diff --git a/.github/scripts/checkout_pr.py b/.github/scripts/checkout_pr.py
new file mode 100644
index 0000000..064f416
--- /dev/null
+++ b/.github/scripts/checkout_pr.py
@@ -0,0 +1,181 @@
+#!/usr/bin/env python3
+
+# Copyright (c) 2026 SandAI. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Robust PR checkout: tarball (GitHub API) → git-fetch fallback.
+
+Env: REPO_URL, BASE_SHA, HEAD_SHA, GITHUB_TOKEN, GITHUB_REPOSITORY.
+"""
+
+from __future__ import annotations
+
+import glob  # noqa: E401
+import os
+import re
+import shutil
+import subprocess
+import sys
+import tarfile
+import time
+
+RETRIES, SLEEP = 10, 5
+REPO_URL = os.environ["REPO_URL"]
+BASE_SHA = os.environ["BASE_SHA"]
+HEAD_SHA = os.environ["HEAD_SHA"]
+TOKEN = os.environ["GITHUB_TOKEN"]
+REPO = os.environ.get("GITHUB_REPOSITORY") or re.sub(r".*github\.com/", "", REPO_URL).removesuffix(".git")
+
+
+def log(msg: str) -> None:
+    print(f"[checkout] {msg}", flush=True)
+
+
+def sh(cmd: str, **kw) -> int:
+    kw.setdefault("check", True)
+    return subprocess.run(cmd, shell=True, **kw).returncode
+
+
+def env_no_proxy() -> dict[str, str]:
+    return {k: v for k, v in os.environ.items() if k.lower() not in ("http_proxy", "https_proxy")}
+
+
+# ── tarball checkout ─────────────────────────────────────────────────
+
+
+def _curl(sha: str, dest: str) -> bool:
+    url = f"https://api.github.com/repos/{REPO}/tarball/{sha}"
+    for i in range(1, RETRIES + 1):
+        via = "proxy" if i % 2 else "direct"
+        log(f"tarball({sha[:8]}) attempt {i}/{RETRIES} via {via}")
+        rc = subprocess.run(
+            [
+                "curl",
+                "-fSL",
+                "--retry",
+                "2",
+                "--connect-timeout",
+                "15",
+                "--max-time",
+                "180",
+                "-H",
+                f"Authorization: Bearer {TOKEN}",
+                "-H",
+                "Accept: application/vnd.github+json",
+                "-o",
+                dest,
+                url,
+            ],
+            env=env_no_proxy() if via == "direct" else None,
+        ).returncode
+        if rc == 0:
+            return True
+        if i < RETRIES:
+            time.sleep(SLEEP)
+    return False
+
+
+def _extract(tar_path: str) -> None:
+    with tarfile.open(tar_path, "r:gz") as tf:
+        prefix = os.path.commonprefix(tf.getnames()).rstrip("/")
+        for m in tf.getmembers():
+            m.name = m.name[len(prefix) :].lstrip("/") if prefix else m.name
+            if m.name:
+                tf.extract(m, ".", filter="data")
+
+
+def tarball_checkout() -> tuple[str, str] | None:
+    """Download HEAD & BASE tarballs, build synthetic two-commit repo.
+
+    Returns ``(local_base_sha, local_head_sha)`` or ``None``.
+    """
+    h, b = "/tmp/_head.tar.gz", "/tmp/_base.tar.gz"
+    try:
+        if not _curl(HEAD_SHA, h) or not _curl(BASE_SHA, b):
+            return None
+        shutil.rmtree(".git", ignore_errors=True)
+        sh("git init . && git config user.email ci@sandai.org && git config user.name CI")
+        for tar, msg, date in [
+            (b, f"base {BASE_SHA}", "2000-01-01T00:00:00Z"),
+            (h, f"head {HEAD_SHA}", "2000-01-02T00:00:00Z"),
+        ]:
+            if tar == h:
+                sh("git rm -rf . 2>/dev/null || true", check=False)
+            _extract(tar)
+            sh("git add -A")
+            env = {**os.environ, "GIT_COMMITTER_DATE": date, "GIT_AUTHOR_DATE": date}
+            sh(f'git commit --allow-empty -m "{msg}"', env=env)
+        base_l = subprocess.check_output(["git", "rev-parse", "HEAD~1"], text=True).strip()
+        head_l = subprocess.check_output(["git", "rev-parse", "HEAD"], text=True).strip()
+        log(f"synthetic: {BASE_SHA[:8]}→{base_l[:8]}, {HEAD_SHA[:8]}→{head_l[:8]}")
+        return base_l, head_l
+    finally:
+        for f in (h, b):
+            try:
+                os.remove(f)
+            except FileNotFoundError:
+                pass
+
+
+# ── git-fetch fallback ───────────────────────────────────────────────
+
+
+def git_fetch_checkout() -> bool:
+    if sh("git rev-parse --is-inside-work-tree", check=False) != 0:
+        sh("git init .")
+    if sh(f"git remote set-url origin {REPO_URL}", check=False) != 0:
+        sh(f"git remote add origin {REPO_URL}")
+    for i in range(1, RETRIES + 1):
+        for f in glob.glob(".git/*.lock"):
+            os.remove(f)
+        log(f"fetch attempt {i}/{RETRIES}")
+        if i % 2:
+            cmd = f"timeout 120 git -c http.lowSpeedLimit=100 -c http.lowSpeedTime=10 fetch --no-tags --prune --depth=1 origin {BASE_SHA} {HEAD_SHA}"
+            env = None
+        else:
+            cmd = f"timeout 120 git -c http.proxy= -c https.proxy= -c http.lowSpeedLimit=1 -c http.lowSpeedTime=30 fetch --no-tags --prune --depth=1 origin {BASE_SHA} {HEAD_SHA}"
+            env = env_no_proxy()
+        if sh(cmd, check=False, env=env) == 0:
+            sh(f"git checkout --force {HEAD_SHA} && git clean -fdx && git reset --hard {HEAD_SHA}")
+            return True
+        if i < RETRIES:
+            time.sleep(SLEEP)
+    return False
+
+
+# ── main ─────────────────────────────────────────────────────────────
+
+
+def main() -> None:
+    log(f"HEAD={HEAD_SHA}, BASE={BASE_SHA}")
+    result = tarball_checkout()
+    if result:
+        base_ref, head_ref = result
+        log("tarball succeeded")
+    elif git_fetch_checkout():
+        base_ref, head_ref = BASE_SHA, HEAD_SHA
+        log("git-fetch fallback succeeded")
+    else:
+        log("all methods failed")
+        sys.exit(1)
+    out = os.environ.get("GITHUB_OUTPUT")
+    if out:
+        with open(out, "a") as f:
+            f.write(f"base_ref={base_ref}\nhead_ref={head_ref}\n")
+    sh(f"git diff --stat {base_ref} {head_ref} | tail -3")
+    log("done")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/.github/workflows/integration_test.yml b/.github/workflows/integration_test.yml
index b3f7be3..1bd83d8 100644
--- a/.github/workflows/integration_test.yml
+++ b/.github/workflows/integration_test.yml
@@ -36,28 +36,44 @@ jobs:
                 git config --global http.proxy "${{ secrets.HTTP_PROXY }}"
                 git config --global https.proxy "${{ secrets.HTTPS_PROXY }}"
 
-        -   name: Checkout base branch
-            uses: actions/checkout@v6
-            timeout-minutes: 5
-            with:
-                ref: ${{ github.event.pull_request.base.sha }}
-                fetch-depth: 1
-                persist-credentials: false
+        -   name: Checkout PR head with retry
+            id: checkout
+            timeout-minutes: 10
+            env:
+                REPO_URL: https://github.com/${{ github.repository }}.git
+                GITHUB_REPOSITORY: ${{ github.repository }}
+                GITHUB_TOKEN: ${{ github.token }}
+                BASE_SHA: ${{ github.event.pull_request.base.sha }}
+                HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+            run: |
+                set -euo pipefail
+
+                script_path=".github/scripts/checkout_pr.py"
+                api_url="https://api.github.com/repos/${GITHUB_REPOSITORY}/contents/${script_path}?ref=${HEAD_SHA}"
+
+                mkdir -p .github/scripts
+                for attempt in $(seq 1 10); do
+                    echo "[bootstrap] download checkout script attempt ${attempt}/10"
+                    if curl -fsSL \
+                        -H "Authorization: Bearer ${GITHUB_TOKEN}" \
+                        -H "Accept: application/vnd.github.raw" \
+                        "$api_url" -o "$script_path"; then
+                        break
+                    fi
+                    if [ "$attempt" -eq 10 ]; then
+                        echo "[bootstrap] failed to download checkout script"
+                        exit 1
+                    fi
+                    sleep 3
+                done
 
-        -   name: Checkout PR head
-            uses: actions/checkout@v6
-            timeout-minutes: 5
-            with:
-                ref: ${{ github.event.pull_request.head.sha }}
-                fetch-depth: 1
-                persist-credentials: false
-                clean: false
+                python3 "$script_path"
 
         -   name: Check Chinese Characters
             run: python3 .github/workflows/check_chinese_chars.py
             env:
-                BASE_REF: ${{ github.event.pull_request.base.sha }}
-                HEAD_REF: ${{ github.event.pull_request.head.sha }}
+                BASE_REF: ${{ steps.checkout.outputs.base_ref || github.event.pull_request.base.sha }}
+                HEAD_REF: ${{ steps.checkout.outputs.head_ref || github.event.pull_request.head.sha }}
 
         -   name: Check Code Style
             run: pre-commit run --show-diff-on-failure --color=always --all-files