tree are copied
+# to the other so the file sets match. The copy won't load on the wrong
+# arch at runtime, but the owning package (e.g. PyYAML) falls back to
+# its pure-Python implementation, so this is safe.
 set -euo pipefail
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd)"
@@ -14,11 +20,28 @@ ARM64_DIR="$ROOT/build/exe/darwin-arm64"
 X64_DIR="$ROOT/build/exe/darwin-x64"
 
 if [[ ! -d "$ARM64_DIR" || ! -d "$X64_DIR" ]]; then
-    echo "unify-backend-plain-files: one or both backend dirs missing, skipping"
+    echo "unify-backend: one or both backend dirs missing, skipping"
     exit 0
 fi
 
 unified=0
+synced=0
+
+copy_missing() {
+    local src_dir="$1" dst_dir="$2" label="$3"
+    while IFS= read -r -d '' rel; do
+        rel="${rel#./}"
+        if [[ ! -f "$dst_dir/$rel" ]]; then
+            mkdir -p "$dst_dir/$(dirname "$rel")"
+            cp "$src_dir/$rel" "$dst_dir/$rel"
+            echo "  synced ($label): $rel"
+            synced=$((synced + 1))
+        fi
+    done < <(cd "$src_dir" && find . -type f -print0)
+}
+
+copy_missing "$ARM64_DIR" "$X64_DIR" "arm64 -> x64"
+copy_missing "$X64_DIR" "$ARM64_DIR" "x64 -> arm64"
 
 while IFS= read -r -d '' rel; do
     rel="${rel#./}"
@@ -41,8 +64,9 @@ while IFS= read -r -d '' rel; do
     unified=$((unified + 1))
 done < <(cd "$ARM64_DIR" && find . -type f -print0)
 
-if [[ $unified -gt 0 ]]; then
-    echo "unify-backend-plain-files: copied $unified file(s) from arm64 → x64"
+total=$((unified + synced))
+if [[ $total -gt 0 ]]; then
+    echo "unify-backend: synced $synced missing file(s), unified $unified differing file(s)"
 else
-    echo "unify-backend-plain-files: all non-binary files already identical"
+    echo "unify-backend: all files already identical and present in both trees"
 fi