diff --git a/README.md b/README.md
index 2968782..dbe65ed 100644
--- a/README.md
+++ b/README.md
@@ -44,7 +44,7 @@
 - **🌐 内嵌浏览器**：多开网页 tab，后退 / 前进 / 刷新；内容运行在沙箱 iframe；外链默认按协议分流——HTTP 在侧边栏打开、HTTPS 走系统浏览器（设置页可分别调整）
 - **💻 真实终端**：xterm.js + node-pty 真实 shell，断线重连回放；可选为模型注入 `terminal_*` 工具
 - **📂 模型侧边栏打开（可选）**：全局设置开启后注入 `sidebar_open` 工具——模型可主动在侧边栏打开文件 / 文件夹（树以该目录为根）/ HTTP(S) 网页
-- **🌿 文件变动**：Git 视角（真 diff / 历史 / 暂存·提交·还原 / worktree·子仓库选择）与本轮文件视角（模型读 / 写 / 编辑实时追踪，按文件分组、按类型筛选）**双视角合一**；统一 diff 渲染（改蓝配对 + 行内字符级高亮 + 语法着色（含 mjs/cjs/mts/cts、CSS/SCSS/Less、HTML/XML/SVG/Vue、GraphQL、JSONC/JSON5）+ 上下文折叠），底部可拖拽预览面板，可一键展开为独立 diff tab（默认自由浮窗，可在设置改为面板下半 split）；`.md` 操作（读 / 写 / 编辑）预览头部可切换**阅读模式**——经共享 MarkdownText 渲染 GFM 表格 / 任务列表 / 删除线 / 脚注 / 数学公式，本地图片自动改写为 `/sidebar/file` 媒体路由（与编辑器预览同一渲染栈）
+- **🌿 文件变动**：Git 视角（真 diff / 历史 / 暂存·提交·还原 / worktree·子仓库选择）与本轮文件视角（模型读 / 写 / 编辑实时追踪，按文件分组、按类型筛选）**双视角合一**；统一 diff 渲染（改蓝配对 + 行内字符级高亮 + 语法着色（含 mjs/cjs/mts/cts、CSS/SCSS/Less、HTML/XML/SVG/Vue、GraphQL、JSONC/JSON5）+ 上下文折叠），底部可拖拽预览面板，可一键展开为独立 diff tab（默认自由浮窗，可在设置改为面板下半 split）；`.md` 操作（读 / 写 / 编辑）预览头部可切换**阅读模式**——经共享 MarkdownText 渲染 GFM 表格 / 任务列表 / 删除线 / 脚注 / 数学公式，本地图片自动改写为 `/sidebar/file` 媒体路由；含 ```mermaid 围栏时走编辑器同款懒加载 mermaid 渲染器（图可点击缩放 / 平移）
 - **🧩 后台任务页**：subagent 拓扑 + 后台任务（退出码 / 实时输出 / 强制终止）
 - **💬 侧边对话(beta)**：Codex 风格的侧边线程——继承主会话完整上下文（含进行中的回合与工具调用）独立运行，不进入主会话；线程内可持续追问，一键「保存为新会话」提升为顶层会话
 - **🪟 双工作台**：右侧栏 + 底部面板；拖 Tab 拆分 / 合并分栏（可跨面板），移动端自动合并全宽抽屉
diff --git a/README_EN.md b/README_EN.md
index 52f39fd..75b4c65 100644
--- a/README_EN.md
+++ b/README_EN.md
@@ -44,7 +44,7 @@
 - **🌐 Embedded Browser**: multiple web tabs with back / forward / refresh; content runs in a sandboxed iframe; external links are routed by protocol by default — HTTP opens in the sidebar, HTTPS goes to the system browser (both adjustable in settings)
 - **💻 Real Terminal**: xterm.js + node-pty real shell, reconnect with transcript replay; optionally injects `terminal_*` tools for the model
 - **📂 Model-driven sidebar opens (opt-in)**: with the global setting on, the `sidebar_open` tool lets the model actively open files / folders (tree rooted there) / HTTP(S) pages in the sidebar
-- **🌿 Changes**: one tab, two lenses — **Git** (real diff / history / stage·commit·revert / worktree & child-repo selection) and **This Session** (live tracking of every file the model reads / writes / edits, grouped by file with kind filters); a unified diff renderer (mod pairing + intra-line character highlights + syntax coloring incl. mjs/cjs/mts/cts, CSS/SCSS/Less, HTML/XML/SVG/Vue, GraphQL, JSONC/JSON5 + context folding), a draggable bottom preview pane, and one-click expansion into a dedicated diff tab; `.md` ops (read / write / edit) offer a **reading-mode** toggle in the preview header — the shared MarkdownText renders GFM tables / task lists / strikethrough / footnotes / math, with local images rewritten through the /sidebar/file media route (the same rendering stack as the editor preview)
+- **🌿 Changes**: one tab, two lenses — **Git** (real diff / history / stage·commit·revert / worktree & child-repo selection) and **This Session** (live tracking of every file the model reads / writes / edits, grouped by file with kind filters); a unified diff renderer (mod pairing + intra-line character highlights + syntax coloring incl. mjs/cjs/mts/cts, CSS/SCSS/Less, HTML/XML/SVG/Vue, GraphQL, JSONC/JSON5 + context folding), a draggable bottom preview pane, and one-click expansion into a dedicated diff tab; `.md` ops (read / write / edit) offer a **reading-mode** toggle in the preview header — the shared MarkdownText renders GFM tables / task lists / strikethrough / footnotes / math, with local images rewritten through the /sidebar/file media route; documents with ```mermaid fences render through the editor's lazy mermaid renderer (click-to-zoom / pan diagrams)
 - **🧩 Background Tasks**: agent topology + background tasks (exit codes / live output / force-kill)
 - **💬 Side Chat (beta)**: Codex-style side threads — the child inherits the parent's FULL context (completed turns + the pending question + the in-progress turn's assistant output and tool activity, honestly frozen as "interrupted") and runs independently without entering the main conversation; threads support continuous follow-ups (auto-resumed after a DSH restart) and one-click "Save as new session" promotion to a top-level session
 - **🪟 Dual Workbench**: right sidebar + bottom panel; drag tabs to split / merge panes (cross-panel), mobile auto-merges into a full-width drawer
diff --git a/src/client/MarkdownHtml.tsx b/src/client/MarkdownHtml.tsx
index 70011ab..ce291bb 100644
--- a/src/client/MarkdownHtml.tsx
+++ b/src/client/MarkdownHtml.tsx
@@ -31,11 +31,11 @@ import type { SessionScope } from './api.ts'
 import css from './sidebar.module.css'
 
 /** The chunk-resident markdown renderer (mermaid lazy chunk), shared with the
- *  legacy no-HTML preview path in TextEditor. */
-export const LazyMermaidMarkdown = lazyChunkComponent<MermaidMarkdownProps>(
-  'mermaid',
-  (mod) => mod.MermaidMarkdown as ComponentType<MermaidMarkdownProps> | undefined,
-)
+ *  legacy no-HTML preview path in TextEditor. Defined in mermaid-lazy.tsx (a
+ *  light module) so core-bundle consumers can import the stub without
+ *  dragging DOMPurify along. */
+export { LazyMermaidMarkdown } from './mermaid-lazy.tsx'
+import { LazyMermaidMarkdown } from './mermaid-lazy.tsx'
 
 /** Everything the sanitizers need to resolve local media + scope the route. */
 export interface MarkdownHtmlMedia {
diff --git a/src/client/changes/DiffPane.tsx b/src/client/changes/DiffPane.tsx
index 939a221..e00bdf3 100644
--- a/src/client/changes/DiffPane.tsx
+++ b/src/client/changes/DiffPane.tsx
@@ -23,6 +23,8 @@ import { buildDiffSegments, diffLines, diffStats, parseUnifiedDiff, unifiedSegme
 import { parseReadContent, parseReadLines, type FileOp } from './ops.ts'
 import { rewriteLocalImageUrls } from '../markdown-images.ts'
 import { markdownTextProps } from '../markdown-labels.tsx'
+import { splitMermaidBlocks } from '../mermaid-blocks.ts'
+import { LazyMermaidMarkdown } from '../mermaid-lazy.tsx'
 import css from './changes.module.css'
 import diffCss from '../diff/diff.module.css'
 
@@ -183,6 +185,13 @@ export function DiffPane({ target, scope, height, onHeightCommit, onClose, onExp
       : ''),
     [mdOp, reading, readingSrc, scope, target],
   )
+  // Mermaid fences render through the same chunk-resident renderer the editor
+  // preview uses (one MarkdownText pass with the mermaid fences lifted out);
+  // the plain single-pass path stays byte-for-byte for documents without any.
+  const readingHasMermaid = useMemo(
+    () => (readingText !== '' ? splitMermaidBlocks(readingText).some((block) => block.kind === 'mermaid') : false),
+    [readingText],
+  )
   const codeLabels = { copyLabel: t('copy'), copiedLabel: t('copied') }
 
   // Header stats for git targets come off the parsed patch text.
@@ -310,7 +319,9 @@ export function DiffPane({ target, scope, height, onHeightCommit, onClose, onExp
         ? (
           <div className={css.paneBody}>
             <div className={css.mdBody}>
-              <MarkdownText {...markdownTextProps(readingText, codeLabels)} />
+              {readingHasMermaid
+                ? <LazyMermaidMarkdown text={readingText} codeLabels={codeLabels} />
+                : <MarkdownText {...markdownTextProps(readingText, codeLabels)} />}
             </div>
           </div>
         )
diff --git a/src/client/mermaid-lazy.tsx b/src/client/mermaid-lazy.tsx
new file mode 100644
index 0000000..3a56793
--- /dev/null
+++ b/src/client/mermaid-lazy.tsx
@@ -0,0 +1,16 @@
+/**
+ * The chunk-resident markdown renderer (mermaid lazy chunk): a light stub
+ * module so core-bundle consumers (the changes tab's reading mode) can mount
+ * the mermaid-aware renderer without statically pulling MarkdownHtml's
+ * DOMPurify/HTML-analysis machinery into the core bundle. The chunk itself
+ * (src/client/chunks/mermaid) loads on first render via /sidebar/bundle.
+ */
+import type { ComponentType } from 'react'
+import { lazyChunkComponent } from './lazy-chunk.tsx'
+import type { MermaidMarkdownProps } from './mermaid-blocks.ts'
+
+/** Shared with the legacy no-HTML preview path in TextEditor (re-exported). */
+export const LazyMermaidMarkdown = lazyChunkComponent<MermaidMarkdownProps>(
+  'mermaid',
+  (mod) => mod.MermaidMarkdown as ComponentType<MermaidMarkdownProps> | undefined,
+)