diff --git a/client/src/lib/changelog.ts b/client/src/lib/changelog.ts
index 75ae0db..8201ebb 100644
--- a/client/src/lib/changelog.ts
+++ b/client/src/lib/changelog.ts
@@ -19,6 +19,8 @@ export const CHANGE_LOG_ENTRIES: ChangeLogEntry[] = [
       "同步观看的 relay 缓存时长改为按会话配置，范围 10 秒到 5 分钟，默认 2 分钟；viewer 文案、徽标和设置面板都会实时显示当前缓存窗口",
       "owner 端合成画布录制改为每 10 秒上传一次 relay 分片，同时继续维持每 60 秒一段的自动归档录像，因此观看端切到短缓存时不需要再等满 60 秒才出现平滑视频",
       "media 服务会按各自 relay 会话的缓存窗口裁剪预览分段，并在从磁盘恢复旧会话时自动归一化缓存秒数，避免旧数据继续按固定 60 秒窗口工作",
+      "同步端渲染远端 recentSegments 时新增旧快照归一化，`keyFrames`、`issueSummary` 等数组字段缺失时也会自动补默认值，避免再出现 `Cannot read properties of undefined (reading 'length')`",
+      "同步观看界面新增“已累积 / 还需多久才能看到首段回放 / 距离目标缓存还差多少”的提示，观看端不再只显示笼统的等待文案",
       "线上 smoke 已确认 `https://te.hao.work/` 已经提供本次新构建，而不是旧资源版本；首页、主样式和 `pose` 模块都已切到本次发布的最新资源 revision",
     ],
     tests: [
diff --git a/client/src/pages/LiveCamera.tsx b/client/src/pages/LiveCamera.tsx
index 6ae645c..76b3305 100644
--- a/client/src/pages/LiveCamera.tsx
+++ b/client/src/pages/LiveCamera.tsx
@@ -255,6 +255,7 @@ const MERGE_GAP_MS = 900;
 const MIN_SEGMENT_MS = 1_200;
 const ANALYSIS_RECORDING_SEGMENT_MS = 60_000;
 const RELAY_UPLOAD_SEGMENT_MS = 10_000;
+const VIEWER_MIN_PLAYABLE_BUFFER_SECONDS = RELAY_UPLOAD_SEGMENT_MS / 1000;
 const RELAY_BUFFER_OPTIONS = [
   { value: 10, label: "10 秒缓存" },
   { value: 30, label: "30 秒缓存" },
@@ -821,6 +822,106 @@ function formatRelayBufferLabel(seconds: number | null | undefined) {
   return `${minutes} 分 ${remainSeconds} 秒`;
 }
 
+function formatElapsedSecondsLabel(seconds: number | null | undefined) {
+  const normalized = Math.max(0, Math.floor(seconds ?? 0));
+  if (normalized < 60) {
+    return `${normalized} 秒`;
+  }
+  if (normalized % 60 === 0) {
+    return `${normalized / 60} 分钟`;
+  }
+  const minutes = Math.floor(normalized / 60);
+  const remainSeconds = normalized % 60;
+  return `${minutes} 分 ${remainSeconds} 秒`;
+}
+
+function normalizeActionSegment(input: Partial<ActionSegment> | null | undefined) {
+  const actionType = ACTION_META[(input?.actionType as ActionType) ?? "unknown"]
+    ? ((input?.actionType as ActionType) ?? "unknown")
+    : "unknown";
+  const startMs = Math.max(0, Number(input?.startMs ?? 0));
+  const endMs = Math.max(startMs, Number(input?.endMs ?? startMs));
+  const durationMs = Math.max(
+    0,
+    Number(input?.durationMs ?? Math.max(0, endMs - startMs))
+  );
+  const keyFrames = Array.isArray(input?.keyFrames)
+    ? input!.keyFrames
+        .map(value => Number(value))
+        .filter(value => Number.isFinite(value) && value >= 0)
+    : [];
+  const issueSummary = Array.isArray(input?.issueSummary)
+    ? input!.issueSummary.filter(
+        (value): value is string => typeof value === "string" && value.length > 0
+      )
+    : [];
+
+  return {
+    actionType,
+    isUnknown: Boolean(input?.isUnknown ?? actionType === "unknown"),
+    startMs,
+    endMs,
+    durationMs,
+    confidenceAvg: Number(input?.confidenceAvg ?? 0),
+    score: Number(input?.score ?? 0),
+    peakScore: Number(input?.peakScore ?? input?.score ?? 0),
+    frameCount: Math.max(0, Number(input?.frameCount ?? 0)),
+    issueSummary,
+    keyFrames,
+    clipLabel:
+      typeof input?.clipLabel === "string" && input.clipLabel.length > 0
+        ? input.clipLabel
+        : `${ACTION_META[actionType].label} ${formatDuration(startMs)} - ${formatDuration(endMs)}`,
+  } satisfies ActionSegment;
+}
+
+function normalizeActionSegments(input: Partial<ActionSegment>[] | null | undefined) {
+  if (!Array.isArray(input)) {
+    return [];
+  }
+
+  return input.map(segment => normalizeActionSegment(segment));
+}
+
+function formatViewerAccumulationHint({
+  bufferedSeconds,
+  relayBufferSeconds,
+  previewReady,
+}: {
+  bufferedSeconds: number;
+  relayBufferSeconds: number;
+  previewReady: boolean;
+}) {
+  const normalizedBufferedSeconds = Math.max(0, bufferedSeconds);
+  const remainingToPlayableSeconds = Math.max(
+    0,
+    VIEWER_MIN_PLAYABLE_BUFFER_SECONDS - normalizedBufferedSeconds
+  );
+  const remainingToTargetSeconds = Math.max(
+    0,
+    relayBufferSeconds - normalizedBufferedSeconds
+  );
+  const bufferedLabel = formatElapsedSecondsLabel(normalizedBufferedSeconds);
+  const playableLabel = formatElapsedSecondsLabel(remainingToPlayableSeconds);
+  const targetLabel = formatRelayBufferLabel(relayBufferSeconds);
+  const targetRemainingLabel = formatElapsedSecondsLabel(
+    remainingToTargetSeconds
+  );
+
+  if (previewReady) {
+    if (remainingToTargetSeconds > 0) {
+      return `已累积 ${bufferedLabel}，已经可以观看；距离当前 ${targetLabel} 缓存目标还差 ${targetRemainingLabel}。`;
+    }
+    return `已累积 ${bufferedLabel}，当前缓存窗口已达到 ${targetLabel}。`;
+  }
+
+  if (remainingToPlayableSeconds > 0) {
+    return `已累积 ${bufferedLabel}，预计还需 ${playableLabel} 才会出现首段可观看回放；当前缓存目标为 ${targetLabel}。`;
+  }
+
+  return `首段缓存已到达，正在生成可观看回放；当前缓存目标为 ${targetLabel}。`;
+}
+
 export default function LiveCamera() {
   const { user } = useAuth();
   const utils = trpc.useUtils();
@@ -1018,12 +1119,14 @@ export default function LiveCamera() {
           ),
     [segmentFilter, visibleSegments]
   );
+  const normalizedViewerRecentSegments = useMemo(
+    () => normalizeActionSegments(runtimeSnapshot?.recentSegments),
+    [runtimeSnapshot?.recentSegments]
+  );
   const viewerRecentSegments = useMemo(
     () =>
-      (runtimeSnapshot?.recentSegments ?? []).filter(
-        segment => !segment.isUnknown
-      ),
-    [runtimeSnapshot?.recentSegments]
+      normalizedViewerRecentSegments.filter(segment => !segment.isUnknown),
+    [normalizedViewerRecentSegments]
   );
   const displayVisibleSegments =
     runtimeRole === "viewer" ? viewerRecentSegments : visibleSegments;
@@ -2260,7 +2363,13 @@ export default function LiveCamera() {
   const displayScore =
     runtimeRole === "viewer" ? (runtimeSnapshot?.liveScore ?? null) : liveScore;
   const displayFeedback =
-    runtimeRole === "viewer" ? (runtimeSnapshot?.feedback ?? []) : feedback;
+    runtimeRole === "viewer"
+      ? Array.isArray(runtimeSnapshot?.feedback)
+        ? runtimeSnapshot.feedback.filter(
+            (item): item is string => typeof item === "string" && item.length > 0
+          )
+        : []
+      : feedback;
   const displayDurationMs =
     runtimeRole === "viewer" ? (runtimeSnapshot?.durationMs ?? 0) : durationMs;
   const displayStabilityMeta =
@@ -2331,6 +2440,18 @@ export default function LiveCamera() {
   const viewerBufferReady =
     runtimeRole === "viewer" &&
     Boolean(viewerMediaSession?.playback.previewUrl);
+  const viewerBufferedSeconds = Math.max(
+    0,
+    Math.floor((viewerMediaSession?.durationMs ?? 0) / 1000)
+  );
+  const viewerAccumulationHint =
+    runtimeRole === "viewer"
+      ? formatViewerAccumulationHint({
+          bufferedSeconds: viewerBufferedSeconds,
+          relayBufferSeconds: displayRelayBufferSeconds,
+          previewReady: viewerBufferReady,
+        })
+      : "";
   const hasVideoFeed =
     runtimeRole === "viewer" ? viewerConnected : cameraActive;
   const heroAction = ACTION_META[displayAction];
@@ -2705,6 +2826,7 @@ export default function LiveCamera() {
             。当前设备不会占用本地摄像头，也不能再次开启分析；同步画面会通过
             media 服务中转，并以最近 {displayRelayBufferLabel}
             缓存视频方式平滑回放，动作、评分与会话信息会按心跳自动同步。
+            {viewerAccumulationHint ? ` ${viewerAccumulationHint}` : ""}
           </AlertDescription>
         </Alert>
       ) : null}
@@ -2860,8 +2982,8 @@ export default function LiveCamera() {
                       <div className="text-sm text-white/60">
                         {runtimeRole === "viewer"
                           ? viewerBufferReady
-                            ? `${viewerModeLabel}，当前设备只能观看同步内容；正在载入最近 ${displayRelayBufferLabel} 缓存回放。`
-                            : `${viewerModeLabel}，当前设备只能观看同步内容；持有端累计出缓存片段后会自动出现平滑视频。`
+                            ? `${viewerModeLabel}，当前设备只能观看同步内容；正在载入最近 ${displayRelayBufferLabel} 缓存回放。${viewerAccumulationHint}`
+                            : `${viewerModeLabel}，当前设备只能观看同步内容；${viewerAccumulationHint}`
                           : "先完成拍摄校准，再开启自动动作识别。"}
                       </div>
                     </div>
@@ -3023,6 +3145,10 @@ export default function LiveCamera() {
                               }
                             </div>
                             <div>同步缓存：{displayRelayBufferLabel}</div>
+                            <div>
+                              已累积：
+                              {formatElapsedSecondsLabel(viewerBufferedSeconds)}
+                            </div>
                             <div>
                               虚拟形象：
                               {displayAvatarEnabled
@@ -3045,6 +3171,9 @@ export default function LiveCamera() {
                                 )
                               : "等待首个心跳"}
                           </div>
+                          <div className="mt-2 text-xs leading-5 text-muted-foreground">
+                            {viewerAccumulationHint}
+                          </div>
                         </div>
                       </div>
                     </div>
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index e274119..d6d8376 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -8,7 +8,9 @@
 - 多端同步观看的 relay 缓存窗口改为按会话配置，默认 `2` 分钟，可选最短 `10` 秒、最长 `5` 分钟；viewer 页面、徽标和设置卡都会同步显示当前缓存时长
 - owner 端分析录制在继续保持“每 `60` 秒自动归档”之外，会额外每 `10` 秒上传一次 relay 分片，因此短缓存模式下其他端不需要等待整整 `60` 秒才看到平滑同步视频
 - media 服务会按各自 relay 会话的缓存秒数裁剪 preview 分段；从磁盘恢复旧 relay 会话时也会自动归一化到合法范围，避免旧会话继续沿用固定 `60` 秒窗口
-- 线上 smoke 已确认 `https://te.hao.work/` 正在提供本次新构建，而不是旧资源版本；当前公开站点资源 revision 为 `assets/index-2-BhvFom.js`、`assets/index-BHHHsAWc.css`、`assets/pose-BPcIm7Xa.js`
+- 同步端渲染远端 `recentSegments` 时新增旧快照归一化；即使历史快照缺少 `keyFrames`、`issueSummary` 等数组字段，也会自动补默认值，不再触发 `Cannot read properties of undefined (reading 'length')`
+- 同步观看界面新增“已累积多少缓存、预计还需多久才能看到首段回放、距离目标缓存还差多少”的提示，观看端等待阶段会给出更明确的可观察时间说明
+- 线上 smoke 已确认 `https://te.hao.work/` 正在提供本次新构建，而不是旧资源版本；当前公开站点资源 revision 为 `assets/index-CYpJPG0R.js`、`assets/index-BHHHsAWc.css`、`assets/pose-C93FSit6.js`
 
 ### 测试
 
@@ -19,14 +21,14 @@
 - `pnpm exec playwright test tests/e2e/app.spec.ts`
 - `playwright-skill` 线上 smoke：登录 `H1` 后访问 `https://te.hao.work/live-camera`，完成校准、启用假摄像头并点击“开始分析”，确认页面进入分析中状态、默认显示“缓存 2 分钟”，且无控制台与页面级错误
 - `curl -I https://te.hao.work/`
-- `curl -I https://te.hao.work/assets/index-2-BhvFom.js`
+- `curl -I https://te.hao.work/assets/index-CYpJPG0R.js`
 - `curl -I https://te.hao.work/assets/index-BHHHsAWc.css`
-- `curl -I https://te.hao.work/assets/pose-BPcIm7Xa.js`
+- `curl -I https://te.hao.work/assets/pose-C93FSit6.js`
 
 ### 线上 smoke
 
 - `https://te.hao.work/` 已切换到本次新构建，而不是旧资源版本
-- 当前公开站点前端资源 revision：`assets/index-2-BhvFom.js`、`assets/index-BHHHsAWc.css`、`assets/pose-BPcIm7Xa.js`
+- 当前公开站点前端资源 revision：`assets/index-CYpJPG0R.js`、`assets/index-BHHHsAWc.css`、`assets/pose-C93FSit6.js`
 - 已确认首页、主 JS、主 CSS 与 `pose` 模块均返回 `200`，且 MIME 分别为 `text/html`、`application/javascript`、`text/css`、`application/javascript`
 - 真实浏览器验证已通过：登录 `H1` 后进入 `/live-camera`，能够完成校准、启用摄像头并点击“开始分析”；页面会进入“分析进行中”状态，默认显示“缓存 2 分钟”，且未再出现 `Pose` 模块解构异常
 
diff --git a/tests/e2e/app.spec.ts b/tests/e2e/app.spec.ts
index 32abca1..d75b85c 100644
--- a/tests/e2e/app.spec.ts
+++ b/tests/e2e/app.spec.ts
@@ -88,12 +88,54 @@ test("live camera switches into viewer mode when another device already owns ana
   await expect(page.getByTestId("live-camera-viewer-sync-card")).toContainText(
     "均衡模式"
   );
+  await expect(page.getByTestId("live-camera-viewer-sync-card")).toContainText(
+    "已累积"
+  );
   await expect(page.getByTestId("live-camera-viewer-sync-card")).toContainText(
     "猩猩"
   );
   await expect(page.getByTestId("live-camera-score-overall")).toBeVisible();
 });
 
+test("live camera viewer tolerates legacy segments and shows remaining buffer hint", async ({
+  page,
+}) => {
+  const state = await installAppMocks(page, {
+    authenticated: true,
+    liveViewerMode: true,
+  });
+
+  if (state.liveRuntime.runtimeSession?.snapshot) {
+    state.liveRuntime.runtimeSession.snapshot.recentSegments = [
+      {
+        actionType: "forehand",
+        isUnknown: false,
+        startMs: 1200,
+        endMs: 3600,
+        durationMs: 2400,
+        confidenceAvg: 0.82,
+        score: 81,
+        peakScore: 86,
+        frameCount: 18,
+      } as any,
+    ];
+  }
+
+  if (state.mediaSession) {
+    state.mediaSession.durationMs = 4_000;
+    state.mediaSession.playback.previewUrl = undefined;
+  }
+
+  await page.goto("/live-camera");
+  await expect(page.getByText("同步观看模式")).toBeVisible();
+  await expect(
+    page
+      .getByTestId("live-camera-viewer-sync-card")
+      .getByText(/预计还需 6 秒 才会出现首段可观看回放/)
+  ).toBeVisible();
+  await expect(page.getByText("关键帧 0")).toBeVisible();
+});
+
 test("live camera recovers mojibake viewer titles before rendering", async ({
   page,
 }) => {
diff --git a/tests/e2e/helpers/mockApp.ts b/tests/e2e/helpers/mockApp.ts
index 4beb957..af4a9a3 100644
--- a/tests/e2e/helpers/mockApp.ts
+++ b/tests/e2e/helpers/mockApp.ts
@@ -754,13 +754,17 @@ async function handleMedia(route: Route, state: MockAppState) {
 
   if (path === `/media/sessions/${state.mediaSession.id}`) {
     if (state.mediaSession.purpose === "relay") {
-      state.mediaSession.previewStatus = "ready";
+      state.mediaSession.previewStatus = state.mediaSession.playback.previewUrl
+        ? "ready"
+        : "processing";
       state.mediaSession.previewUpdatedAt = nowIso();
       state.mediaSession.playback = {
-        ready: true,
-        webmUrl: "/media/assets/sessions/session-e2e/preview.webm",
-        webmSize: 1_800_000,
-        previewUrl: "/media/assets/sessions/session-e2e/preview.webm",
+        ready: Boolean(state.mediaSession.playback.previewUrl),
+        webmUrl:
+          state.mediaSession.playback.webmUrl ??
+          "/media/assets/sessions/session-e2e/preview.webm",
+        webmSize: state.mediaSession.playback.webmSize ?? 1_800_000,
+        previewUrl: state.mediaSession.playback.previewUrl,
       };
     } else {
       state.mediaSession.status = "archived";