diff --git a/cloudflare-worker/src/lb.js b/cloudflare-worker/src/lb.js
index a41ea3d..8e14af5 100644
--- a/cloudflare-worker/src/lb.js
+++ b/cloudflare-worker/src/lb.js
@@ -139,8 +139,21 @@ export async function forwardWithFailover(origins, request, opts) {
   });
 }
 
-/** 在响应上加轻量可观测头（不改 body/状态语义；origin 非敏感，含在响应头中便于排障）。 */
+/**
+ * 在响应上加轻量可观测头（不改 body/状态语义；origin 非敏感，含在响应头中便于排障）。
+ *
+ * ⚠️ 跨域且不带 CORS 允许头的响应，其 `headers` 的 guard 是 immutable，`set` 会抛
+ * `TypeError: Can't modify immutable headers.`（workerd 实测：后端 `/` 返回 200 但无
+ * `access-control-allow-origin` 即触发）。LB 代理的响应几乎都属于这种——若让该异常冒泡到
+ * `forwardWithFailover` 的 catch，一个健康的 200 会被记成 `backend ... unavailable (TypeError)`
+ * 并耗尽重试、返回 503（2026-09-30 生产事故根因）。观测头是尽力而为的元数据，
+ * 不可变时静默放弃，绝不阻塞响应返回。
+ */
 function mark(resp, origin) {
-  resp.headers.set("x-lb-backend", origin);
+  try {
+    resp.headers.set("x-lb-backend", origin);
+  } catch {
+    /* 响应 headers 不可变（跨域且无 CORS）→ 放弃观测头，照常返回响应。 */
+  }
   return resp;
 }
diff --git a/cloudflare-worker/test/lb.test.js b/cloudflare-worker/test/lb.test.js
index 237d4e3..ba0499a 100644
--- a/cloudflare-worker/test/lb.test.js
+++ b/cloudflare-worker/test/lb.test.js
@@ -10,6 +10,7 @@ import { forwardWithFailover, joinUrl, isRetryableError } from "../src/lb.js";
  * 可控 fetch mock：steps[i] 每次调用消费一个；
  * - {status, body?} → 返回该 Response；
  * - {error: Error} → 抛出。
+ * - {response: Response} → 原样返回该对象（用于模拟 workerd 特有行为，如不可变响应头）；
  */
 function mockFetch(steps) {
   const calls = [];
@@ -18,6 +19,7 @@ function mockFetch(steps) {
     calls.push({ url, init });
     const step = steps[Math.min(i, steps.length - 1)];
     if (step.error) throw step.error;
+    if (step.response) return step.response;
     return new Response(step.body ?? "ok", { status: step.status, headers: { "content-type": "text/plain" } });
   };
   return { fetch, calls };
@@ -26,6 +28,24 @@ function mockFetch(steps) {
 /** 测试用计时器：不真触发超时。 */
 const noOpTimers = { setTimeout: () => 0, clearTimeout: () => {} };
 
+/**
+ * 模拟「不可变响应头」：跨域且不带 CORS 允许头的响应，其 headers guard 是 immutable，
+ * 任何写操作都抛 `TypeError: Can't modify immutable headers.`（workerd 实测行为）。
+ * 生产 LB 代理后端 `/` 正是这种响应。
+ */
+function immutableResponse(status, body = "healthy") {
+  return {
+    status,
+    headers: {
+      get() { return null; },
+      set() { throw new TypeError("Can't modify immutable headers."); },
+      append() { throw new TypeError("Can't modify immutable headers."); },
+    },
+    body: undefined,
+    text: async () => body,
+  };
+}
+
 function req(path = "/reconcile") {
   return new Request(`https://lb.example${path}`, {
     method: "POST",
@@ -70,6 +90,38 @@ test("transparent passthrough forwards headers + body + method (SAF-LB-PASSTHRU)
   assert.equal(res.headers.get("x-lb-backend"), "https://a.example");
 });
 
+test("immutable response headers never mask a healthy 2xx as 503 (跨域无 CORS)", async () => {
+  // 回归：2026-09-30 生产事故。后端返回健康的 200，但该响应跨域且无 CORS 头，
+  // headers guard 为 immutable；mark() 写 `x-lb-backend` 抛 TypeError，被外层 catch
+  // 记成 `backend ... unavailable (TypeError)` → 重试耗尽 → 503。/healthz 自建响应
+  // 不调 mark()，所以一直正常，掩盖了问题。
+  const { fetch, calls } = mockFetch([{ response: immutableResponse(200) }]);
+  const res = await forwardWithFailover(["https://a.example"], req("/"), {
+    ...noOpTimers,
+    fetch,
+    maxAttempts: 2,
+    timeoutMs: 5000,
+    rng: () => 0,
+  });
+  assert.equal(res.status, 200, "healthy 200 must be returned as-is");
+  assert.equal(calls.length, 1, "a healthy response must not trigger failover");
+  assert.equal(await res.text(), "healthy");
+});
+
+test("immutable headers are tolerated on the 5xx failover path too", async () => {
+  const { fetch, calls } = mockFetch([{ response: immutableResponse(503) }, { status: 200, body: "ok" }]);
+  const res = await forwardWithFailover(["https://a.example", "https://b.example"], req("/"), {
+    ...noOpTimers,
+    fetch,
+    maxAttempts: 2,
+    timeoutMs: 5000,
+    rng: () => 0,
+  });
+  assert.equal(res.status, 200, "503 with immutable headers still fails over");
+  assert.equal(calls.length, 2);
+  assert.equal(res.headers.get("x-lb-backend"), "https://b.example", "marker still added on a mutable response");
+});
+
 test("GET/HEAD do not forward a body", async () => {
   const { fetch, calls } = mockFetch([{ status: 200 }]);
   const get = new Request("https://lb.example/ready", { method: "GET", headers: { authorization: "Bearer x" } });