diff --git a/README.md b/README.md
index 6a6a8c4..06cb830 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,7 @@
 - **🌿 Git 面板**：真 diff + VSCode 式 diff tab、懒加载历史、右键暂存/放弃/提交/还原/捡取
 - **🧩 Subagent 页面**：主会话完整 agent 拓扑、点击直达执行记录、实时工具调用轮询、新子代理自动展开
 - **🪟 底部面板**：独立的第二个工作台（与右侧栏同类的标签页），只挤占中间 Agent 输出区、不覆盖左右侧边栏；**首次展开自动开一个新终端**（终端卡片二级设置可关）；右上角 x 一键折叠
+- **📱 移动端**：视口 < 1024px 时（与 DSH 自身的窄屏断点一致）自动合并为**单面板抽屉**——右侧栏与底部面板两个工作台上下同显（中间可拖分隔条，高度与桌面底部面板互通），右上角只剩一枚开关；新会话默认收起，聊天里点文件/外链自动展开，不挤压对话区
 - **🔧 分栏工作台**：拖 Tab 拆分/合并分栏（可**跨面板拖 Tab**）、分隔线调比例；右上角持久按钮簇（底栏 + 侧拉 glyph）折叠/展开两个面板；两面板共享拐角双向拖动调节尺寸，拖动 rAF 直写 DOM 保持流畅
 - **🔁 会话隔离**：布局/分栏/Tab/两面板状态按会话持久化（localStorage），陈旧状态自动净化；聊天「产出文件」改在侧边栏打开
 - **⚙️ 声明式设置**：设置页「侧边卡片」分区按注册表渲染功能清单（小卡片网格，高亮 = 启用），每项可独立开/关；二级设置（子代理自动展开、终端工具、底部面板首展自动开终端、沙箱开关等）经齿轮按钮在原生弹窗中编辑
@@ -118,6 +119,7 @@ pnpm watch        # tsdown --watch
 - `.xlsx` 预览不保留单元格样式（SheetJS 社区版限制）；Office/PPTX 预览内联进 client bundle（约 23MB），首次加载较慢
 - 浏览器沙箱无登录态/第三方 Cookie 受限，部分站点登录需走弹窗；被 `X-Frame-Options`/`frame-ancestors` 拒绝嵌入的站点（如 arxiv.org）显示原因面板（含「在浏览器中打开」）；iframe 内部跳转不进后退栈
 - HTML 预览渲染的是已保存文件（不反映未保存草稿）
+- 移动端（<1024px）无底部面板概念：底部面板的标签页并入抽屉下工作台，桌面端的 `bottomOpen` 状态只在宽视口下生效；移动端底部首展自动开终端不触发
 
 ## 🖥️ 平台支持
 
diff --git a/docs/plans/2026-08-12-mobile-layout-design.md b/docs/plans/2026-08-12-mobile-layout-design.md
new file mode 100644
index 0000000..ab98f29
--- /dev/null
+++ b/docs/plans/2026-08-12-mobile-layout-design.md
@@ -0,0 +1,73 @@
+# 移动端（窄视口 <1024px）侧边栏布局设计
+
+**日期**：2026-08-12
+**状态**：已实施（本文档含实施偏差记录）
+**目标版本**：v0.7.0
+
+## 1. 目标
+
+按**视口宽度**（<1024px，与 DSH 宿主 `SIDEBAR_AUTO_COLLAPSE` 一致）切换移动端侧边栏体验：
+
+1. 移动端**不提供底部面板按钮**（右上角按钮簇只剩一枚右侧面板开关）。
+2. **底部面板与右侧面板合并显示**：右侧面板成为全屏抽屉，内部上下堆叠两个工作台——上 = 原右侧栏工作台（`state.splits`），下 = 原底部面板工作台（`state.bottomSplits`），中间可拖分隔条。底部面板浮层（关闭按钮 / 顶部拖动条 / 共享拐角）在移动端整体不渲染。
+3. 附带移动端优化：新会话默认收起抽屉、文件/外链打开自动展开、宽度拖动条禁用、布局不挤压（抽屉悬浮）、窄屏标签收窄。
+
+## 2. 非目标
+
+- 不改持久化 schema：`bottomOpen` / `bottomHeight` / 两棵树字段原样保留，跨断点不迁移状态。
+- 不改桌面行为：≥1024px 时双面板布局与之前完全一致（含布局挤压、拐角、底部首展自动终端）。
+- 不改公开服务 API、PrefsSchema、host 半。
+- 不处理 DSH 详情栏（宿主让步链在 <~996px 自动关闭详情栏，与本改动无交互）。
+
+## 3. 现状回顾
+
+- `Sidebar.tsx` 渲染两个独立浮层：右侧面板（全高、宽度可拖、布局挤压 `--dsh-sidebar-width`）与底部面板（只挤中间列、高度可拖、右上角按钮簇双按钮、共享拐角）。
+- 底部面板状态字段：`bottomOpen` / `bottomHeight`（钳制 `[BOTTOM_MIN, innerHeight-PANEL_MIN]`）/ `bottomOpenedOnce` / `bottomSplits`。
+- 首次展开底部面板自动开终端（`bottomPanelAutoTerminal` pref）。
+
+## 4. 设计
+
+### 4.1 断点模块（新 `src/client/breakpoints.ts`）
+
+- `NARROW_MAX_WIDTH = 1024`；`isNarrowWidth(width)` 纯函数；`useNarrowViewport()` hook（初始读 `window.innerWidth`，resize + rAF 节流，`typeof window` 守卫；不用 matchMedia——jsdom 未实现）。
+- CSS 侧配对 `@media (max-width: 1023px)`（1023 ≡ <1024），两端注释互指。
+
+### 4.2 合并布局（`Sidebar.tsx` + `split-pane.tsx` `MobileWorkbench`）
+
+- 窄屏下：面板宽度 `100vw`（全屏抽屉）；`--dsh-sidebar-*` 布局变量恒 0（抽屉悬浮覆盖，不挤压 AppFrame）；宽度拖动条、底部浮层、`bottomClose`、拐角、底部面板按钮全部不渲染。
+- `MobileWorkbench`：column flex → 上区 `Workbench(state.splits)`（flex:1）→ 分隔条（复用 `.dividerCol` 命中区 + hairline）→ 下区 `Workbench(state.bottomSplits)`（高度 = `state.bottomHeight`，`flex:none`）。
+- **下区高度复用 `bottomHeight`**：桌面底部面板高度语义延续到移动端；移动端拖分隔条写回同一字段并持久化，回桌面后一致。
+- 分隔条拖动：镜像现有面板拖动模式（pointer capture + rAF 批处理 DOM 直写 + 松手一次 `setBottomHeight` commit），避免终端/编辑器逐帧重渲染。
+- 两个工作台**始终渲染**（空底部树显示欢迎卡片）；跨工作台拖 Tab 由既有跨树 `moveTabToEdge` / `moveTab` 支持。
+- 底部树 tab 的 `visible` 在窄屏 = `panelOpen && active`（抽屉开合决定），不再看 `bottomOpen`。
+
+### 4.3 行为修正
+
+- `state.ts` `loadState`：窄屏新会话默认 `panelOpen=false`（首开才生效；用户手动展开后照常持久化）。
+- `service.ts` `openTab`：窄屏且 seed 带 `path`/`url` 且面板收起 → 自动展开抽屉（dedupe-focus 分支同样生效）；纯类型打开不展开；宽屏行为不变。
+- 底部首展自动终端 effect 加 `narrow` 守卫（桌面专属行为）。
+- 子代理自动展开 / jump-back 不动（展开 `panelOpen` = 展开抽屉，语义正确）。
+
+### 4.4 样式（`sidebar.module.css`）
+
+- `@media (max-width: 1023px)`：开放面板标签条预留宽度 72px → 40px（按钮簇只剩一枚）；`.tab` min/max 宽收窄（48/128px）。
+- 新增 `.mobileWorkbench` / `.mobilePane` / `.mobilePaneBottom`；分隔条复用 `.dividerCol`。
+
+### 4.5 测试
+
+- `tests/breakpoints.spec.ts`：断点边界。
+- `tests/service.spec.ts`：窄屏 auto-expand 5 例（path/url 展开、类型不展开、宽屏不展开、dedupe 聚焦仍展开）。
+- `tests/unit.spec.ts`：窄屏新会话默认收起（stub `window.innerWidth=390`，try/finally 还原）。
+- `tests/mobile-workbench.spec.tsx`：双树并显 + 分隔条 + 底部高度（renderToString 结构断言）。
+- 分隔条拖动 = 指针捕获逻辑，人工验证（与桌面拖动同策略）；store 钳制由既有 `setBottomHeight` 测试覆盖。
+
+## 5. 边界情况
+
+- 跨断点往返：状态零迁移；桌面 `bottomOpen=false` 但底部树有标签的会话，移动端仍显示（合并语义），回桌面仍隐藏。
+- 底部高度越界：渲染直接用 `state.bottomHeight`（store 写入已钳制）；拖动中按面板体高度钳制，松手再走 store 钳制。
+- jsdom 兼容：hook 不用 matchMedia；既有测试 stub `innerWidth=1024` 自动走桌面路径，零行为变化。
+
+## 6. 实施偏差记录
+
+- 计划时考虑过「空底部树隐藏 + 拖 Tab 创建」方案，最终采用**始终渲染两工作台**（用户确认的选项语义：上下并显 + 可调分隔；空底部树显示欢迎卡片，发现性更好）。
+- 计划时考虑过 matchMedia 方案，最终用 resize + rAF（jsdom 兼容，仓库既有拖动模式同款）。
diff --git a/src/client/Sidebar.tsx b/src/client/Sidebar.tsx
index 83a2713..56b24e4 100644
--- a/src/client/Sidebar.tsx
+++ b/src/client/Sidebar.tsx
@@ -16,8 +16,16 @@
  * content to the views. New tabs come from the + menu (explorer / git /
  * terminal; editors open from the explorer). Tabs live in one tree only —
  * they never cross panels; only the panel sizes drag against each other.
+ *
+ * Narrow (mobile, <1024px) viewports collapse the two panels into one: the
+ * right panel becomes a full-width drawer holding BOTH workbenches stacked
+ * (MobileWorkbench, see split-pane.tsx), the bottom panel and its toggle
+ * button disappear, and the layout push is disabled (the drawer floats).
+ * Crossing the breakpoint never rewrites per-session state — bottomOpen /
+ * bottomHeight / both trees survive untouched, so desktop restores exactly
+ * what the user left.
  */
-import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
+import { useCallback, useEffect, useMemo, useRef, useState, type ReactNode } from 'react'
 import { useSyncExternalStore } from 'react'
 import clsx from 'clsx'
 import { IconCloseFill14, Tooltip } from '@deepseek-ai/dsh-client-ui-primitives'
@@ -29,7 +37,8 @@ import {
   type DropZone, type SidebarState, type SidebarStore, type SidebarTab, type SplitNode,
 } from './state.ts'
 import { IconPanelBottomOutline16, IconPanelRightOutline16 } from './icons.tsx'
-import { Workbench, type WorkbenchActions } from './split-pane.tsx'
+import { MobileWorkbench, Workbench, type WorkbenchActions } from './split-pane.tsx'
+import { useNarrowViewport } from './breakpoints.ts'
 import type { NewTabOption } from './TabBar.tsx'
 import type { TabDragPayload } from './TabBar.tsx'
 import { relativeTo } from './paths.ts'
@@ -94,6 +103,15 @@ function buildNewTabOptions(state: SidebarState, ctx: Context, scope: SessionSco
 export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
   const { ctx, store } = props
 
+  // Narrow (mobile) viewports collapse the two panels into one: the right
+  // panel becomes a full-width drawer holding BOTH workbenches, the bottom
+  // panel (and its toggle button) disappears, and the layout push is
+  // disabled (the drawer floats over the app shell). Crossing the
+  // breakpoint never rewrites the per-session state — bottomOpen /
+  // bottomHeight / both trees survive untouched, so desktop restores
+  // exactly what the user left.
+  const narrow = useNarrowViewport()
+
   // Current conversation (the sessions list feed).
   const sessionList = useSyncExternalStore(
     useMemo(() => (callback: () => void) => ctx.sessions.list.subscribe(callback), [ctx]),
@@ -268,6 +286,10 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
    */
   const bottomWasOpenRef = useRef<boolean | undefined>(undefined)
   useEffect(() => {
+    // The bottom panel does not exist on narrow viewports (the two
+    // workbenches merge into one panel), so the first-expansion auto
+    // terminal is a desktop-only behavior.
+    if (narrow) return
     if (state === undefined) return
     const wasOpen = bottomWasOpenRef.current
     bottomWasOpenRef.current = state.bottomOpen
@@ -279,7 +301,7 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
     // atomically so later expansions never repeat the auto-open.
     store.reduce(s => ({ ...s, activePane: firstLeaf(s.bottomSplits).id, bottomOpenedOnce: true }))
     ctx.betterSidebar?.openTab({ type: 'terminal' })
-  }, [state, store, ctx])
+  }, [state, store, ctx, narrow])
 
   // Panel drags: the right panel's width (left edge strip), the bottom
   // panel's height (top edge strip), and the shared corner (both at once).
@@ -356,16 +378,18 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
   // are squeezed instead of covered. The margins are capped at the viewport
   // so a stale persisted size (e.g. fullscreen on a bigger window) can never
   // crush the app shell to zero. Dragging disables the layout transition.
+  // On NARROW viewports the drawer FLOATS over the app shell — no push, the
+  // conversation keeps the full width behind the drawer.
   useEffect(() => {
-    const width = snapshot.state?.panelOpen === true
+    const width = !narrow && snapshot.state?.panelOpen === true
       ? Math.min(snapshot.state.width, window.innerWidth)
       : 0
-    const height = snapshot.state?.bottomOpen === true
+    const height = !narrow && snapshot.state?.bottomOpen === true
       ? Math.min(snapshot.state.bottomHeight, window.innerHeight)
       : 0
     document.documentElement.style.setProperty('--dsh-sidebar-width', `${width}px`)
     document.documentElement.style.setProperty('--dsh-sidebar-height', `${height}px`)
-  }, [snapshot.state?.panelOpen, snapshot.state?.width, snapshot.state?.bottomOpen, snapshot.state?.bottomHeight])
+  }, [narrow, snapshot.state?.panelOpen, snapshot.state?.width, snapshot.state?.bottomOpen, snapshot.state?.bottomHeight])
   useEffect(() => {
     if (anyDragging) document.body.setAttribute('data-dsh-sidebar-dragging', '')
     else document.body.removeAttribute('data-dsh-sidebar-dragging')
@@ -450,11 +474,13 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
   if (state === undefined || sessionId === undefined) {
     return (
       <div className={css.toggleCluster}>
-        <Tooltip label={t('noSession')} side="bottom" delayMs={500}>
-          <button type="button" className={css.toggleButton} disabled aria-label={t('noSession')}>
-            <IconPanelBottomOutline16 />
-          </button>
-        </Tooltip>
+        {!narrow && (
+          <Tooltip label={t('noSession')} side="bottom" delayMs={500}>
+            <button type="button" className={css.toggleButton} disabled aria-label={t('noSession')}>
+              <IconPanelBottomOutline16 />
+            </button>
+          </Tooltip>
+        )}
         <Tooltip label={t('noSession')} side="bottom" delayMs={500}>
           <button type="button" className={css.toggleButton} disabled aria-label={t('noSession')}>
             <IconPanelRightOutline16 />
@@ -478,6 +504,13 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
    * ctx and the conversation input service at click time; a missing service
    * or scope degrades to a logged no-op, never a crash.
    */
+  /** The tab icon from the tab-type registry (shared by every workbench). */
+  const tabIconOf = (tab: SidebarTab): ReactNode => {
+    const descriptor = ctx.betterSidebar?.getTab(tab.type)
+    if (descriptor === undefined) return null
+    return typeof descriptor.icon === 'function' ? descriptor.icon(14) : descriptor.icon
+  }
+
   /**
    * Render one tab's content. `active` (from the workbench) tells whether
    * this tab is the active one in its pane; combined with the panel's
@@ -495,7 +528,10 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
       onReferenceFile={referenceInChat}
       ctx={ctx}
       store={store}
-      visible={bottom ? state.bottomOpen && active : state.panelOpen && active}
+      // On narrow viewports BOTH trees live in the one drawer, so the bottom
+      // tree's visibility is the drawer's (panelOpen), not the bottom
+      // panel's — the bottom panel itself does not exist there.
+      visible={narrow ? state.panelOpen && active : bottom ? state.bottomOpen && active : state.panelOpen && active}
       onSubagentJump={(childSessionId) => { subagentJumpRef.current = childSessionId }}
       onOpenDiff={(diffTab) => { store.reduce(s => openDiffTab(s, paneId, diffTab)) }}
     />
@@ -512,16 +548,22 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
         so the tabs genuinely yield space to it.
       */}
       <div className={css.toggleCluster}>
-        <Tooltip label={state.bottomOpen ? t('collapseBottomPanel') : t('expandBottomPanel')} side="bottom" delayMs={500}>
-          <button
-            type="button"
-            className={css.toggleButton}
-            aria-label={state.bottomOpen ? t('collapseBottomPanel') : t('expandBottomPanel')}
-            onClick={() => { store.reduce(toggleBottomPanel) }}
-          >
-            <IconPanelBottomOutline16 />
-          </button>
-        </Tooltip>
+        {/*
+          Narrow viewports merge the two workbenches into the one drawer —
+          there is no bottom panel, so its toggle button is not offered.
+        */}
+        {!narrow && (
+          <Tooltip label={state.bottomOpen ? t('collapseBottomPanel') : t('expandBottomPanel')} side="bottom" delayMs={500}>
+            <button
+              type="button"
+              className={css.toggleButton}
+              aria-label={state.bottomOpen ? t('collapseBottomPanel') : t('expandBottomPanel')}
+              onClick={() => { store.reduce(toggleBottomPanel) }}
+            >
+              <IconPanelBottomOutline16 />
+            </button>
+          </Tooltip>
+        )}
         <Tooltip label={state.panelOpen ? t('collapse') : t('expand')} side="bottom" delayMs={500}>
           <button
             type="button"
@@ -538,50 +580,65 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
         the slide in/out can animate; visibility hides it after the slide
         settles. Its bottom edge follows the bottom panel's height (0 while
         the bottom panel is closed) — the VSCode-style "sidebar above panel".
+        On NARROW viewports it is a full-width drawer holding both
+        workbenches (see MobileWorkbench); the width drag strip is not
+        offered there — a full-screen sheet has nothing to drag.
       */}
       <div
         ref={panelRef}
         className={clsx(css.panel, !state.panelOpen && css.panelHidden)}
-        style={{ width: Math.min(state.width, window.innerWidth) }}
+        style={{ width: narrow ? '100vw' : Math.min(state.width, window.innerWidth) }}
         data-dragging={anyDragging || undefined}
       >
-          <div
-            className={clsx(css.panelResize, draggingWidth && css.panelResizeActive)}
-            onPointerDown={(event) => {
-              event.preventDefault()
-              event.currentTarget.setPointerCapture(event.pointerId)
-              widthDrag.current = { startX: event.clientX, startWidth: state.width }
-              setDraggingWidth(true)
-            }}
-            onPointerMove={(event) => {
-              if (!event.currentTarget.hasPointerCapture(event.pointerId)) return
-              const { startX, startWidth } = widthDrag.current
-              const width = clampWidth(startWidth + (startX - event.clientX))
-              const height = state.bottomOpen ? Math.min(state.bottomHeight, window.innerHeight) : 0
-              scheduleDrag(width, height)
-            }}
-            onPointerUp={(event) => {
-              if (!event.currentTarget.hasPointerCapture(event.pointerId)) return
-              event.currentTarget.releasePointerCapture(event.pointerId)
-              const { startX, startWidth } = widthDrag.current
-              stopDragScheduling()
-              store.reduce(s => setWidth(s, startWidth + (startX - event.clientX)))
-              setDraggingWidth(false)
-            }}
-          />
+          {!narrow && (
+            <div
+              className={clsx(css.panelResize, draggingWidth && css.panelResizeActive)}
+              onPointerDown={(event) => {
+                event.preventDefault()
+                event.currentTarget.setPointerCapture(event.pointerId)
+                widthDrag.current = { startX: event.clientX, startWidth: state.width }
+                setDraggingWidth(true)
+              }}
+              onPointerMove={(event) => {
+                if (!event.currentTarget.hasPointerCapture(event.pointerId)) return
+                const { startX, startWidth } = widthDrag.current
+                const width = clampWidth(startWidth + (startX - event.clientX))
+                const height = state.bottomOpen ? Math.min(state.bottomHeight, window.innerHeight) : 0
+                scheduleDrag(width, height)
+              }}
+              onPointerUp={(event) => {
+                if (!event.currentTarget.hasPointerCapture(event.pointerId)) return
+                event.currentTarget.releasePointerCapture(event.pointerId)
+                const { startX, startWidth } = widthDrag.current
+                stopDragScheduling()
+                store.reduce(s => setWidth(s, startWidth + (startX - event.clientX)))
+                setDraggingWidth(false)
+              }}
+            />
+          )}
         <div className={css.panelBody}>
-          <Workbench
-            state={state}
-            newTabOptions={buildNewTabOptions(state, ctx, { sessionId, cwd })}
-            actions={actions}
-            onNewTab={onNewTab}
-            renderTab={renderTab}
-            getTabIcon={(tab) => {
-              const descriptor = ctx.betterSidebar?.getTab(tab.type)
-              if (descriptor === undefined) return null
-              return typeof descriptor.icon === 'function' ? descriptor.icon(14) : descriptor.icon
-            }}
-          />
+          {narrow
+            ? (
+              <MobileWorkbench
+                state={state}
+                newTabOptions={buildNewTabOptions(state, ctx, { sessionId, cwd })}
+                actions={actions}
+                onNewTab={onNewTab}
+                renderTab={(tab, active, paneId) => renderTab(tab, active, paneId, true)}
+                getTabIcon={tabIconOf}
+                onBottomHeightCommit={(height) => { store.reduce(s => setBottomHeight(s, height)) }}
+              />
+            )
+            : (
+              <Workbench
+                state={state}
+                newTabOptions={buildNewTabOptions(state, ctx, { sessionId, cwd })}
+                actions={actions}
+                onNewTab={onNewTab}
+                renderTab={renderTab}
+                getTabIcon={tabIconOf}
+              />
+            )}
         </div>
       </div>
       {/*
@@ -590,8 +647,10 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
         shell's own left sidebar and ends at the right panel's left edge —
         neither sidebar gives up any position (the right panel keeps its
         full height). Its resize strip is the top edge; hidden by sliding
-        down like the right panel.
+        down like the right panel. On NARROW viewports it does not exist —
+        the bottom workbench lives inside the drawer (MobileWorkbench).
       */}
+      {!narrow && (
       <div
         ref={bottomRef}
         className={clsx(css.bottomPanel, !state.bottomOpen && css.bottomPanelHidden)}
@@ -655,21 +714,19 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
             actions={actions}
             onNewTab={onNewTab}
             renderTab={(tab, active, paneId) => renderTab(tab, active, paneId, true)}
-            getTabIcon={(tab) => {
-              const descriptor = ctx.betterSidebar?.getTab(tab.type)
-              if (descriptor === undefined) return null
-              return typeof descriptor.icon === 'function' ? descriptor.icon(14) : descriptor.icon
-            }}
+            getTabIcon={tabIconOf}
           />
         </div>
       </div>
+      )}
       {/*
         The shared corner (only while BOTH panels are open): the intersection
         of the right panel's left edge and the bottom panel's top edge.
         Horizontal drags resize the right panel's width, vertical drags the
         bottom panel's height — the two panels drag against each other.
+        (Never on narrow viewports: the bottom panel does not exist there.)
       */}
-      {state.panelOpen && state.bottomOpen && (
+      {!narrow && state.panelOpen && state.bottomOpen && (
         <div
           ref={cornerRef}
           className={css.cornerHandle}
diff --git a/src/client/breakpoints.ts b/src/client/breakpoints.ts
new file mode 100644
index 0000000..a600f6b
--- /dev/null
+++ b/src/client/breakpoints.ts
@@ -0,0 +1,50 @@
+/**
+ * Narrow-viewport ("mobile") breakpoint for the sidebar. Width-based, shared
+ * by the layout logic (JS) and the style gates (CSS). The CSS side pairs
+ * with this file via `@media (max-width: 1023px)` rules (sidebar.module.css)
+ * — 1023px ≡ widths below NARROW_MAX_WIDTH, documented at both ends.
+ *
+ * The value matches the DSH app shell's own narrow breakpoint
+ * (SIDEBAR_AUTO_COLLAPSE = 1024, ui-layout/columns.ts), so the sidebar's
+ * mobile layout switches at exactly the width where the host app collapses
+ * its own sidebar to the rail.
+ */
+import { useEffect, useState } from 'react'
+
+/** Viewport widths strictly below this are "mobile" (paired CSS: max-width: 1023px). */
+export const NARROW_MAX_WIDTH = 1024
+
+/** Whether a viewport width is narrow (mobile). */
+export function isNarrowWidth(width: number): boolean {
+  return width < NARROW_MAX_WIDTH
+}
+
+/**
+ * Live narrow-viewport flag for components. Reads `window.innerWidth` and
+ * re-measures on resize (rAF-throttled, the repo's existing drag pattern).
+ * Deliberately avoids `matchMedia` (jsdom does not implement it) — the
+ * resize listener is equally exact for a breakpoint that never changes
+ * while the page is open.
+ */
+export function useNarrowViewport(): boolean {
+  const [narrow, setNarrow] = useState(
+    () => typeof window !== 'undefined' && isNarrowWidth(window.innerWidth),
+  )
+  useEffect(() => {
+    if (typeof window === 'undefined') return
+    let frame: number | null = null
+    const measure = (): void => {
+      frame = null
+      setNarrow(isNarrowWidth(window.innerWidth))
+    }
+    const onResize = (): void => {
+      if (frame === null) frame = requestAnimationFrame(measure)
+    }
+    window.addEventListener('resize', onResize)
+    return () => {
+      window.removeEventListener('resize', onResize)
+      if (frame !== null) cancelAnimationFrame(frame)
+    }
+  }, [])
+  return narrow
+}
diff --git a/src/client/service.ts b/src/client/service.ts
index 2cfed4c..a08a160 100644
--- a/src/client/service.ts
+++ b/src/client/service.ts
@@ -22,9 +22,10 @@
 import type { ReactNode } from 'react'
 import type { Context } from '../context-types.ts'
 import {
-  activateTab, allLeaves, closeTab as closeTabReducer, openTabInActivePane, patchTab,
+  activateTab, allLeaves, closeTab as closeTabReducer, openTabInActivePane, patchTab, togglePanel,
   type SidebarState, type SidebarStore, type SidebarTab,
 } from './state.ts'
+import { isNarrowWidth } from './breakpoints.ts'
 import type { SessionScope } from './api.ts'
 
 /** One declarative boolean setting of a tab/viewer, rendered as a nested
@@ -335,13 +336,29 @@ export function createBetterSidebarService(store: SidebarStore): BetterSidebarSe
       // A URL seed pre-fills the tab's path (the browser tab navigates to it
       // on mount). An explicit seed.title also wins over a createTab-minted
       // default title (e.g. the sidebar-browser's hostname title).
+      let landed: SidebarState
       if (seed.url !== undefined) {
-        return patchTab(next, tab.id, {
+        landed = patchTab(next, tab.id, {
           path: seed.url,
           ...(seed.title !== undefined ? { title: seed.title } : {}),
         })
+      } else {
+        landed = next
       }
-      return next
+      // Narrow-viewport drawer: a CONTENT open (file / browser) must be
+      // visible — expand the collapsed drawer so the open lands in sight.
+      // Type-only opens (+ menu, agent-terminal auto-tabs) never expand; on
+      // wide viewports nothing here changes (the panel behavior is the
+      // caller's business).
+      if (
+        typeof window !== 'undefined'
+        && isNarrowWidth(window.innerWidth)
+        && (seed.path !== undefined || seed.url !== undefined)
+        && !landed.panelOpen
+      ) {
+        return togglePanel(landed)
+      }
+      return landed
     })
   }
 
diff --git a/src/client/sidebar.module.css b/src/client/sidebar.module.css
index 5975c2d..b63b1db 100644
--- a/src/client/sidebar.module.css
+++ b/src/client/sidebar.module.css
@@ -264,6 +264,38 @@
   display: flex;
 }
 
+/* The narrow-viewport workbench (MobileWorkbench): BOTH workbench trees
+   stacked in the one drawer — the right panel's tree flexes after the
+   fixed-height bottom tree, with a draggable divider (the .dividerCol
+   hit-strip + hairline) between them. */
+.mobileWorkbench {
+  flex: 1;
+  min-width: 0;
+  min-height: 0;
+  display: flex;
+  flex-direction: column;
+}
+
+.mobilePane {
+  position: relative;
+  display: flex;
+  min-width: 0;
+  min-height: 0;
+}
+
+/* The bottom tree of the merged layout: fixed height (state.bottomHeight),
+   flex: none so the top tree takes the remainder. */
+.mobilePaneBottom {
+  flex: none;
+}
+
+/* The divider of the merged layout sits between the two panes; .dividerCol
+   already supplies the 7px hit strip, row-resize cursor and hairline. */
+.mobileWorkbench > .dividerCol {
+  flex: none;
+  margin: 0;
+}
+
 .split {
   flex: 1;
   min-width: 0;
@@ -1864,3 +1896,28 @@
     animation: none;
   }
 }
+
+/* ── Narrow (mobile) viewport ──────────────────────────────────────────── */
+
+/* Pairs with NARROW_MAX_WIDTH = 1024 in src/client/breakpoints.ts: widths
+   strictly below 1024px (i.e. max-width: 1023px) switch the sidebar to the
+   mobile layout — one drawer holding both workbenches, one toggle button.
+   The structural switches (which panels render, the merged MobileWorkbench)
+   are JS-driven via useNarrowViewport; this block only adapts the pure
+   style details. */
+@media (max-width: 1023px) {
+  /* The toggle cluster holds a single button on narrow viewports (the
+     bottom panel button is not rendered), so the open panel's tab strip
+     reserves 40px instead of the desktop 72px — the tabs and + menu still
+     yield space to the one remaining control. */
+  .panel:not(.panelHidden) .tabBar {
+    padding-right: 40px;
+  }
+
+  /* Narrower tabs: a phone-width drawer fits more open tabs before the
+     strip scrolls. */
+  .tab {
+    min-width: 48px;
+    max-width: 128px;
+  }
+}
diff --git a/src/client/split-pane.tsx b/src/client/split-pane.tsx
index ee339a3..649aefc 100644
--- a/src/client/split-pane.tsx
+++ b/src/client/split-pane.tsx
@@ -12,7 +12,7 @@
 import { Fragment, useEffect, useRef, useState } from 'react'
 import type { ReactNode } from 'react'
 import clsx from 'clsx'
-import type { SidebarState, SidebarTab, SplitNode } from './state.ts'
+import { BOTTOM_MIN, type SidebarState, type SidebarTab, type SplitNode } from './state.ts'
 import type { DropZone } from './state.ts'
 import { TabBar, type NewTabOption, parseDrag, type TabDragPayload } from './TabBar.tsx'
 import css from './sidebar.module.css'
@@ -290,3 +290,114 @@ export function Workbench(props: {
     </div>
   )
 }
+
+/**
+ * The narrow-viewport workbench: BOTH workbench trees stacked in one panel —
+ * the right panel's tree on top, the bottom panel's tree below, with a
+ * draggable divider between them (the desktop "two panels" collapse into one
+ * on mobile; see the breakpoint module). The bottom tree's height is the
+ * shared `state.bottomHeight` — the same field the desktop bottom panel uses
+ * — so a height set on mobile carries over to the desktop bottom panel and
+ * vice versa. Tabs stay in their own trees; the cross-tree drag logic
+ * (state.moveTabToEdge / moveTab) already handles moving a tab between the
+ * two panes.
+ *
+ * The divider writes the height DIRECTLY to the DOM while dragging (the top
+ * tree flexes after the fixed-height bottom tree), committing to the store
+ * once on release — the same pattern as the desktop panel drags, so
+ * terminals/editors never re-render on every pointer move.
+ */
+export function MobileWorkbench(props: {
+  state: SidebarState
+  actions: WorkbenchActions
+  newTabOptions: NewTabOption[]
+  onNewTab: (optionId: string) => void
+  renderTab: (tab: SidebarTab, active: boolean, paneId: string) => ReactNode
+  getTabIcon?: (tab: SidebarTab) => ReactNode
+  /** Commit a new bottom-tree height (store-backed, clamped by setBottomHeight). */
+  onBottomHeightCommit: (height: number) => void
+}) {
+  const {
+    state, actions, newTabOptions, onNewTab, renderTab, getTabIcon, onBottomHeightCommit,
+  } = props
+  const bottomRef = useRef<HTMLDivElement | null>(null)
+  const dividerRef = useRef<HTMLDivElement | null>(null)
+  const dragRef = useRef({ startY: 0, startHeight: 0 })
+  const [dragging, setDragging] = useState(false)
+  // One DOM write per frame (like the desktop panel drags): pointer events
+  // outrun the display refresh, and each write reflows the pane content.
+  const dragFrame = useRef<number | null>(null)
+  const pending = useRef<number | null>(null)
+  const scheduleHeight = (height: number): void => {
+    pending.current = height
+    if (dragFrame.current !== null) return
+    dragFrame.current = requestAnimationFrame(() => {
+      dragFrame.current = null
+      const next = pending.current
+      if (next !== null) bottomRef.current?.style.setProperty('height', `${next}px`)
+    })
+  }
+
+  return (
+    <div className={css.mobileWorkbench}>
+      <div className={css.mobilePane}>
+        <Workbench
+          state={state}
+          newTabOptions={newTabOptions}
+          actions={actions}
+          onNewTab={onNewTab}
+          renderTab={renderTab}
+          getTabIcon={getTabIcon}
+        />
+      </div>
+      <div
+        ref={dividerRef}
+        className={clsx(css.divider, css.dividerCol, dragging && css.dividerActive)}
+        onPointerDown={(event) => {
+          event.preventDefault()
+          event.currentTarget.setPointerCapture(event.pointerId)
+          dragRef.current = { startY: event.clientY, startHeight: state.bottomHeight }
+          setDragging(true)
+        }}
+        onPointerMove={(event) => {
+          if (!event.currentTarget.hasPointerCapture(event.pointerId)) return
+          const { startY, startHeight } = dragRef.current
+          const box = dividerRef.current?.parentElement?.getBoundingClientRect()
+          const bodyHeight = box === undefined || box.height === 0 ? window.innerHeight : box.height
+          const next = Math.min(
+            Math.max(BOTTOM_MIN, startHeight + (event.clientY - startY)),
+            Math.max(BOTTOM_MIN, bodyHeight - BOTTOM_MIN),
+          )
+          scheduleHeight(next)
+        }}
+        onPointerUp={(event) => {
+          if (!event.currentTarget.hasPointerCapture(event.pointerId)) return
+          event.currentTarget.releasePointerCapture(event.pointerId)
+          if (dragFrame.current !== null) {
+            cancelAnimationFrame(dragFrame.current)
+            dragFrame.current = null
+          }
+          const final = pending.current
+          pending.current = null
+          setDragging(false)
+          if (final !== null) onBottomHeightCommit(final)
+        }}
+      />
+      <div
+        ref={bottomRef}
+        className={clsx(css.mobilePane, css.mobilePaneBottom)}
+        style={{ height: state.bottomHeight }}
+      >
+        <Workbench
+          state={state}
+          tree={state.bottomSplits}
+          newTabOptions={newTabOptions}
+          actions={actions}
+          onNewTab={onNewTab}
+          renderTab={renderTab}
+          getTabIcon={getTabIcon}
+        />
+      </div>
+    </div>
+  )
+}
diff --git a/src/client/state.ts b/src/client/state.ts
index 9e15031..3a6dd7b 100644
--- a/src/client/state.ts
+++ b/src/client/state.ts
@@ -10,6 +10,7 @@
  * operations are pure functions over the node, unit-tested in tests/state.spec.ts.
  */
 import { SIDEBAR_PREFS_DEFAULTS, type SidebarPrefs } from '../prefs-shared.ts'
+import { isNarrowWidth } from './breakpoints.ts'
 
 /**
  * Tab type identifier. Builtins register their ids (explorer / git / editor
@@ -721,12 +722,18 @@ function loadState(sessionId: string, prefs: SidebarPrefs): SidebarState {
   // chosen percent of the window (clamped to the panel floor and the
   // viewport so a huge percent can never crush the app shell), the panel
   // starts open only when the preference says so, and the default explorer
-  // tab is skipped when the user disabled the explorer tab type.
+  // tab is skipped when the user disabled the explorer tab type. On a
+  // NARROW viewport a brand-new session starts collapsed instead — the
+  // panel is a full-screen drawer there, and auto-opening it on first
+  // paint would cover the conversation before the user asked. Only the
+  // first seeding is affected: once the user expands the drawer,
+  // `panelOpen: true` persists like any other state.
   const viewport = typeof window !== 'undefined' ? window.innerWidth : undefined
   const width = viewport === undefined
     ? PANEL_DEFAULT
     : defaultWidthFor(viewport, prefs.defaultWidthPercent)
-  return makeDefaultState(width, prefs.openByDefault, prefs.tabsEnabled['explorer'] !== false)
+  const openByDefault = prefs.openByDefault && (viewport === undefined || !isNarrowWidth(viewport))
+  return makeDefaultState(width, openByDefault, prefs.tabsEnabled['explorer'] !== false)
 }
 
 /**
diff --git a/tests/breakpoints.spec.ts b/tests/breakpoints.spec.ts
new file mode 100644
index 0000000..827caeb
--- /dev/null
+++ b/tests/breakpoints.spec.ts
@@ -0,0 +1,23 @@
+/**
+ * Narrow-viewport breakpoint tests: the boundary the mobile sidebar layout
+ * keys off (paired with the CSS @media (max-width: 1023px) gates in
+ * sidebar.module.css — 1023px ≡ widths strictly below NARROW_MAX_WIDTH).
+ */
+import { describe, expect, it } from 'vitest'
+import { isNarrowWidth, NARROW_MAX_WIDTH } from '../src/client/breakpoints.ts'
+
+describe('narrow-viewport breakpoint', () => {
+  it('treats widths below NARROW_MAX_WIDTH as narrow (mobile)', () => {
+    expect(NARROW_MAX_WIDTH).toBe(1024)
+    expect(isNarrowWidth(320)).toBe(true)
+    expect(isNarrowWidth(390)).toBe(true)
+    expect(isNarrowWidth(768)).toBe(true)
+    expect(isNarrowWidth(1023)).toBe(true)
+  })
+
+  it('treats NARROW_MAX_WIDTH and above as wide (desktop)', () => {
+    expect(isNarrowWidth(1024)).toBe(false)
+    expect(isNarrowWidth(1280)).toBe(false)
+    expect(isNarrowWidth(1920)).toBe(false)
+  })
+})
diff --git a/tests/mobile-workbench.spec.tsx b/tests/mobile-workbench.spec.tsx
new file mode 100644
index 0000000..517ff0a
--- /dev/null
+++ b/tests/mobile-workbench.spec.tsx
@@ -0,0 +1,68 @@
+/**
+ * MobileWorkbench tests: the narrow-viewport merged workbench — BOTH trees
+ * (right + bottom) stacked in one panel with a draggable divider between
+ * them, the bottom tree at the shared state.bottomHeight. Structure
+ * assertions via renderToString (the repo's component-test pattern). The
+ * divider drag itself is pointer-capture logic — exercised manually, like
+ * the desktop panel drags; the store clamp it commits through
+ * (setBottomHeight) is unit-tested in unit.spec.ts.
+ */
+import { describe, expect, it } from 'vitest'
+import { renderToString } from 'react-dom/server'
+import { MobileWorkbench, type WorkbenchActions } from '../src/client/split-pane.tsx'
+import { makeDefaultState } from '../src/client/state.ts'
+
+const noopActions: WorkbenchActions = {
+  closeTab: () => {},
+  activateTab: () => {},
+  focusPane: () => {},
+  moveTabToEdge: () => {},
+  moveTabBefore: () => {},
+  resizeSplit: () => {},
+}
+
+describe('MobileWorkbench (narrow merged layout)', () => {
+  it('renders BOTH trees stacked, with the divider and the shared bottom height', () => {
+    const state = makeDefaultState()
+    // A terminal tab in the bottom tree (a desktop bottom-panel session).
+    const bottomPane = state.bottomSplits as { tabs: Array<{ id: string; type: string; title: string }> }
+    bottomPane.tabs = [{ id: 'tab:b1', type: 'terminal', title: 'Terminal' }]
+    const html = renderToString((
+      <MobileWorkbench
+        state={state}
+        actions={noopActions}
+        newTabOptions={[]}
+        onNewTab={() => {}}
+        renderTab={(tab) => <div data-testid={`tab-${tab.id}`}>{tab.title}</div>}
+        onBottomHeightCommit={() => {}}
+      />
+    ))
+    // The right tree's default explorer tab renders (tab strip).
+    expect(html).toContain('Explorer')
+    // The bottom tree's terminal tab renders — the merge shows both trees.
+    expect(html).toContain('Terminal')
+    // The divider sits between the two workbenches.
+    expect(html).toContain('divider')
+    // The bottom pane carries the shared bottomHeight (the desktop bottom
+    // panel's height field drives the mobile bottom tree's height).
+    expect(html).toContain(`height:${state.bottomHeight}px`)
+  })
+
+  it('renders an empty bottom tree with its welcome cards (both workbenches always visible)', () => {
+    const state = makeDefaultState()
+    const html = renderToString((
+      <MobileWorkbench
+        state={state}
+        actions={noopActions}
+        newTabOptions={[{ id: 'explorer', label: 'Explorer' }, { id: 'terminal', label: 'Terminal' }]}
+        onNewTab={() => {}}
+        renderTab={() => null}
+        onBottomHeightCommit={() => {}}
+      />
+    ))
+    // The top tree's explorer TAB and the empty bottom pane's welcome cards
+    // both render (each option card carries its label).
+    expect(html).toContain('Explorer')
+    expect(html).toContain('Terminal')
+  })
+})
diff --git a/tests/service.spec.ts b/tests/service.spec.ts
index 63d4164..9c42d66 100644
--- a/tests/service.spec.ts
+++ b/tests/service.spec.ts
@@ -465,3 +465,81 @@ describe('service.openTab across the two panels', () => {
     expect(allLeaves(after.bottomSplits).flatMap(l => l.tabs).some(t => t.id === gitTab.id)).toBe(false)
   })
 })
+
+describe('service.openTab narrow-viewport auto-expand', () => {
+  /** The window stub is a plain object (see the file header), so the width is writable. */
+  const setWidth = (width: number): void => {
+    ;(g.window as { innerWidth: number }).innerWidth = width
+  }
+
+  it('expands the collapsed drawer for a path (file) open on a narrow viewport', () => {
+    setWidth(390)
+    try {
+      const store = createSidebarStore()
+      const service = createBetterSidebarService(store)
+      service.registerTab({ id: 'editor', title: 'Editor', component: () => null })
+      store.setSession('s1')
+      store.reduce(s => ({ ...s, panelOpen: false }))
+      service.openTab({ type: 'editor', title: 'main.ts', path: '/p/main.ts' })
+      expect(store.getSnapshot().state?.panelOpen).toBe(true)
+    } finally {
+      setWidth(1024)
+    }
+  })
+
+  it('expands the collapsed drawer for a URL (browser) open on a narrow viewport', () => {
+    setWidth(390)
+    try {
+      const store = createSidebarStore()
+      const service = createBetterSidebarService(store)
+      service.registerTab({ id: 'browser', title: 'Browser', component: () => null })
+      store.setSession('s1')
+      store.reduce(s => ({ ...s, panelOpen: false }))
+      service.openTab({ type: 'browser', url: 'https://example.com', title: 'example.com' })
+      expect(store.getSnapshot().state?.panelOpen).toBe(true)
+    } finally {
+      setWidth(1024)
+    }
+  })
+
+  it('keeps a collapsed drawer for a type-only open on a narrow viewport', () => {
+    setWidth(390)
+    try {
+      const store = createSidebarStore()
+      const service = createBetterSidebarService(store)
+      service.registerTab({ id: 'explorer', title: 'Explorer', component: () => null })
+      store.setSession('s1')
+      store.reduce(s => ({ ...s, panelOpen: false }))
+      service.openTab({ type: 'explorer', title: 'Explorer' })
+      expect(store.getSnapshot().state?.panelOpen).toBe(false)
+    } finally {
+      setWidth(1024)
+    }
+  })
+
+  it('never expands on a wide viewport (path open keeps the caller’s choice)', () => {
+    const store = createSidebarStore()
+    const service = createBetterSidebarService(store)
+    service.registerTab({ id: 'editor', title: 'Editor', component: () => null })
+    store.setSession('s1')
+    store.reduce(s => ({ ...s, panelOpen: false }))
+    service.openTab({ type: 'editor', title: 'main.ts', path: '/p/main.ts' })
+    expect(store.getSnapshot().state?.panelOpen).toBe(false)
+  })
+
+  it('expands on a narrow viewport even when the open focuses an existing tab (id dedupe)', () => {
+    setWidth(390)
+    try {
+      const store = createSidebarStore()
+      const service = createBetterSidebarService(store)
+      service.registerTab({ id: 'editor', title: 'Editor', component: () => null })
+      store.setSession('s1')
+      service.openTab({ type: 'editor', title: 'main.ts', path: '/p/main.ts' })
+      store.reduce(s => ({ ...s, panelOpen: false }))
+      service.openTab({ type: 'editor', title: 'main.ts', path: '/p/main.ts' })
+      expect(store.getSnapshot().state?.panelOpen).toBe(true)
+    } finally {
+      setWidth(1024)
+    }
+  })
+})
diff --git a/tests/unit.spec.ts b/tests/unit.spec.ts
index 9a6ba5b..f6ca3e5 100644
--- a/tests/unit.spec.ts
+++ b/tests/unit.spec.ts
@@ -1187,6 +1187,29 @@ describe('side card preferences', () => {
     expect(openStore.getSnapshot().state?.panelOpen).toBe(true)
   })
 
+  it('seeds a brand-new session COLLAPSED on narrow viewports (the panel is a full-screen drawer there)', () => {
+    // Stub a narrow window (the file otherwise runs without one): only
+    // innerWidth is read while seeding a fresh session.
+    const original = (globalThis as Record<string, unknown>).window
+    ;(globalThis as Record<string, unknown>).window = {
+      innerWidth: 390,
+      clearTimeout: () => {},
+      setTimeout: (_fn: () => void) => 0,
+    }
+    try {
+      const store = createSidebarStore()
+      // Default prefs say openByDefault: true — the narrow viewport overrides
+      // it for the FIRST seeding only (a later user expansion persists).
+      store.setSession('narrow-fresh')
+      expect(store.getSnapshot().state?.panelOpen).toBe(false)
+      // The width seeding still follows the window (clamped to the floor).
+      expect(store.getSnapshot().state?.width).toBe(280)
+    } finally {
+      if (original === undefined) delete (globalThis as Record<string, unknown>).window
+      else (globalThis as Record<string, unknown>).window = original
+    }
+  })
+
   it('skips the default explorer tab when the explorer type is disabled', () => {
     const store = createSidebarStore()
     store.setPrefs({ openByDefault: true, defaultWidthPercent: 30, autoOpenSubagent: true, agentTerminalTools: false, bottomPanelAutoTerminal: true, interceptOpenPath: true, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, tabsEnabled: { explorer: false }, viewersEnabled: {} })