diff --git a/cli/src/App.tsx b/cli/src/App.tsx
index 9ed673ae..c561efd6 100644
--- a/cli/src/App.tsx
+++ b/cli/src/App.tsx
@@ -493,7 +493,14 @@ export default function App() {
     const renderState = createEventRenderState();
     for (;;) {
       const result = await api.pollTask(taskId, offset);
-      offset = result.next_offset;
+      // 事件窗口缺口检测（协议 §5.2）：中间事件已被裁剪，提示用户并对齐窗口继续
+      const windowStart = Number(result.window_start ?? 0);
+      if (offset > 0 && offset < windowStart) {
+        addItem({ kind: 'system', body: `部分实时输出已被裁剪（窗口起点 ${windowStart}），以最终结果为准` });
+        offset = windowStart;
+      } else {
+        offset = result.next_offset;
+      }
       for (const event of result.events || []) {
         if (event.type === 'tool_approval_required') {
           const approval = event.data?.approval || event.data;
diff --git a/cli/src/api.ts b/cli/src/api.ts
index 65964d65..0038c718 100644
--- a/cli/src/api.ts
+++ b/cli/src/api.ts
@@ -17,6 +17,8 @@ export type TaskPollResult = {
   conversation_id?: string;
   events: Array<{ idx: number; type: string; data: any; ts?: number }>;
   next_offset: number;
+  // 事件窗口当前最小 idx（协议 §5.2）：offset 落后于此值说明中间事件已被裁剪
+  window_start?: number;
 };
 
 export class ApiClient {
diff --git a/static/src/app/methods/taskPolling/lifecycle.ts b/static/src/app/methods/taskPolling/lifecycle.ts
index 79ac1640..44ba29ae 100644
--- a/static/src/app/methods/taskPolling/lifecycle.ts
+++ b/static/src/app/methods/taskPolling/lifecycle.ts
@@ -392,6 +392,21 @@ export const lifecycleMethods = {
         if (this._rebuildingFromScratch) break;
         this.handleRuntimeQueueSync(eventData);
         break;
+      case 'event_window_gap':
+        // 事件窗口缺口（协议 §5.2/§5.4）：中间事件已被服务端裁剪，
+        // 以会话 JSON 为权威强制重读历史对账。重建期间历史本就是新加载的，跳过。
+        if (this._rebuildingFromScratch) break;
+        debugLog('[TaskPolling] 事件窗口缺口，触发会话快照对账:', eventData);
+        if ((window as any).__vueApp?.uiPushToast) {
+          (window as any).__vueApp.uiPushToast({
+            title: t('stores.eventWindowGap'),
+            message: t('stores.eventWindowGapReload'),
+            type: 'warning',
+            duration: 5000
+          });
+        }
+        this.fetchAndDisplayHistory({ force: true });
+        break;
 
       default:
         debugLog(`[TaskPolling] 未知事件类型: ${eventType}`);
diff --git a/static/src/locales/en-US/stores.ts b/static/src/locales/en-US/stores.ts
index 0f9bdf7e..5000adb9 100644
--- a/static/src/locales/en-US/stores.ts
+++ b/static/src/locales/en-US/stores.ts
@@ -60,6 +60,8 @@ export default {
   fetchTaskListFailed: 'Failed to fetch task list',
   pollingFluctuation: 'Polling fluctuation',
   pollingUnstableRetry: 'Message updates are temporarily unstable, retrying automatically',
+  eventWindowGap: 'Sync gap detected',
+  eventWindowGapReload: 'Some live updates were trimmed; resyncing conversation content',
   requestFailed: 'Request failed',
   fileTreeUnavailableHostMode: 'File tree unavailable in host mode',
   unlimited: 'Unlimited',
diff --git a/static/src/locales/zh-CN/stores.ts b/static/src/locales/zh-CN/stores.ts
index 4426adaf..7a078913 100644
--- a/static/src/locales/zh-CN/stores.ts
+++ b/static/src/locales/zh-CN/stores.ts
@@ -60,6 +60,8 @@ export default {
   fetchTaskListFailed: '获取任务列表失败',
   pollingFluctuation: '轮询波动',
   pollingUnstableRetry: '消息更新暂时不稳定，正在自动重试',
+  eventWindowGap: '消息同步缺口',
+  eventWindowGapReload: '部分实时更新已被裁剪，正在重新同步对话内容',
   requestFailed: '请求失败',
   fileTreeUnavailableHostMode: '宿主机模式下文件树不可用',
   unlimited: '未限制',
diff --git a/static/src/stores/task.ts b/static/src/stores/task.ts
index 3888b2f0..4ec6c534 100644
--- a/static/src/stores/task.ts
+++ b/static/src/stores/task.ts
@@ -258,6 +258,34 @@ export const useTaskStore = defineStore('task', {
         let sawTaskCompleteEvent = false;
         let sawTaskStoppedEvent = false;
 
+        // 事件窗口缺口检测（协议 §5.2）：请求 offset 落后于窗口起点，说明中间事件
+        // 已被裁剪，禁止当作“无新事件”静默吞掉——通知上层走会话快照对账（§5.4），
+        // 本批返回的事件（idx >= window_start）是连续段，游标照常推进。
+        const windowStart = Number(data?.window_start ?? 0);
+        if (fromOffset > 0 && fromOffset < windowStart) {
+          debugLog('[Task] 事件窗口缺口:', { from: fromOffset, windowStart });
+          taskPollDiag('task-poll-window-gap', {
+            requestId,
+            taskId,
+            from: fromOffset,
+            windowStart,
+            nextOffset: data?.next_offset
+          });
+          try {
+            eventHandler({
+              type: 'event_window_gap',
+              data: {
+                task_id: data?.task_id || this.currentTaskId,
+                conversation_id: data?.conversation_id || null,
+                from_offset: fromOffset,
+                window_start: windowStart
+              }
+            });
+          } catch (err) {
+            console.warn('[Task] 处理 event_window_gap 失败:', err);
+          }
+        }
+
         // 处理新事件
         if (data.events && data.events.length > 0) {
           debugLog(`[Task] 收到 ${data.events.length} 个新事件`);