diff --git a/sdk/demo/index.html b/sdk/demo/index.html
new file mode 100644
index 0000000..bece85a
--- /dev/null
+++ b/sdk/demo/index.html
@@ -0,0 +1,5 @@
+<!doctype html><html><head><meta charset="utf-8"><title>CTK Demo 1</title>
+<script>window.ctk = window.ctk || function(){(ctk.q=ctk.q||[]).push(arguments)};
+ctk("init", { siteId: "site_x", endpoint: "http://127.0.0.1:8000/api/collect/batch" });</script>
+<script async src="../dist/v1.js"></script></head>
+<body><h1>Demo 页 1</h1><a href="page2.html">去第 2 页</a></body></html>
diff --git a/sdk/demo/page2.html b/sdk/demo/page2.html
new file mode 100644
index 0000000..fb7565b
--- /dev/null
+++ b/sdk/demo/page2.html
@@ -0,0 +1,5 @@
+<!doctype html><html><head><meta charset="utf-8"><title>CTK Demo 2</title>
+<script>window.ctk = window.ctk || function(){(ctk.q=ctk.q||[]).push(arguments)};
+ctk("init", { siteId: "site_x", endpoint: "http://127.0.0.1:8000/api/collect/batch" });</script>
+<script async src="../dist/v1.js"></script></head>
+<body><h1>Demo 页 2</h1><a href="index.html">返回第 1 页</a></body></html>
diff --git a/sdk/src/events.ts b/sdk/src/events.ts
new file mode 100644
index 0000000..06936af
--- /dev/null
+++ b/sdk/src/events.ts
@@ -0,0 +1,65 @@
+import type { Transport } from "./transport";
+import { pageContext } from "./context";
+import { uuid } from "./util";
+
+// 有效停留（spec §4.3）：页面可见且 30s 内有交互活动才按秒累计 activeMs；
+// 隐藏（visibilitychange→hidden）或离开（pagehide）时发 page_stay 并 flush(true)。
+// close 可能连续触发（hidden 后紧跟 pagehide）：activeMs 清零后不再重发 stay。
+// 红线：任何回调异常不得冒泡到宿主页（spec §4.5）。
+export function startPageTracking(t: Transport) {
+  let activeMs = 0;
+  let lastAct = Date.now();
+  const act = () => {
+    lastAct = Date.now();
+  };
+  ["pointerdown", "keydown", "wheel", "touchstart"].forEach((e) =>
+    document.addEventListener(e, act, { passive: true, capture: true })
+  );
+  setInterval(() => {
+    try {
+      if (!document.hidden && Date.now() - lastAct < 30_000) activeMs += 1000;
+    } catch {
+      /* 静默 */
+    }
+  }, 1000);
+  // emit 内 pageContext() 只取一次（pc）复用于 path/title——避免多次读取间
+  // location 瞬变导致同一事件内字段不一致；page_view 另将 pc 传入复用
+  const emit = (
+    event_name: string,
+    properties: Record<string, unknown> = {},
+    pc = pageContext()
+  ) => {
+    t.push({
+      event_id: uuid(),
+      event_name,
+      occurred_at: new Date().toISOString(),
+      path: pc.path,
+      title: pc.title,
+      properties,
+    });
+  };
+  const pc = pageContext();
+  emit(
+    "page_view",
+    { referrer: pc.referrer, referrer_domain: pc.referrer_domain, utm: pc.utm },
+    pc
+  );
+  const close = () => {
+    try {
+      // path 必须与 page_view 同源同清洗（pageContext().path 含 query），
+      // 否则带 UTM 进落地页时服务端"page_stay 合并进相邻 page_view"匹配失败
+      if (activeMs > 0) {
+        const path = pageContext().path;
+        emit("page_stay", { active_ms: activeMs, path });
+      }
+      activeMs = 0;
+      t.flush(true);
+    } catch {
+      /* 静默 */
+    }
+  };
+  document.addEventListener("visibilitychange", () => {
+    if (document.hidden) close();
+  });
+  window.addEventListener("pagehide", close);
+}
diff --git a/sdk/src/index.ts b/sdk/src/index.ts
new file mode 100644
index 0000000..84efd7a
--- /dev/null
+++ b/sdk/src/index.ts
@@ -0,0 +1,54 @@
+import { uuid } from "./util";
+import { mergeConfig } from "./config";
+import { Transport } from "./transport";
+import { startPageTracking } from "./events";
+import { buildEnvContext, pageContext } from "./context";
+import type { CtkConfig } from "./config";
+
+// IIFE 打包入口（build.mjs）：模块加载即接管全局 ctk 命令队列并重放。
+// 红线：dispatch 内任何异常静默，绝不冒泡到宿主页（spec §4.5）。
+type Cmd = [string, unknown?];
+type CtkFn = ((cmd: string, arg?: unknown) => void) & { q?: Cmd[] };
+const g = globalThis as unknown as {
+  ctk?: CtkFn;
+  __ctk_ctx?: unknown;
+};
+const q: Cmd[] = (g.ctk?.q as Cmd[] | undefined) || [];
+let transport: Transport | null = null;
+
+function dispatch(cmd: string, arg?: unknown) {
+  try {
+    if (cmd === "init" && !transport) {
+      const cfg = mergeConfig((arg ?? {}) as CtkConfig);
+      // 首触归因：批次 context 除设备环境外必须带 referrer_domain/utm/path——
+      // 服务端只从 context 读首触，缺了则首触/会话来源恒空
+      const pc = pageContext();
+      g.__ctk_ctx = {
+        ...buildEnvContext(),
+        referrer_domain: pc.referrer_domain,
+        utm: pc.utm,
+        path: pc.path,
+      };
+      transport = new Transport(cfg);
+      transport.push({
+        event_id: uuid(),
+        event_name: "sdk_loaded",
+        occurred_at: new Date().toISOString(),
+        path: pc.path,
+        title: pc.title,
+        properties: { sdk_version: "0.1.0" },
+      });
+      startPageTracking(transport);
+    } else if (cmd === "track" && transport) {
+      transport.push(arg as object);
+    } else if (cmd === "flush" && transport) {
+      transport.flush();
+    }
+  } catch {
+    /* 任何异常静默，绝不冒泡到宿主页（spec §4.5） */
+  }
+}
+
+// 必须先替换全局入口再重放队列：重放期间宿主新调用直接进 dispatch，不再排队
+g.ctk = dispatch;
+q.forEach(([c, a]) => dispatch(c, a));
diff --git a/sdk/src/transport.ts b/sdk/src/transport.ts
index 4683a8f..113bc66 100644
--- a/sdk/src/transport.ts
+++ b/sdk/src/transport.ts
@@ -7,8 +7,8 @@ export type TransportConfig = Required<CtkConfig>;
 // 批量传输：满 batchSize 或 flushMs 到期即发；4xx（除 429）不重试；
 // 指数退避 + 抖动重试；重试耗尽按本批事件数计 dropped；queueMax 溢出丢最旧。
 // 红线：任何异常不得冒泡到宿主页（定时器/事件回调内的异常无人捕获）。
-// 审查留档：events 目前均为 SDK 自产可序列化对象；若未来引入受控外的 events
-// 来源，需处理 stringify 抛错时已换出批次的计数（当前静默丢弃不计 dropped）。
+// 审查留档（Task 14 结论）：index.ts 的 track 分支已引入宿主可控对象——
+// stringify 抛错（如循环引用）时已换出的批次按事件数计 dropped。
 export class Transport {
   private q: object[] = [];
   private timer: ReturnType<typeof setTimeout> | null = null;
@@ -33,9 +33,11 @@ export class Transport {
       this.timer = null;
     }
     if (!this.q.length) return;
+    let n = 0;
     try {
       const events = this.q;
       this.q = [];
+      n = events.length; // 换出后先记数：stringify 抛错时本批已丢，须计 dropped
       const body = JSON.stringify({
         batch_id: uuid(),
         site_id: this.cfg.siteId,
@@ -55,7 +57,9 @@ export class Transport {
       }
       this.send(body, events.length);
     } catch {
-      // 定时器/事件回调内的任何异常不得冒泡到宿主页
+      // 定时器/事件回调内的任何异常不得冒泡到宿主页；
+      // stringify 抛错时批次已换出（宿主可控对象），按事件数计丢弃
+      this.dropped += n;
     }
   }
 
diff --git a/sdk/test/events.test.ts b/sdk/test/events.test.ts
new file mode 100644
index 0000000..d441b26
--- /dev/null
+++ b/sdk/test/events.test.ts
@@ -0,0 +1,145 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { Transport } from "../src/transport";
+import { mergeConfig } from "../src/config";
+import { startPageTracking } from "../src/events";
+
+// 有效停留（spec §4.3）：可见 + 30s 内有活动才按秒累计。
+// 注意：startPageTracking 在 document/window 上注册的监听跨用例存留（匿名函数无法摘除），
+// 故每个用例用独立 siteId，并按 body.site_id 过滤断言，避免旧 tracker 的 close 串扰计数。
+// happy-dom 的 navigator.sendBeacon 会发真实网络请求——beforeEach 里整体 stubGlobal 替换
+// （Task 14 注释警示），凡 close→flush(true) 路径均不得触网。
+
+const UUID_RE = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/;
+
+let fetchMock: ReturnType<typeof vi.fn>;
+let beacon: ReturnType<typeof vi.fn>;
+
+function stubNavigator() {
+  beacon = vi.fn().mockReturnValue(true);
+  vi.stubGlobal("navigator", {
+    userAgent:
+      "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36",
+    language: "zh-CN",
+    hardwareConcurrency: 8,
+    sendBeacon: beacon,
+  });
+}
+
+function mkTransport(siteId: string) {
+  return new Transport(
+    mergeConfig({ siteId, endpoint: "/api/collect/batch", batchSize: 10, flushMs: 60000 })
+  );
+}
+
+/** 从 beacon 调用里取指定 site 的请求体（过滤掉旧用例 tracker 的串扰批次） */
+async function beaconBodies(siteId: string) {
+  const out = [];
+  for (const [, blob] of beacon.mock.calls as unknown as [string, Blob][]) {
+    const body = JSON.parse(await blob.text());
+    if (body.site_id === siteId) out.push(body);
+  }
+  return out as { site_id: string; events: { event_name: string; path: string; properties: Record<string, unknown> }[] }[];
+}
+
+describe("events / startPageTracking", () => {
+  beforeEach(() => {
+    localStorage.clear();
+    vi.useFakeTimers();
+    fetchMock = vi.fn().mockResolvedValue({ ok: true, status: 200 } as Response);
+    vi.stubGlobal("fetch", fetchMock);
+    stubNavigator();
+  });
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    vi.useRealTimers(); // 旧 tracker 的 1s interval 挂在各自 fake clock 上，随钟一起废弃
+    delete (document as { hidden?: boolean }).hidden; // 还原 happy-dom 原型 getter
+    delete (document as { referrer?: string }).referrer;
+  });
+
+  it("page_view 含 path/title/referrer_domain/utm（path 同源清洗）", async () => {
+    document.title = "落地页";
+    Object.defineProperty(document, "referrer", {
+      value: "https://news.example/article/1",
+      configurable: true,
+    });
+    window.history.pushState({}, "", "/land?utm_source=ns&utm_medium=cpc&email=a@b.c");
+    const t = mkTransport("site-pv");
+    startPageTracking(t);
+    t.flush();
+
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+    const body = JSON.parse(fetchMock.mock.calls[0][1].body);
+    const pv = body.events.find((e: { event_name: string }) => e.event_name === "page_view");
+    expect(pv.event_id).toMatch(UUID_RE);
+    expect(pv.occurred_at).toMatch(/^\d{4}-\d{2}-\d{2}T/);
+    expect(pv.path).toBe("/land?utm_source=ns&utm_medium=cpc"); // email 参数被清洗，utm 保留
+    expect(pv.title).toBe("落地页");
+    expect(pv.properties).toEqual({
+      referrer: "https://news.example/article/1",
+      referrer_domain: "news.example",
+      utm: { utm_source: "ns", utm_medium: "cpc" },
+    });
+  });
+
+  it("有效停留：可见 + pointerdown 后 3s → close 发 page_stay(active_ms=3000)", async () => {
+    window.history.pushState({}, "", "/p4");
+    const t = mkTransport("site-t4");
+    startPageTracking(t);
+    document.dispatchEvent(new Event("pointerdown"));
+    await vi.advanceTimersByTimeAsync(3000); // 1s interval 走 3 拍
+
+    window.dispatchEvent(new Event("pagehide")); // 触发 close
+    expect(fetchMock).not.toHaveBeenCalled(); // close 走 flush(true) → sendBeacon 而非 fetch
+    const bodies = await beaconBodies("site-t4");
+    expect(bodies).toHaveLength(1);
+    expect(bodies[0].events.map((e) => e.event_name)).toEqual(["page_view", "page_stay"]);
+    const stay = bodies[0].events[1];
+    expect(stay.properties.active_ms).toBe(3000);
+    expect(stay.path).toBe("/p4"); // 顶层 path 与 page_view 同源
+    expect(stay.properties.path).toBe("/p4"); // properties.path 供服务端合并相邻 page_view
+  });
+
+  it("hidden 后不累计：visibilitychange→close 发 page_stay 并 flush(true)；再次 close 不重发", async () => {
+    window.history.pushState({}, "", "/p5");
+    const t = mkTransport("site-t5");
+    startPageTracking(t);
+    document.dispatchEvent(new Event("pointerdown"));
+    await vi.advanceTimersByTimeAsync(2000); // 累计 2000
+
+    Object.defineProperty(document, "hidden", { value: true, configurable: true });
+    document.dispatchEvent(new Event("visibilitychange")); // close 第 1 次
+    await vi.advanceTimersByTimeAsync(5000); // hidden 期间不累计（若累计则下步会多出一批）
+    window.dispatchEvent(new Event("pagehide")); // close 第 2 次（双触发）
+
+    const bodies = await beaconBodies("site-t5");
+    expect(bodies).toHaveLength(1); // 第 2 次 close：activeMs=0 不发 stay，空队列 no-op
+    const stay = bodies[0].events.find((e) => e.event_name === "page_stay")!;
+    expect(stay.properties.active_ms).toBe(2000); // 恰为隐藏前累计值
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+
+  it("隐藏→恢复→再隐藏：两段 stay 各自正确（第二段从 0 重新累计）", async () => {
+    window.history.pushState({}, "", "/p6");
+    const t = mkTransport("site-t6");
+    startPageTracking(t);
+    document.dispatchEvent(new Event("pointerdown"));
+    await vi.advanceTimersByTimeAsync(2000); // 段 1：2000ms
+    Object.defineProperty(document, "hidden", { value: true, configurable: true });
+    document.dispatchEvent(new Event("visibilitychange")); // close → 第 1 批
+
+    delete (document as { hidden?: boolean }).hidden; // 恢复可见（还原原型 getter）
+    document.dispatchEvent(new Event("visibilitychange")); // 非 hidden → 不 close
+    document.dispatchEvent(new Event("pointerdown")); // 新活动
+    await vi.advanceTimersByTimeAsync(3000); // 段 2：3000ms
+    Object.defineProperty(document, "hidden", { value: true, configurable: true });
+    document.dispatchEvent(new Event("visibilitychange")); // close → 第 2 批
+
+    const bodies = await beaconBodies("site-t6");
+    expect(bodies).toHaveLength(2);
+    const s1 = bodies[0].events.find((e) => e.event_name === "page_stay")!;
+    const s2 = bodies[1].events.find((e) => e.event_name === "page_stay")!;
+    expect(s1.properties.active_ms).toBe(2000);
+    expect(s2.properties.active_ms).toBe(3000); // 从 0 重新起算，非 5000
+    expect(bodies[1].events).toHaveLength(1); // 第 2 批只有 page_stay
+  });
+});
diff --git a/sdk/test/index.test.ts b/sdk/test/index.test.ts
new file mode 100644
index 0000000..533e674
--- /dev/null
+++ b/sdk/test/index.test.ts
@@ -0,0 +1,177 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+
+// index.ts 是 IIFE 打包入口：模块加载即读取全局队列、替换 g.ctk 并重放。
+// 隔离手段：vi.resetModules() + 动态 import，每用例拿到全新模块级 transport 状态。
+// startPageTracking 注册的 document/window 监听跨用例存留，用独立 siteId 过滤批次。
+
+type Cmd = [string, unknown?];
+type CtkFn = ((cmd: string, arg?: unknown) => void) & { q?: Cmd[] };
+const g = globalThis as unknown as {
+  ctk?: CtkFn;
+  __ctk_ctx?: Record<string, unknown>;
+};
+const UUID_RE = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/;
+
+let fetchMock: ReturnType<typeof vi.fn>;
+
+/** 模拟宿主snippet：先以桩函数排队，再动态加载 index 模块（触发重放） */
+async function loadSdk(q: Cmd[] = []) {
+  vi.resetModules();
+  const stub = (cmd: string, arg?: unknown) => {
+    q.push([cmd, arg]);
+  };
+  stub.q = q;
+  g.ctk = stub;
+  await import("../src/index");
+}
+
+function bodiesOf(siteId: string) {
+  return fetchMock.mock.calls
+    .map((c) => JSON.parse(c[1].body as string))
+    .filter((b: { site_id: string }) => b.site_id === siteId);
+}
+
+describe("index 入口", () => {
+  beforeEach(() => {
+    localStorage.clear();
+    vi.useFakeTimers();
+    fetchMock = vi.fn().mockResolvedValue({ ok: true, status: 200 } as Response);
+    vi.stubGlobal("fetch", fetchMock);
+    // happy-dom 的 sendBeacon 会发真实网络请求——整体替换（Task 14 注释警示）
+    vi.stubGlobal("navigator", {
+      userAgent:
+        "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36",
+      language: "zh-CN",
+      hardwareConcurrency: 8,
+      sendBeacon: vi.fn().mockReturnValue(true),
+    });
+  });
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    vi.useRealTimers();
+    delete g.__ctk_ctx;
+    delete (document as { hidden?: boolean }).hidden;
+    delete (document as { referrer?: string }).referrer;
+  });
+
+  it("init 后 sdk_loaded 入队（含 sdk_version）", async () => {
+    await loadSdk([["init", { siteId: "site-1", endpoint: "/e", flushMs: 60000 }]]);
+    g.ctk!("flush");
+
+    const bodies = bodiesOf("site-1");
+    expect(bodies).toHaveLength(1);
+    const loaded = bodies[0].events.find((e: { event_name: string }) => e.event_name === "sdk_loaded");
+    expect(loaded).toBeTruthy();
+    expect(loaded.properties.sdk_version).toBe("0.1.0");
+    expect(loaded.event_id).toMatch(UUID_RE);
+  });
+
+  it("批次 context 含设备环境 + referrer_domain/utm/path（首触归因链路）", async () => {
+    Object.defineProperty(document, "referrer", {
+      value: "https://news.example/a",
+      configurable: true,
+    });
+    window.history.pushState({}, "", "/land?utm_source=ns&utm_campaign=m1");
+    await loadSdk([["init", { siteId: "site-2", endpoint: "/e", flushMs: 60000 }]]);
+
+    // __ctk_ctx 四类键齐全：设备环境 + referrer_domain + utm + path
+    const ctx = g.__ctk_ctx!;
+    expect(ctx.device_type).toBe("PC");
+    expect(ctx.os).toBe("Windows");
+    expect(ctx.browser).toBe("Chrome");
+    expect(ctx.language).toBe("zh-CN");
+    expect(typeof ctx.timezone).toBe("string");
+    expect(ctx.screen).toMatch(/^\d+x\d+$/);
+    expect(ctx.referrer_domain).toBe("news.example");
+    expect(ctx.utm).toEqual({ utm_source: "ns", utm_campaign: "m1" });
+    expect(ctx.path).toBe("/land?utm_source=ns&utm_campaign=m1");
+
+    g.ctk!("flush"); // 原样透传进批次 body.context
+    const bodies = bodiesOf("site-2");
+    expect(bodies).toHaveLength(1);
+    expect(bodies[0].context).toEqual(ctx);
+  });
+
+  it("localStorage 抛异常：15s 定时 flush 无未捕获异常，身份降级内存后事件照常发出", async () => {
+    vi.stubGlobal("localStorage", {
+      getItem: () => {
+        throw new Error("denied");
+      },
+      setItem: () => {
+        throw new Error("denied");
+      },
+      removeItem: () => {
+        throw new Error("denied");
+      },
+      clear: () => {
+        throw new Error("denied");
+      },
+    });
+    const unhandled: unknown[] = [];
+    const onRej = (e: unknown) => unhandled.push(e);
+    // tsconfig 无 @types/node，经 globalThis 取 process（vitest node worker 内必在）
+    const proc = (
+      globalThis as {
+        process?: {
+          on(ev: "unhandledRejection", cb: (e: unknown) => void): void;
+          off(ev: "unhandledRejection", cb: (e: unknown) => void): void;
+        };
+      }
+    ).process;
+    proc?.on("unhandledRejection", onRej);
+    await loadSdk([["init", { siteId: "site-7", endpoint: "/e", flushMs: 15000 }]]);
+
+    let syncErr: unknown;
+    try {
+      await vi.advanceTimersByTimeAsync(15000); // 定时 flush 到期
+    } catch (e) {
+      syncErr = e; // fake timers 会把定时回调内的同步异常抛到 advance 调用方
+    }
+    proc?.off("unhandledRejection", onRej);
+    expect(syncErr).toBeUndefined();
+    expect(unhandled).toEqual([]);
+
+    const bodies = bodiesOf("site-7");
+    expect(bodies).toHaveLength(1);
+    expect(bodies[0].visitor_id).toMatch(UUID_RE); // 内存降级身份仍可用
+    expect(bodies[0].session_id).toMatch(UUID_RE);
+    expect(bodies[0].events.map((e: { event_name: string }) => e.event_name)).toEqual([
+      "sdk_loaded",
+      "page_view",
+    ]);
+  });
+
+  it("init 前队列重放：排队的 track 事件入队，g.ctk 已替换为 dispatch", async () => {
+    const trackEv = {
+      event_id: "e-custom",
+      event_name: "cta_click",
+      occurred_at: new Date().toISOString(),
+      path: "/x",
+      title: "t",
+      properties: { k: 1 },
+    };
+    await loadSdk([
+      ["init", { siteId: "site-8", endpoint: "/e", flushMs: 60000 }],
+      ["track", trackEv],
+    ]);
+
+    expect(typeof g.ctk).toBe("function");
+    expect(g.ctk!.q).toBeUndefined(); // 桩（带 q）已被 dispatch 替换
+    g.ctk!("flush");
+    let bodies = bodiesOf("site-8");
+    expect(bodies).toHaveLength(1);
+    expect(bodies[0].events.map((e: { event_name: string }) => e.event_name)).toEqual([
+      "sdk_loaded",
+      "page_view",
+      "cta_click",
+    ]);
+    expect(bodies[0].events[2].properties).toEqual({ k: 1 });
+
+    // 重放结束后宿主新调用直接进 dispatch（不再排队丢失）
+    g.ctk!("track", { ...trackEv, event_id: "e-2", event_name: "cta_click2" });
+    g.ctk!("flush");
+    bodies = bodiesOf("site-8");
+    expect(bodies).toHaveLength(2);
+    expect(bodies[1].events.map((e: { event_name: string }) => e.event_name)).toEqual(["cta_click2"]);
+  });
+});
diff --git a/sdk/test/transport.test.ts b/sdk/test/transport.test.ts
index 7a0887b..689574c 100644
--- a/sdk/test/transport.test.ts
+++ b/sdk/test/transport.test.ts
@@ -176,6 +176,22 @@ describe("transport", () => {
     await vi.advanceTimersByTimeAsync(100);
     expect(t.dropped).toBe(1); // maxRetry=0 耗尽 → 按事件数丢弃
   });
+
+  it("stringify 抛错（track 引入宿主可控对象）：已换出批次按事件数计 dropped", () => {
+    const t = new Transport(mkCfg({ batchSize: 10, flushMs: 60000 }));
+    const bad: { event_id: string; self?: unknown } = { event_id: "bad" };
+    bad.self = bad; // 循环引用 → JSON.stringify 抛 TypeError
+    t.push(bad);
+    t.push({ event_id: "ok" });
+    expect(() => t.flush()).not.toThrow();
+    expect(t.dropped).toBe(2); // 换出的 2 条全部落丢，不能静默不计
+    expect(fetchMock).not.toHaveBeenCalled();
+
+    t.push({ event_id: "e3" }); // 队列已换出，后续事件照常
+    t.flush();
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+    expect(JSON.parse(fetchMock.mock.calls[0][1].body).events).toEqual([{ event_id: "e3" }]);
+  });
 });
 
 describe("mergeConfig", () => {