diff --git a/web/scripts/check-changed-formatting.mjs b/web/scripts/check-changed-formatting.mjs
index 1a3225354..f1a9a91dd 100644
--- a/web/scripts/check-changed-formatting.mjs
+++ b/web/scripts/check-changed-formatting.mjs
@@ -12,8 +12,19 @@ export async function checkChangedFormatting({ cwd = process.cwd(), baseSha = pr
         return result.stdout;
     };
     const head = git(["rev-parse", "--verify", `${headSha}^{commit}`]).trim();
-    // 新分支/tag 的 before 为全零；首次提交没有父提交，改为检查整棵树。
-    const base = baseSha && !/^0+$/.test(baseSha) ? git(["rev-parse", "--verify", `${baseSha}^{commit}`]).trim() : git(["rev-list", "--parents", "-n", "1", head]).trim().split(" ")[1];
+    const parent = git(["rev-list", "--parents", "-n", "1", head]).trim().split(" ")[1] || null;
+    let base = parent;
+    if (baseSha && !/^0+$/.test(baseSha)) {
+        // 强制改写提交历史后，push 事件里的 before 可能已不在 checkout 中；
+        // 对合法但失效的 SHA 退回当前父提交，仍校验本次提交的完整改动。
+        if (!/^[0-9a-f]{40}$/i.test(baseSha)) throw new Error(`Invalid base SHA: ${baseSha}`);
+        const resolved = spawnSync("git", ["rev-parse", "--verify", `${baseSha}^{commit}`], { cwd, encoding: "utf8" });
+        if (resolved.status === 0) {
+            base = resolved.stdout.trim();
+        } else {
+            log(`Base SHA ${baseSha} is unavailable; checking against the head parent instead.`);
+        }
+    }
     const files = (base ? git(["diff", "--relative", "--name-only", "--diff-filter=ACMR", "-z", base, head, "--", "."]) : git(["ls-tree", "-r", "--name-only", "-z", head, "--", "."]))
         .split("\0")
         .filter((file) => /\.(css|html|json|js|jsx|md|mdx|mjs|cjs|ts|tsx|yaml|yml)$/.test(file));
diff --git a/web/test/ci-formatting.test.mjs b/web/test/ci-formatting.test.mjs
index 948eecce8..06aa0f201 100644
--- a/web/test/ci-formatting.test.mjs
+++ b/web/test/ci-formatting.test.mjs
@@ -71,9 +71,17 @@ describe("changed-file formatting gate", () => {
         expect(repo.messages).toEqual([]);
     });
 
-    test("invalid explicit base SHA fails closed", async () => {
+    test("unavailable explicit base SHA falls back to the head parent", async () => {
         const repo = fixture();
-        await expect(repo.check({ baseSha: "f".repeat(40) })).rejects.toThrow("git rev-parse");
+        repo.write("changed.ts", "const value=1;\n");
+        repo.commit();
+        await expect(repo.check({ baseSha: "f".repeat(40) })).rejects.toThrow("changed.ts");
+        expect(repo.messages).toContain(`Base SHA ${"f".repeat(40)} is unavailable; checking against the head parent instead.`);
+    });
+
+    test("malformed explicit base SHA fails closed", async () => {
+        const repo = fixture();
+        await expect(repo.check({ baseSha: "not-a-sha" })).rejects.toThrow("Invalid base SHA");
     });
 
     test("zero before SHA compares the parent commit", async () => {