diff --git a/docker-build-runner.sh b/docker-build-runner.sh
index f6f1160..20be7a8 100755
--- a/docker-build-runner.sh
+++ b/docker-build-runner.sh
@@ -157,9 +157,16 @@ seed_third_party_from_host() {
   local p
   for p in libvpx opus speex wireguard-android; do
     if [[ -d "${host_tree}/third-party/${p}" ]] && third_party_incomplete "$iso_tree" "$p"; then
-      log "[runner] seed third-party/${p} from host checkout"
+      log "[runner] seed third-party/${p} from host checkout (no .git metadata)"
       rm -rf "${iso_tree}/third-party/${p}"
-      cp -r "${host_tree}/third-party/${p}" "${iso_tree}/third-party/"
+      mkdir -p "${iso_tree}/third-party"
+      if command -v rsync >/dev/null 2>&1; then
+        rsync -a --delete --exclude='.git' --exclude='.git/' \
+          "${host_tree}/third-party/${p}/" "${iso_tree}/third-party/${p}/"
+      else
+        cp -a "${host_tree}/third-party/${p}" "${iso_tree}/third-party/"
+        find "${iso_tree}/third-party/${p}" -name .git -exec rm -rf {} + 2>/dev/null || true
+      fi
     fi
   done
 }
diff --git a/init-third-party-submodules.sh b/init-third-party-submodules.sh
index 54647f6..db92cc2 100755
--- a/init-third-party-submodules.sh
+++ b/init-third-party-submodules.sh
@@ -91,6 +91,11 @@ for path in "${SUB_PATHS[@]}"; do
     echo "error: $path not in .gitmodules" >&2
     exit 1
   fi
+  if validate_path "$path"; then
+    short="$(git -C "$path" rev-parse --short HEAD 2>/dev/null || echo seeded)"
+    echo "    $path @ $short (pre-seeded)"
+    continue
+  fi
   ensure_gitlink "$path"
   git submodule sync "$path"
   git submodule update --init --recursive "$path"