diff --git a/product-source/hololake-platform/guanghu-os/scripts/replace-jd-native-final-resident-candidate.sh b/product-source/hololake-platform/guanghu-os/scripts/replace-jd-native-final-resident-candidate.sh
index eb82a8a..4019983 100755
--- a/product-source/hololake-platform/guanghu-os/scripts/replace-jd-native-final-resident-candidate.sh
+++ b/product-source/hololake-platform/guanghu-os/scripts/replace-jd-native-final-resident-candidate.sh
@@ -43,8 +43,21 @@ dd if=/dev/vda bs=512 skip=141 count=1 status=none |
   grep -aq '^GHOS_GHCIP_INDEX_V1'
 dd if=/dev/vda bs=512 skip=142 count=1 status=none |
   grep -aq '^GHOS_GHCIP_ROOT_V1'
-grep -q '^GRUB_DEFAULT=guanghu-jd-native-once$' /etc/default/grub
-grep -q 'set default="guanghu-jd-native-once"' /boot/grub/grub.cfg
+grub_default=$(awk -F= '$1 == "GRUB_DEFAULT" { print $2; found = 1 } END { if (!found) exit 1 }' \
+  /etc/default/grub)
+case "${grub_default}" in
+  guanghu-jd-native-once)
+    grep -q 'set default="guanghu-jd-native-once"' /boot/grub/grub.cfg
+    ;;
+  gnulinux-simple-9e4550a0-452b-4f28-b5a5-d5364aa450f6)
+    grep -q 'set default="gnulinux-simple-9e4550a0-452b-4f28-b5a5-d5364aa450f6"' \
+      /boot/grub/grub.cfg
+    ;;
+  *)
+    echo "unsupported GRUB_DEFAULT for bounded native candidate replacement: ${grub_default}" >&2
+    exit 65
+    ;;
+esac
 
 mkdir -m 0700 "${recovery_root}"
 printf '%s\n' "${partition_dump}" >"${recovery_root}/sfdisk-before.txt"
@@ -85,7 +98,7 @@ preserved:
   world_and_code_store_lba_135_138: true
   recovery_beacon_lba_139_140: true
   gestational_index_lba_141_142: true
-  grub_default_native: true
+  grub_default_unchanged: ${grub_default}
 rollback:
   recovery_root: ${recovery_root}
   first_and_last_2mib_backed_up: true
diff --git a/product-source/hololake-platform/guanghu-os/scripts/test-jd-final-candidate-replacement-contract.sh b/product-source/hololake-platform/guanghu-os/scripts/test-jd-final-candidate-replacement-contract.sh
new file mode 100755
index 0000000..2bc43f2
--- /dev/null
+++ b/product-source/hololake-platform/guanghu-os/scripts/test-jd-final-candidate-replacement-contract.sh
@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+[[ $# -eq 1 ]] || {
+  echo "usage: test-jd-final-candidate-replacement-contract.sh <source-root>" >&2
+  exit 64
+}
+
+source_root=$(cd "$1" && pwd -P)
+script="${source_root}/scripts/replace-jd-native-final-resident-candidate.sh"
+
+grep -Fq 'guanghu-jd-native-once)' "${script}"
+grep -Fq 'gnulinux-simple-9e4550a0-452b-4f28-b5a5-d5364aa450f6)' "${script}"
+grep -Fq 'unsupported GRUB_DEFAULT for bounded native candidate replacement' "${script}"
+grep -Fq 'grub_default_unchanged: ${grub_default}' "${script}"
+! grep -Fq 'grub_default_native: true' "${script}"
+
+echo "JD_FINAL_CANDIDATE_REPLACEMENT_CONTRACT_OK"