diff --git a/iso/builder/build.sh b/iso/builder/build.sh
index 5ff446f..fce5b46 100755
--- a/iso/builder/build.sh
+++ b/iso/builder/build.sh
@@ -126,6 +126,37 @@ resolve_iso_version() {
     resolve_audit_version
 }
 
+sync_builder_workdir() {
+    src_dir="$1"
+    dst_dir="$2"
+
+    mkdir -p "$dst_dir"
+
+    # Historical bug: old workdirs could keep config/bootloaders/grub-pc even
+    # after the source tree moved to grub-efi only. Remove bootloaders eagerly
+    # so reused workdirs cannot leak stale templates into a new ISO build.
+    rm -rf "$dst_dir/config/bootloaders"
+
+    rsync -a --delete \
+        --exclude='cache/' \
+        --exclude='chroot/' \
+        --exclude='.build/' \
+        --exclude='*.iso' \
+        --exclude='*.packages' \
+        --exclude='*.contents' \
+        --exclude='*.files' \
+        "$src_dir/" "$dst_dir/"
+
+    if [ ! -f "$dst_dir/config/bootloaders/grub-efi/grub.cfg" ]; then
+        echo "ERROR: staged workdir is missing config/bootloaders/grub-efi/grub.cfg" >&2
+        exit 1
+    fi
+    if [ -e "$dst_dir/config/bootloaders/grub-pc" ]; then
+        echo "ERROR: stale config/bootloaders/grub-pc remained in staged workdir" >&2
+        exit 1
+    fi
+}
+
 iso_list_files() {
     iso_path="$1"
 
@@ -1181,15 +1212,7 @@ echo "=== preparing staged overlay (${BUILD_VARIANT}) ==="
 mkdir -p "${BUILD_WORK_DIR}" "${OVERLAY_STAGE_DIR}"
 
 # Sync builder config into variant work dir, preserving lb cache.
-rsync -a --delete \
-    --exclude='cache/' \
-    --exclude='chroot/' \
-    --exclude='.build/' \
-    --exclude='*.iso' \
-    --exclude='*.packages' \
-    --exclude='*.contents' \
-    --exclude='*.files' \
-    "${BUILDER_DIR}/" "${BUILD_WORK_DIR}/"
+sync_builder_workdir "${BUILDER_DIR}" "${BUILD_WORK_DIR}"
 
 # Share deb package cache across variants.
 # Restore: populate work dir cache from shared cache before build.