diff --git a/web/src/App.tsx b/web/src/App.tsx
index 8fca6e4d..0f182bd5 100644
--- a/web/src/App.tsx
+++ b/web/src/App.tsx
@@ -4126,6 +4126,7 @@ export default function App() {
             )}
             <div style={{ position: 'relative', flex: 1, minHeight: 0, display: 'flex', flexDirection: 'column' }}>
               <MessageStream
+                sessionId={activeSessionId}
                 dayStack={chatWindow.displayDayStack}
                 isLoadingOlder={chatWindow.phase === 'loading_older'}
                 hasMoreBefore={chatWindow.hasMoreBefore}
diff --git a/web/src/components/chat/MessageStream.tsx b/web/src/components/chat/MessageStream.tsx
index 260cdc90..25af67dc 100644
--- a/web/src/components/chat/MessageStream.tsx
+++ b/web/src/components/chat/MessageStream.tsx
@@ -22,6 +22,8 @@ export interface MessageStreamProps {
   onQuote?: (message: ChatMessage) => void;
   onNavigateToAnchor?: (messageId: number) => void;
   selectedCategory?: MessageCategory | 'all';
+  /** 会话切换时重置滚动基线，避免旧会话消息数挡住新会话自动滚底 */
+  sessionId?: string;
   /** P3 / Step 5: 未读消息 ID 集合 */
   unreadMessageIds?: number[];
   onSaveToPrivate?: (message: ChatMessage) => void;
@@ -43,6 +45,7 @@ export const MessageStream: React.FC<MessageStreamProps> = ({
   onQuote,
   onNavigateToAnchor,
   selectedCategory = 'all',
+  sessionId,
   unreadMessageIds = [],
   onSaveToPrivate,
   onExportDocx,
@@ -154,6 +157,11 @@ export const MessageStream: React.FC<MessageStreamProps> = ({
     }
   }, [totalMessagesCount, handle]);
 
+  useEffect(() => {
+    prevMsgCountRef.current = 0;
+    isInitialMountRef.current = true;
+  }, [sessionId]);
+
   return (
     <div
       ref={containerRef}
diff --git a/web/src/components/doc/preview/renderers/ExcelViewer.tsx b/web/src/components/doc/preview/renderers/ExcelViewer.tsx
index 272d45ff..70db8a0c 100644
--- a/web/src/components/doc/preview/renderers/ExcelViewer.tsx
+++ b/web/src/components/doc/preview/renderers/ExcelViewer.tsx
@@ -12,8 +12,9 @@ export const ExcelViewer: React.FC<ExcelViewerProps> = ({ fileRaw, fileUrl }) =>
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState<string | null>(null);
   const [sheetNames, setSheetNames] = useState<string[]>([]);
-  const [activeSheetIndex, setActiveSheetIndex] = useState(0);
   const [sheetData, setSheetData] = useState<Record<string, (string | number)[][]>>({});
+  const [activeSheetIndex, setActiveSheetIndex] = useState(0);
+  const [truncatedSheets, setTruncatedSheets] = useState<Record<string, boolean>>({});
 
   useEffect(() => {
     let isMounted = true;
@@ -40,6 +41,8 @@ export const ExcelViewer: React.FC<ExcelViewerProps> = ({ fileRaw, fileUrl }) =>
         }
 
         const parsedSheets: Record<string, (string | number)[][]> = {};
+        const truncated: Record<string, boolean> = {};
+        const previewRowLimit = 1000;
         for (const name of workbook.SheetNames) {
           const ws = workbook.Sheets[name];
           if (ws) {
@@ -48,7 +51,8 @@ export const ExcelViewer: React.FC<ExcelViewerProps> = ({ fileRaw, fileUrl }) =>
               defval: '',
               blankrows: false,
             });
-            parsedSheets[name] = rawRows;
+            truncated[name] = rawRows.length > previewRowLimit;
+            parsedSheets[name] = rawRows.slice(0, previewRowLimit);
           } else {
             parsedSheets[name] = [];
           }
@@ -57,6 +61,7 @@ export const ExcelViewer: React.FC<ExcelViewerProps> = ({ fileRaw, fileUrl }) =>
         if (isMounted) {
           setSheetNames(workbook.SheetNames);
           setSheetData(parsedSheets);
+          setTruncatedSheets(truncated);
           setActiveSheetIndex(0);
           setLoading(false);
         }
@@ -173,6 +178,9 @@ export const ExcelViewer: React.FC<ExcelViewerProps> = ({ fileRaw, fileUrl }) =>
           <span>工作表：<strong>{currentSheetName}</strong></span>
           <span>·</span>
           <span>{rows.length} 行 × {maxCols} 列</span>
+          {truncatedSheets[currentSheetName] && (
+            <span style={{ color: '#b45309' }}>· 仅预览前 1000 行</span>
+          )}
         </div>
         <div style={{ fontSize: '11px', color: '#94a3b8' }}>
           共 {sheetNames.length} 个工作表
diff --git a/web/src/engine/renderMarkdown.test.ts b/web/src/engine/renderMarkdown.test.ts
index 267029d2..565a2ac7 100644
--- a/web/src/engine/renderMarkdown.test.ts
+++ b/web/src/engine/renderMarkdown.test.ts
@@ -17,6 +17,10 @@ test('renderInlineMarkdown: escapes raw HTML tags to prevent XSS', () => {
   assert.ok(!scriptHtml.includes('<script'));
   assert.ok(scriptHtml.includes('&lt;script&gt;'));
 
+  const forged = renderInlineMarkdown('XMATHTOKENX0XENDX<script>alert(1)</script>');
+  assert.ok(!forged.includes('<script'));
+  assert.ok(forged.includes('XMATHTOKENX0XENDX'));
+
   const imgHtml = renderInlineMarkdown('<img src=x onerror=alert(1)>');
   assert.ok(!/<img[\s>]/i.test(imgHtml), 'must not emit executable img tag');
   assert.ok(imgHtml.includes('&lt;img'));
diff --git a/web/src/engine/renderMarkdown.ts b/web/src/engine/renderMarkdown.ts
index 47c7e705..ab21a58d 100644
--- a/web/src/engine/renderMarkdown.ts
+++ b/web/src/engine/renderMarkdown.ts
@@ -28,7 +28,9 @@ export function renderInlineMarkdown(content: string, options: RenderMarkdownOpt
   // 1. 占位保护预处理：优先解析并提取数学公式，避免公式内部的 * 或 _ 被 Markdown 解析器误伤
   // 注意：占位符严禁使用包含 _ 或 * 等 Markdown 特殊字符，采用纯大写字母标识符
   const mathPlaceholders: string[] = [];
-  const placeholderPrefix = 'XMATHTOKENX';
+  // 每次渲染换盐，避免用户文本伪造 XMATHTOKENX0XENDX 在 escapeHtml 之后被还原成 HTML
+  const tokenSalt = Math.random().toString(36).slice(2, 10).toUpperCase();
+  const placeholderPrefix = `XMATHTOKENX${tokenSalt}X`;
 
   // 1.1 块级独立公式 $$ ... $$
   text = text.replace(/\$\$([\s\S]*?)\$\$/g, (_, math) => {
@@ -42,7 +44,9 @@ export function renderInlineMarkdown(content: string, options: RenderMarkdownOpt
       return `${placeholderPrefix}${idx}XENDX`;
     } catch {
       const idx = mathPlaceholders.length;
-      mathPlaceholders.push(`<span style="color:var(--danger)">$$${math}$$</span>`);
+      mathPlaceholders.push(
+        `<span style="color:var(--danger)">$$${escapeHtml(math)}$$</span>`,
+      );
       return `${placeholderPrefix}${idx}XENDX`;
     }
   });
@@ -59,14 +63,14 @@ export function renderInlineMarkdown(content: string, options: RenderMarkdownOpt
       return `${placeholderPrefix}${idx}XENDX`;
     } catch {
       const idx = mathPlaceholders.length;
-      mathPlaceholders.push(`$${math}$`);
+      mathPlaceholders.push(`$${escapeHtml(math)}$`);
       return `${placeholderPrefix}${idx}XENDX`;
     }
   });
 
   // 2. 行内代码保护 `code`
   const codePlaceholders: string[] = [];
-  const codePlaceholderPrefix = 'XCODETOKENX';
+  const codePlaceholderPrefix = `XCODETOKENX${tokenSalt}X`;
   text = text.replace(/`([^`\n]+)`/g, (_, code) => {
     const rendered = `<code style="font-family:var(--font-mono);font-size:12px;background:var(--bg-subtle);color:var(--brand-500);padding:2px 5px;border-radius:4px;border:1px solid var(--border)">${escapeHtml(code)}</code>`;
     const idx = codePlaceholders.length;
diff --git a/web/src/hooks/useUnreadMarks.ts b/web/src/hooks/useUnreadMarks.ts
index e244c684..b73ae5ce 100644
--- a/web/src/hooks/useUnreadMarks.ts
+++ b/web/src/hooks/useUnreadMarks.ts
@@ -185,6 +185,8 @@ export function useUnreadMarks(
     // 0. 本地持久化记录已读
     saveLocalReadId(sessionId, targetId);
 
+    const marksSnapshot = unreadMarks;
+
     // 1. 乐观更新未读集合
     const remainingIds = unreadMessageIds.filter((id) => id !== targetId);
     setUnreadMessageIds(remainingIds);
@@ -217,7 +219,7 @@ export function useUnreadMarks(
 
     // 2. 异步上报后端已读位点
     try {
-      await fetch('/api/v1/chat/read', {
+      const res = await fetch('/api/v1/chat/read', {
         method: 'POST',
         headers: getBearerAuthHeaders('application/json'),
         body: JSON.stringify({
@@ -226,12 +228,20 @@ export function useUnreadMarks(
           read_message_ids: [targetId],
         }),
       });
+      if (!res.ok) {
+        throw new Error(`mark read failed: ${res.status}`);
+      }
     } catch {
-      // 错误时不强行重滚，下次拉取自愈
+      // 上报失败回滚乐观更新，避免前后端未读数长期分叉
+      setUnreadMessageIds(unreadMessageIds);
+      setNextMessageId(nextMessageId);
+      setUnreadTotal(unreadTotal);
+      setUnreadMarks(marksSnapshot);
+      setIsAllReadCompleted(false);
     }
 
     return targetId;
-  }, [sessionId, userId, nextMessageId, unreadMessageIds, unreadTotal]);
+  }, [sessionId, userId, nextMessageId, unreadMessageIds, unreadTotal, unreadMarks]);
 
   // 批量标记已读 (如滚动进入视口阅读或一键已读)
   const markMessagesRead = useCallback(