diff --git a/webui/src/App.tsx b/webui/src/App.tsx
index d655d4f05..46311b496 100644
--- a/webui/src/App.tsx
+++ b/webui/src/App.tsx
@@ -12,8 +12,26 @@ import { Eye, EyeOff, Moon, PanelLeft, ShieldCheck, Sun, X } from "lucide-react"
 import { useTranslation } from "react-i18next";
 import { channelUiPresentation } from "@/channel-plugins/registry";
 import { Sidebar } from "@/components/Sidebar";
+import type { SidebarDeleteItem } from "@/components/ChatList";
 import type { SettingsSectionKey } from "@/components/settings/SettingsView";
 import { ThreadShell } from "@/components/thread/ThreadShell";
+import { PaneWorkbench } from "@/components/workbench/PaneWorkbench";
+import {
+  WORKBENCH_STORAGE_KEY,
+  MAX_WORKBENCH_PANES,
+  addWorkbenchPane,
+  attachWorkbenchPane,
+  detachWorkbenchPane,
+  ensureWorkbenchTab,
+  focusWorkbenchPane,
+  parseWorkbenchState,
+  promoteWorkbenchPane,
+  reconcileWorkbench,
+  setWorkbenchLayout,
+  workbenchChildPaneKeys,
+  workbenchTab,
+  type WorkbenchState,
+} from "@/components/workbench/workbench-model";
 import { floatingSurfaceElevationClassName } from "@/components/ui/floating-surface";
 import { Sheet, SheetContent, SheetTitle } from "@/components/ui/sheet";
 
@@ -120,6 +138,14 @@ const RenameChatDialog = lazy(async () => {
   return { default: module.RenameChatDialog };
 });
 
+function readWorkbenchState(): WorkbenchState {
+  try {
+    return parseWorkbenchState(window.localStorage.getItem(WORKBENCH_STORAGE_KEY));
+  } catch {
+    return parseWorkbenchState(null);
+  }
+}
+
 function SurfaceLoadingFallback() {
   const { t } = useTranslation();
   return (
@@ -1034,9 +1060,18 @@ function Shell({
   const [hostSidebarPreviewOpen, setHostSidebarPreviewOpen] = useState(false);
   const [mobileSidebarOpen, setMobileSidebarOpen] = useState(false);
   const [sessionSearchOpen, setSessionSearchOpen] = useState(false);
+  const [workbenchState, setWorkbenchState] = useState(readWorkbenchState);
+  const [creatingPane, setCreatingPane] = useState(false);
+  const childPaneKeys = useMemo(
+    () => workbenchChildPaneKeys(workbenchState),
+    [workbenchState],
+  );
+  const topicSessions = useMemo(
+    () => sessions.filter((session) => !childPaneKeys.has(session.key)),
+    [childPaneKeys, sessions],
+  );
   const [pendingDelete, setPendingDelete] = useState<{
-    key: string;
-    label: string;
+    items: SidebarDeleteItem[];
     automations?: SessionAutomationJob[];
   } | null>(null);
   const [pendingRename, setPendingRename] = useState<{
@@ -1157,6 +1192,17 @@ function Shell({
     }
   }, [hostSidebarOpen]);
 
+  useEffect(() => {
+    try {
+      window.localStorage.setItem(
+        WORKBENCH_STORAGE_KEY,
+        JSON.stringify(workbenchState),
+      );
+    } catch {
+      // ignore storage errors (private mode, etc.)
+    }
+  }, [workbenchState]);
+
   useEffect(() => {
     writeSessionUpdateChatIds(updatedChatIds);
   }, [updatedChatIds]);
@@ -1220,9 +1266,19 @@ function Shell({
     if (temporarySessions[activeKey]) return temporarySessions[activeKey];
     return sessions.find((s) => s.key === activeKey) ?? null;
   }, [sessions, activeKey, temporarySessions]);
+  const activeTabState = useMemo(() => (
+    activeKey && !temporarySessions[activeKey]
+      ? workbenchTab(workbenchState, activeKey)
+      : null
+  ), [activeKey, temporarySessions, workbenchState]);
+  const activePaneSession = useMemo<ChatSummary | null>(() => {
+    if (!activeTabState) return activeSession;
+    return sessions.find((session) => session.key === activeTabState.activePaneKey)
+      ?? activeSession;
+  }, [activeSession, activeTabState, sessions]);
   const runningChatIdList = useMemo(() => Array.from(runningChatIds), [runningChatIds]);
   const updatedChatIdList = useMemo(() => Array.from(updatedChatIds), [updatedChatIds]);
-  const activeChatId = activeSession?.chatId ?? null;
+  const activeChatId = activePaneSession?.chatId ?? null;
   useEffect(() => {
     activeChatIdRef.current = activeChatId;
     if (!activeChatId) return;
@@ -1242,13 +1298,13 @@ function Shell({
     if (activeChatId && workspaceOverrides[activeChatId]) {
       return workspaceOverrides[activeChatId];
     }
-    if (activeSession?.workspaceScope) {
-      return activeSession.workspaceScope;
+    if (activePaneSession?.workspaceScope) {
+      return activePaneSession.workspaceScope;
     }
     return draftWorkspaceScope ?? workspaces?.default_scope ?? null;
   }, [
     activeChatId,
-    activeSession?.workspaceScope,
+    activePaneSession?.workspaceScope,
     draftWorkspaceScope,
     temporaryChatRequested,
     workspaceOverrides,
@@ -1284,6 +1340,18 @@ function Shell({
     });
   }, [loading, sessions]);
 
+  useEffect(() => {
+    if (loading) return;
+    const validKeys = new Set(sessions.map((session) => session.key));
+    setWorkbenchState((current) => {
+      const reconciled = reconcileWorkbench(current, validKeys);
+      if (!activeKey || temporarySessions[activeKey] || !validKeys.has(activeKey)) {
+        return reconciled;
+      }
+      return ensureWorkbenchTab(reconciled, activeKey);
+    });
+  }, [activeKey, loading, sessions, temporarySessions]);
+
   useEffect(() => {
     if (loading) return;
     const pendingCreatedKey = pendingCreatedSessionKeyRef.current;
@@ -1788,7 +1856,7 @@ function Shell({
       });
       if (activeKey === key && !sidebarState.archived_keys.includes(key)) {
         const archived = new Set([...sidebarState.archived_keys, key]);
-        const next = sessions.find((session) => !archived.has(session.key));
+        const next = topicSessions.find((session) => !archived.has(session.key));
         navigate({
           view: "chat",
           activeKey: next?.key ?? null,
@@ -1796,7 +1864,7 @@ function Shell({
         });
       }
     },
-    [activeKey, navigate, sessions, sidebarState.archived_keys, updateSidebarState],
+    [activeKey, navigate, sidebarState.archived_keys, topicSessions, updateSidebarState],
   );
 
   const onReorderSessions = useCallback(
@@ -1825,6 +1893,47 @@ function Shell({
     setSessionSearchOpen(true);
   }, []);
 
+  const onAddPane = useCallback(async () => {
+    const tabKey = activeKey;
+    if (
+      !tabKey
+      || !activeSession
+      || creatingPane
+      || (activeTabState?.paneKeys.length ?? 0) >= MAX_WORKBENCH_PANES
+      || temporarySessionsRef.current[tabKey]
+    ) return;
+    setMobileSidebarOpen(false);
+    setSessionSearchOpen(false);
+    setCreatingPane(true);
+    try {
+      const scope = activeWorkspaceScope;
+      const chatId = await createChat(scope);
+      const paneKey = `websocket:${chatId}`;
+      setWorkbenchState((current) => addWorkbenchPane(current, tabKey, paneKey));
+      if (scope) {
+        setWorkspaceOverrides((current) => ({
+          ...current,
+          [chatId]: normalizeWorkspaceScope(scope),
+        }));
+      }
+    } catch (error) {
+      console.error("Failed to create pane", error);
+      if (error instanceof Error && error.message.startsWith("workspace_scope_rejected:")) {
+        setWorkspaceError(t("errors.workspaceScopeRejected.body"));
+      }
+    } finally {
+      setCreatingPane(false);
+    }
+  }, [
+    activeKey,
+    activeSession,
+    activeTabState,
+    activeWorkspaceScope,
+    createChat,
+    creatingPane,
+    t,
+  ]);
+
   useEffect(() => {
     const handleKeyDown = (event: globalThis.KeyboardEvent) => {
       if (event.defaultPrevented) return;
@@ -1902,15 +2011,15 @@ function Shell({
     setMobileSidebarOpen(false);
     const nextKey = (() => {
       if (!activeKey) return null;
-      if (sessions.some((session) => session.key === activeKey)) return activeKey;
-      return sessions[0]?.key ?? null;
+      if (topicSessions.some((session) => session.key === activeKey)) return activeKey;
+      return topicSessions[0]?.key ?? null;
     })();
     navigate({
       view: "chat",
       activeKey: nextKey,
       settingsSection: "overview",
     });
-  }, [activeKey, navigate, sessions]);
+  }, [activeKey, navigate, topicSessions]);
 
   const onRestart = useCallback(() => {
     const chatId = activeSession?.chatId ?? client.defaultChatId;
@@ -2017,30 +2126,42 @@ function Shell({
   }, [client, t]);
 
   const onTurnEnd = useDeferredTitleRefresh(
-    temporaryChatActive ? null : activeSession,
+    temporaryChatActive ? null : activePaneSession,
     refresh,
   );
 
   const onConfirmDelete = useCallback(async () => {
     if (!pendingDelete) return;
-    const key = pendingDelete.key;
+    const items = pendingDelete.items;
+    const deletingKeys = new Set(items.map((item) => item.key));
     const hasAutomations = (pendingDelete.automations?.length ?? 0) > 0;
-    const deletingActive = activeKey === key;
-    const currentIndex = sessions.findIndex((s) => s.key === key);
+    const deletingActive = activeKey !== null && deletingKeys.has(activeKey);
+    const currentIndex = topicSessions.findIndex((s) => s.key === activeKey);
     const fallbackKey = deletingActive
-      ? (sessions[currentIndex + 1]?.key ?? sessions[currentIndex - 1]?.key ?? null)
+      ? (
+          topicSessions.slice(currentIndex + 1).find((session) => (
+            !deletingKeys.has(session.key)
+          ))?.key
+          ?? topicSessions.slice(0, Math.max(0, currentIndex)).reverse().find((session) => (
+            !deletingKeys.has(session.key)
+          ))?.key
+          ?? null
+        )
       : activeKey;
     try {
-      const result = await deleteChat(
-        key,
-        hasAutomations ? { deleteAutomations: true } : undefined,
-      );
-      if (result.blocked_by_automations) {
-        setPendingDelete({
-          ...pendingDelete,
-          automations: result.automations ?? [],
-        });
-        return;
+      for (let index = 0; index < items.length; index += 1) {
+        const item = items[index];
+        const result = await deleteChat(
+          item.key,
+          hasAutomations ? { deleteAutomations: true } : undefined,
+        );
+        if (result.blocked_by_automations) {
+          setPendingDelete({
+            items: items.slice(index),
+            automations: result.automations ?? [],
+          });
+          return;
+        }
       }
       setPendingDelete(null);
       if (deletingActive) {
@@ -2053,18 +2174,24 @@ function Shell({
     } catch (e) {
       console.error("Failed to delete session", e);
     }
-  }, [pendingDelete, deleteChat, activeKey, navigate, sessions]);
+  }, [pendingDelete, deleteChat, activeKey, navigate, topicSessions]);
 
-  const onRequestDelete = useCallback(async (key: string, label: string) => {
-    let automations: SessionAutomationJob[] = [];
-    try {
-      automations = await getSessionAutomations(key);
-    } catch {
-      // Delete remains protected by the backend block; prefetch only improves the first prompt.
-    }
-    setPendingDelete({ key, label, automations });
+  const onRequestDeleteMany = useCallback(async (items: SidebarDeleteItem[]) => {
+    const uniqueItems = Array.from(new Map(items.map((item) => [item.key, item])).values());
+    if (uniqueItems.length === 0) return;
+    const automationResults = await Promise.allSettled(
+      uniqueItems.map((item) => getSessionAutomations(item.key)),
+    );
+    const automations = automationResults.flatMap((result) => (
+      result.status === "fulfilled" ? result.value : []
+    ));
+    setPendingDelete({ items: uniqueItems, automations });
   }, [getSessionAutomations]);
 
+  const onRequestDelete = useCallback((key: string, label: string) => {
+    void onRequestDeleteMany([{ key, label }]);
+  }, [onRequestDeleteMany]);
+
   const visiblePairingRequests = useMemo(
     () => {
       const now = Date.now();
@@ -2109,13 +2236,117 @@ function Shell({
     });
   }, []);
 
+  const titleForSession = useCallback((session: ChatSummary) => (
+    sidebarState.title_overrides[session.key]
+    || session.title
+    || deriveTitle(session.preview, t("chat.newChat"))
+  ), [sidebarState.title_overrides, t]);
+
   const headerTitle = temporaryChatActive
     ? deriveTemporaryChatTitle(activeSession?.preview, t("temporaryChat.title"))
     : activeSession
-    ? sidebarState.title_overrides[activeSession.key] ||
-      activeSession.title ||
-      deriveTitle(activeSession.preview, t("chat.newChat"))
+    ? titleForSession(activeSession)
     : t("app.brand");
+  const workbenchPaneSessions = useMemo(() => {
+    if (!activeTabState) return [];
+    const byKey = new Map(sessions.map((session) => [session.key, session]));
+    return activeTabState.paneKeys
+      .map((key) => byKey.get(key))
+      .filter((session): session is ChatSummary => session !== undefined);
+  }, [activeTabState, sessions]);
+  const paneChromeEnabled = Boolean(
+    activeKey && activeSession && !temporaryChatActive && activeTabState,
+  );
+  const renderedWorkbenchPanes = useMemo(() => {
+    if (paneChromeEnabled && activeKey) {
+      return workbenchPaneSessions.map((session) => ({
+        key: session.key,
+        reactKey: session.key === activeKey ? "tab-root" : `pane:${session.key}`,
+        title: titleForSession(session),
+      }));
+    }
+    return [{
+      key: activeKey ?? "new-topic",
+      reactKey: "tab-root",
+      title: headerTitle,
+    }];
+  }, [activeKey, headerTitle, paneChromeEnabled, titleForSession, workbenchPaneSessions]);
+  const renderedActivePaneKey = paneChromeEnabled && activeTabState
+    ? activeTabState.activePaneKey
+    : renderedWorkbenchPanes[0].key;
+  const renderedWorkbenchLayout = paneChromeEnabled && activeTabState
+    ? activeTabState.layout
+    : "columns";
+  const sidebarPaneGroups = useMemo(() => {
+    const sessionsByKey = new Map(sessions.map((session) => [session.key, session]));
+    return Object.fromEntries(topicSessions.map((topic) => {
+      const tab = workbenchTab(workbenchState, topic.key);
+      const panes = tab.paneKeys
+        .map((key) => sessionsByKey.get(key))
+        .filter((session): session is ChatSummary => session !== undefined)
+        .map((session) => ({
+          key: session.key,
+          chatId: session.chatId,
+          title: titleForSession(session),
+        }));
+      return [topic.key, {
+        topicKey: topic.key,
+        activePaneKey: tab.activePaneKey,
+        panes,
+      }];
+    }));
+  }, [sessions, titleForSession, topicSessions, workbenchState]);
+  const attachableTabKeys = useMemo(() => (
+    topicSessions
+      .filter((session) => workbenchTab(workbenchState, session.key).paneKeys.length === 1)
+      .map((session) => session.key)
+  ), [topicSessions, workbenchState]);
+  const paneAcceptingTabKeys = useMemo(() => (
+    topicSessions
+      .filter((session) => (
+        workbenchTab(workbenchState, session.key).paneKeys.length < MAX_WORKBENCH_PANES
+      ))
+      .map((session) => session.key)
+  ), [topicSessions, workbenchState]);
+  const activePaneLimitReached = Boolean(
+    activeTabState && activeTabState.paneKeys.length >= MAX_WORKBENCH_PANES,
+  );
+
+  const onActivateWorkbenchPane = useCallback((paneKey: string) => {
+    if (!activeKey) return;
+    setWorkbenchState((current) => focusWorkbenchPane(current, activeKey, paneKey));
+  }, [activeKey]);
+
+  const onSelectSidebarPane = useCallback((tabKey: string, paneKey: string) => {
+    setWorkbenchState((current) => focusWorkbenchPane(current, tabKey, paneKey));
+    if (activeKey !== tabKey) {
+      navigate({
+        view: "chat",
+        activeKey: tabKey,
+        settingsSection: "overview",
+      });
+    }
+  }, [activeKey, navigate]);
+
+  const onDetachWorkbenchPane = useCallback((tabKey: string, paneKey: string) => {
+    setWorkbenchState((current) => detachWorkbenchPane(current, tabKey, paneKey));
+  }, []);
+
+  const onPromoteWorkbenchPane = useCallback((tabKey: string, paneKey: string) => {
+    setWorkbenchState((current) => promoteWorkbenchPane(current, tabKey, paneKey));
+  }, []);
+
+  const onAttachWorkbenchPane = useCallback((paneKey: string, tabKey: string) => {
+    if (paneKey === tabKey) return;
+    setWorkbenchState((current) => attachWorkbenchPane(current, tabKey, paneKey));
+    if (activeKey === paneKey) {
+      navigate({
+        view: "chat",
+        activeKey: tabKey,
+        settingsSection: "overview",
+      });
+    }
+  }, [activeKey, navigate]);
 
   useEffect(() => {
     if (view === "settings") {
@@ -2148,7 +2379,7 @@ function Shell({
   }, [activeSession, headerTitle, i18n.resolvedLanguage, t, view]);
 
   const sidebarProps = {
-    sessions,
+    sessions: topicSessions,
     temporarySessions: temporarySessionList,
     activeKey: view === "chat" ? activeKey : null,
     loading,
@@ -2157,9 +2388,17 @@ function Shell({
     onSelect: onSelectChat,
     onCloseTemporaryChat,
     onRequestDelete,
+    onRequestDeleteMany,
     onTogglePin,
     onRequestRename,
     onToggleArchive,
+    paneGroups: sidebarPaneGroups,
+    onSelectPane: onSelectSidebarPane,
+    onDetachPane: onDetachWorkbenchPane,
+    onPromotePane: onPromoteWorkbenchPane,
+    attachableTabKeys,
+    paneAcceptingTabKeys,
+    onAttachPane: onAttachWorkbenchPane,
     onReorderSessions,
     onToggleGroup,
     onRequestRenameProject,
@@ -2182,7 +2421,9 @@ function Shell({
     updatedChatIds: updatedChatIdList,
     viewState: sidebarState.view,
     showArchived: sidebarState.view.show_archived,
-    archivedCount: sidebarState.archived_keys.length,
+    archivedCount: topicSessions.filter(
+      (session) => sidebarState.archived_keys.includes(session.key),
+    ).length,
     defaultWorkspacePath: workspaces?.default_scope.project_path ?? null,
   };
   const hostSidebarCollapsed = showHostChrome && !hostSidebarOpen;
@@ -2318,7 +2559,7 @@ function Shell({
               <SessionSearchDialog
                 open
                 onOpenChange={setSessionSearchOpen}
-                sessions={sessions}
+                sessions={topicSessions}
                 activeKey={activeKey}
                 loading={loading}
                 titleOverrides={sidebarState.title_overrides}
@@ -2337,35 +2578,114 @@ function Shell({
                 view !== "chat" && "hidden",
               )}
             >
-              <ThreadShell
-                session={activeSession}
-                sessions={sessions}
-                title={headerTitle}
-                temporary={temporaryChatRequested}
-                temporaryChatIds={temporaryChatIds}
-                temporaryChatEnabled={temporaryChatEnabled}
-                onTemporaryChatEnabledChange={
-                  !activeKey ? onTemporaryChatEnabledChange : undefined
-                }
-                onToggleSidebar={toggleSidebar}
-                onNewChat={onNewChat}
-                onCreateChat={temporaryChatEnabled ? onCreateTemporaryChat : onCreateChat}
-                onForkChat={temporaryChatActive ? undefined : onForkChat}
-                onTurnEnd={onTurnEnd}
-                theme={theme}
-                onToggleTheme={toggle}
-                hideSidebarToggleForHostChrome
-                hostChromeTitleInset={hostSidebarCollapsed}
-                hideHeader={false}
-                workspaceScope={activeWorkspaceScope}
-                workspaceDefaultScope={workspaces?.default_scope ?? null}
-                workspaceControls={workspaces?.controls ?? null}
-                workspaceScopeDisabled={activeChatRunning}
-                workspaceError={workspaceError}
-                onWorkspaceScopeChange={applyWorkspaceScope}
-                settingsSnapshot={settingsSnapshot}
-                onOpenModelSettings={onOpenModelSettings}
-                skills={skills}
+              <PaneWorkbench
+                panes={renderedWorkbenchPanes}
+                activePaneKey={renderedActivePaneKey}
+                layout={renderedWorkbenchLayout}
+                chrome={paneChromeEnabled}
+                addPaneDisabled={creatingPane || activePaneLimitReached}
+                onActivatePane={onActivateWorkbenchPane}
+                onAddPane={onAddPane}
+                onLayoutChange={(layout) => {
+                  if (!activeKey) return;
+                  setWorkbenchState((current) => (
+                    setWorkbenchLayout(current, activeKey, layout)
+                  ));
+                }}
+                renderPane={(pane, context) => {
+                  if (!paneChromeEnabled) {
+                    return (
+                      <ThreadShell
+                        session={activeSession}
+                        sessions={sessions}
+                        title={headerTitle}
+                        temporary={temporaryChatRequested}
+                        temporaryChatIds={temporaryChatIds}
+                        temporaryChatEnabled={temporaryChatEnabled}
+                        onTemporaryChatEnabledChange={
+                          !activeKey ? onTemporaryChatEnabledChange : undefined
+                        }
+                        onToggleSidebar={toggleSidebar}
+                        onNewChat={onNewChat}
+                        onCreateChat={
+                          temporaryChatEnabled ? onCreateTemporaryChat : onCreateChat
+                        }
+                        onForkChat={temporaryChatActive ? undefined : onForkChat}
+                        onTurnEnd={onTurnEnd}
+                        theme={theme}
+                        onToggleTheme={toggle}
+                        hideSidebarToggleForHostChrome
+                        hostChromeTitleInset={hostSidebarCollapsed}
+                        hideHeader={false}
+                        workspaceScope={activeWorkspaceScope}
+                        workspaceDefaultScope={workspaces?.default_scope ?? null}
+                        workspaceControls={workspaces?.controls ?? null}
+                        workspaceScopeDisabled={activeChatRunning}
+                        workspaceError={workspaceError}
+                        onWorkspaceScopeChange={applyWorkspaceScope}
+                        settingsSnapshot={settingsSnapshot}
+                        onOpenModelSettings={onOpenModelSettings}
+                        skills={skills}
+                      />
+                    );
+                  }
+
+                  const paneSession = workbenchPaneSessions.find(
+                    (session) => session.key === pane.key,
+                  );
+                  if (!paneSession) return null;
+                  const paneScope = workspaceOverrides[paneSession.chatId]
+                    ?? paneSession.workspaceScope
+                    ?? workspaces?.default_scope
+                    ?? null;
+                  const paneRunning = runningChatIds.has(paneSession.chatId);
+                  return (
+                    <ThreadShell
+                      session={paneSession}
+                      sessions={sessions}
+                      title={pane.title}
+                      onToggleSidebar={toggleSidebar}
+                      onNewChat={onNewChat}
+                      onCreateChat={onCreateChat}
+                      onForkChat={onForkChat}
+                      onTurnEnd={context.active ? onTurnEnd : () => void refresh()}
+                      theme={theme}
+                      onToggleTheme={toggle}
+                      hideSidebarToggle={!context.active}
+                      hideSidebarToggleForHostChrome={context.active}
+                      hostChromeTitleInset={hostSidebarCollapsed}
+                      hideThemeButton={!context.active}
+                      hideHeaderTitle
+                      headerActions={context.headerActions}
+                      headerPortalTarget={context.headerPortalTarget}
+                      headerActive={context.active}
+                      composerPortalTarget={context.composerPortalTarget}
+                      composerActive={context.active}
+                      composerInputAriaLabel={t("workbench.composerAria", {
+                        defaultValue: "Message {{title}}",
+                        title: pane.title,
+                      })}
+                      workspaceScope={paneScope}
+                      workspaceDefaultScope={workspaces?.default_scope ?? null}
+                      workspaceControls={workspaces?.controls ?? null}
+                      workspaceScopeDisabled={paneRunning}
+                      workspaceError={context.active ? workspaceError : null}
+                      onWorkspaceScopeChange={(scope) => {
+                        if (paneRunning) return;
+                        const next = normalizeWorkspaceScope(scope);
+                        setWorkspaceError(null);
+                        setWorkspaceOverrides((current) => ({
+                          ...current,
+                          [paneSession.chatId]: next,
+                        }));
+                        client.setWorkspaceScope(paneSession.chatId, next);
+                      }}
+                      settingsSnapshot={settingsSnapshot}
+                      onOpenModelSettings={onOpenModelSettings}
+                      skills={skills}
+                    />
+                  );
+                }}
               />
             </div>
             {view !== "chat" && (
@@ -2398,7 +2718,8 @@ function Shell({
           <Suspense fallback={null}>
             <DeleteConfirm
               open
-              title={pendingDelete.label}
+              title={pendingDelete.items[0]?.label ?? ""}
+              count={pendingDelete.items.length}
               automations={pendingDelete.automations}
               onCancel={() => setPendingDelete(null)}
               onConfirm={onConfirmDelete}
diff --git a/webui/src/components/ChatList.tsx b/webui/src/components/ChatList.tsx
index e4abc146f..c79b4a8fb 100644
--- a/webui/src/components/ChatList.tsx
+++ b/webui/src/components/ChatList.tsx
@@ -1,22 +1,33 @@
 import {
   memo,
+  useCallback,
   useEffect,
+  useLayoutEffect,
   useMemo,
   useRef,
   useState,
+  type DragEvent,
   type RefObject,
 } from "react";
 import {
   Archive,
   ArchiveRestore,
+  BringToFront,
+  CornerDownRight,
   Folder,
+  ListChecks,
   MessageCircleDashed,
   MoreHorizontal,
+  PanelsTopLeft,
   Pencil,
   Pin,
   PinOff,
   Plus,
+  Square,
+  SquareCheckBig,
+  SquareMinus,
   Trash2,
+  Unplug,
   X,
 } from "lucide-react";
 import { useTranslation } from "react-i18next";
@@ -25,6 +36,9 @@ import {
   DropdownMenu,
   DropdownMenuContent,
   DropdownMenuItem,
+  DropdownMenuSub,
+  DropdownMenuSubContent,
+  DropdownMenuSubTrigger,
   DropdownMenuTrigger,
 } from "@/components/ui/dropdown-menu";
 import {
@@ -43,7 +57,13 @@ import {
   visibleSessionsForGroup,
   type ChatGroupLabels,
 } from "@/lib/chat-groups";
-import { clearDraggedSession, writeDraggedSession } from "@/lib/session-drag";
+import {
+  clearDraggedSession,
+  writeDraggedPane,
+  writeDraggedSession,
+  type DraggedPane,
+} from "@/lib/session-drag";
+import { MAX_WORKBENCH_PANES } from "@/components/workbench/workbench-model";
 import { deriveTemporaryChatTitle } from "@/lib/temporary-chat";
 import { cn } from "@/lib/utils";
 import type { ChatSummary, SidebarDensity, SidebarSortMode } from "@/lib/types";
@@ -52,6 +72,21 @@ const INITIAL_VISIBLE_SESSIONS = 160;
 const VISIBLE_SESSIONS_INCREMENT = 160;
 const ACTION_MENU_CONTENT_CLASS = "w-[8.5rem] min-w-[8.5rem]";
 
+export interface SidebarPaneGroup {
+  topicKey: string;
+  activePaneKey: string;
+  panes: Array<{
+    key: string;
+    chatId: string;
+    title: string;
+  }>;
+}
+
+export interface SidebarDeleteItem {
+  key: string;
+  label: string;
+}
+
 interface ChatListProps {
   sessions: ChatSummary[];
   temporarySessions?: ChatSummary[];
@@ -59,9 +94,17 @@ interface ChatListProps {
   onSelect: (key: string) => void;
   onCloseTemporaryChat?: (key: string) => void;
   onRequestDelete: (key: string, label: string) => void;
+  onRequestDeleteMany?: (items: SidebarDeleteItem[]) => void;
   onTogglePin: (key: string) => void;
   onRequestRename: (key: string, label: string) => void;
   onToggleArchive: (key: string) => void;
+  paneGroups?: Record<string, SidebarPaneGroup>;
+  onSelectPane?: (tabKey: string, paneKey: string) => void;
+  onDetachPane?: (tabKey: string, paneKey: string) => void;
+  onPromotePane?: (tabKey: string, paneKey: string) => void;
+  attachableTabKeys?: string[];
+  paneAcceptingTabKeys?: string[];
+  onAttachPane?: (paneKey: string, tabKey: string) => void;
   onReorderSessions?: (keys: string[]) => void;
   onToggleGroup?: (groupId: string) => void;
   onRequestRenameProject?: (projectKey: string, label: string) => void;
@@ -92,9 +135,17 @@ export const ChatList = memo(function ChatList({
   onSelect,
   onCloseTemporaryChat,
   onRequestDelete,
+  onRequestDeleteMany,
   onTogglePin,
   onRequestRename,
   onToggleArchive,
+  paneGroups = {},
+  onSelectPane,
+  onDetachPane,
+  onPromotePane,
+  attachableTabKeys = [],
+  paneAcceptingTabKeys = [],
+  onAttachPane,
   onReorderSessions,
   onToggleGroup,
   onRequestRenameProject,
@@ -124,7 +175,49 @@ export const ChatList = memo(function ChatList({
     edge: "before" | "after";
     key: string;
   } | null>(null);
+  const [draggedSessionHeight, setDraggedSessionHeight] = useState(0);
+  const [draggedPane, setDraggedPane] = useState<DraggedPane | null>(null);
+  const [tabAttachTargetKey, setTabAttachTargetKey] = useState<string | null>(null);
+  const tabAttachTargetRef = useRef<string | null>(null);
+  const tabRowRefs = useRef(new Map<string, HTMLLIElement>());
+  const pendingTabRectsRef = useRef<Map<string, DOMRect> | null>(null);
+  const tabLayoutAnimationsRef = useRef(new Map<string, Animation>());
+  const [deleteSelectionMode, setDeleteSelectionMode] = useState(false);
+  const [selectedDeleteKeys, setSelectedDeleteKeys] = useState<Set<string>>(
+    () => new Set(),
+  );
   const activeRowRef = useRef<HTMLDivElement>(null);
+  const selectedPaneGroup = activeKey ? paneGroups[activeKey] : undefined;
+  const selectedRowKey = selectedPaneGroup
+    ? selectedPaneGroup.activePaneKey
+    : activeKey;
+  const attachableTabs = useMemo(() => new Set(attachableTabKeys), [attachableTabKeys]);
+  const paneAcceptingTabs = useMemo(
+    () => new Set(paneAcceptingTabKeys),
+    [paneAcceptingTabKeys],
+  );
+  const deleteItemsByKey = useMemo(() => {
+    const items = new Map<string, SidebarDeleteItem>();
+    for (const group of Object.values(paneGroups)) {
+      for (const pane of group.panes) {
+        items.set(pane.key, { key: pane.key, label: pane.title });
+      }
+    }
+    for (const session of sessions) {
+      if (items.has(session.key)) continue;
+      items.set(session.key, {
+        key: session.key,
+        label: displayTitle(session, titleOverrides, t("chat.newChat")),
+      });
+    }
+    return items;
+  }, [paneGroups, sessions, t, titleOverrides]);
+  const paneMoveTargets = useMemo(() => sessions
+    .filter((session) => paneAcceptingTabs.has(session.key))
+    .map((session) => ({
+      key: session.key,
+      title: deleteItemsByKey.get(session.key)?.label ?? session.title ?? session.chatId,
+    })), [deleteItemsByKey, paneAcceptingTabs, sessions]);
   const labels = useMemo<ChatGroupLabels>(() => ({
     pinned: t("chat.groups.pinned"),
     all: t("chat.groups.all"),
@@ -196,6 +289,80 @@ export const ChatList = memo(function ChatList({
     setVisibleLimit(INITIAL_VISIBLE_SESSIONS);
   }, [showArchived, sort]);
 
+  useEffect(() => {
+    if (!deleteSelectionMode) return;
+    const onKeyDown = (event: KeyboardEvent) => {
+      if (event.key !== "Escape") return;
+      setDeleteSelectionMode(false);
+      setSelectedDeleteKeys(new Set());
+    };
+    window.addEventListener("keydown", onKeyDown);
+    return () => window.removeEventListener("keydown", onKeyDown);
+  }, [deleteSelectionMode]);
+
+  const measureTabRows = useCallback(() => {
+    const rects = new Map<string, DOMRect>();
+    for (const [key, row] of tabRowRefs.current) {
+      rects.set(key, row.getBoundingClientRect());
+    }
+    return rects;
+  }, []);
+
+  const updateTabAttachTarget = useCallback((next: string | null) => {
+    if (tabAttachTargetRef.current === next) return;
+    for (const animation of tabLayoutAnimationsRef.current.values()) animation.cancel();
+    tabLayoutAnimationsRef.current.clear();
+    pendingTabRectsRef.current = measureTabRows();
+    tabAttachTargetRef.current = next;
+    setTabAttachTargetKey(next);
+  }, [measureTabRows]);
+
+  const resetDragState = useCallback(() => {
+    clearDraggedSession();
+    setDraggedSessionKey(null);
+    setDraggedPane(null);
+    setSessionDropTarget(null);
+    updateTabAttachTarget(null);
+    setDraggedSessionHeight(0);
+  }, [updateTabAttachTarget]);
+
+  useLayoutEffect(() => {
+    const previousRects = pendingTabRectsRef.current;
+    if (!previousRects) return;
+    pendingTabRectsRef.current = null;
+    const nextRects = measureTabRows();
+    const reduceMotion = typeof window.matchMedia === "function"
+      && window.matchMedia("(prefers-reduced-motion: reduce)").matches;
+    if (reduceMotion) return;
+    for (const [key, nextRect] of nextRects) {
+      const previousRect = previousRects.get(key);
+      const row = tabRowRefs.current.get(key);
+      if (!previousRect || !row || typeof row.animate !== "function") continue;
+      const deltaY = previousRect.top - nextRect.top;
+      if (Math.abs(deltaY) < 0.5) continue;
+      const animation = row.animate(
+        [
+          { transform: `translateY(${deltaY}px)` },
+          { transform: "translateY(0)" },
+        ],
+        {
+          duration: 180,
+          easing: "cubic-bezier(0.2, 0, 0, 1)",
+        },
+      );
+      tabLayoutAnimationsRef.current.set(key, animation);
+      animation.addEventListener("finish", () => {
+        if (tabLayoutAnimationsRef.current.get(key) === animation) {
+          tabLayoutAnimationsRef.current.delete(key);
+        }
+      }, { once: true });
+    }
+  }, [measureTabRows, tabAttachTargetKey]);
+
+  useEffect(() => () => {
+    for (const animation of tabLayoutAnimationsRef.current.values()) animation.cancel();
+  }, []);
+
   if (loading && sessions.length === 0 && temporarySessions.length === 0) {
     return (
       <div className="px-3 py-6 text-[12px] text-muted-foreground">
@@ -218,10 +385,48 @@ export const ChatList = memo(function ChatList({
   const firstProjectGroupIndex = limitedGroups.findIndex((group) => group.kind === "project");
 
   const canReorderSession = (targetKey: string) => (
-    !!draggedSessionKey
+    !deleteSelectionMode
+    && !!draggedSessionKey
     && draggedSessionKey !== targetKey
     && sessionLanes.get(draggedSessionKey) === sessionLanes.get(targetKey)
   );
+  const beginDeleteSelection = (keys: string[]) => {
+    setDeleteSelectionMode(true);
+    setSelectedDeleteKeys(new Set(keys.filter((key) => deleteItemsByKey.has(key))));
+  };
+  const toggleDeleteSelection = (keys: string[]) => {
+    setSelectedDeleteKeys((current) => {
+      const next = new Set(current);
+      const validKeys = keys.filter((key) => deleteItemsByKey.has(key));
+      const remove = validKeys.length > 0 && validKeys.every((key) => next.has(key));
+      for (const key of validKeys) {
+        if (remove) next.delete(key);
+        else next.add(key);
+      }
+      return next;
+    });
+  };
+  const closeDeleteSelection = () => {
+    setDeleteSelectionMode(false);
+    setSelectedDeleteKeys(new Set());
+  };
+  const requestDeleteItems = (items: SidebarDeleteItem[]) => {
+    if (items.length === 0) return;
+    if (onRequestDeleteMany) onRequestDeleteMany(items);
+    else if (items.length === 1) onRequestDelete(items[0].key, items[0].label);
+  };
+  const requestDeleteKeys = (keys: string[]) => {
+    requestDeleteItems(keys
+      .map((key) => deleteItemsByKey.get(key))
+      .filter((item): item is SidebarDeleteItem => item !== undefined));
+  };
+  const confirmDeleteSelection = () => {
+    requestDeleteKeys(Array.from(selectedDeleteKeys));
+    closeDeleteSelection();
+  };
+  const draggedItemTitle = draggedPane
+    ? deleteItemsByKey.get(draggedPane.paneKey)?.label
+    : draggedSessionKey ? deleteItemsByKey.get(draggedSessionKey)?.label : undefined;
   const reorderSession = (targetKey: string, edge: "before" | "after") => {
     if (!draggedSessionKey || !canReorderSession(targetKey) || !onReorderSessions) return;
     const keys = groups.flatMap((group) => group.sessions.map((session) => session.key));
@@ -240,7 +445,7 @@ export const ChatList = memo(function ChatList({
     <div className="h-full min-h-0 min-w-0 overflow-x-hidden overflow-y-auto overscroll-contain scrollbar-thin scrollbar-track-transparent">
       <SidebarSelectionHighlight
         targetRef={activeRowRef}
-        activeId={activeKey}
+        activeId={draggedSessionKey || draggedPane ? null : selectedRowKey}
         scope="sessions"
         data-chat-list-content
         className="relative min-w-0 space-y-3 px-2 py-1.5"
@@ -265,6 +470,12 @@ export const ChatList = memo(function ChatList({
           );
           const hiddenInGroup = Math.max(0, group.sessions.length - visibleSessions.length);
           const canToggleFold = group.sessions.length > COLLAPSED_CHATS_VISIBLE_COUNT;
+          const reorderOffsets = sessionReorderOffsets(
+            visibleSessions.map((session) => session.key),
+            draggedSessionKey,
+            sessionDropTarget,
+            draggedSessionHeight,
+          );
 
           return (
             <section key={group.id} aria-label={group.label} className="relative z-[1]">
@@ -298,12 +509,28 @@ export const ChatList = memo(function ChatList({
               {group.kind === "project" && collapsedGroups[group.id] ? null : (
                 <ul className="space-y-0.5">
                   {visibleSessions.map((s) => {
-                    const active = s.key === activeKey;
+                    const topicActive = s.key === activeKey;
+                    const paneGroup = paneGroups[s.key];
                     const fallbackTitle = t("chat.fallbackTitle", {
                       id: s.chatId.slice(0, 6),
                     });
                     const generatedTitle = s.title?.trim() || "";
                     const title = displayTitle(s, titleOverrides, t("chat.newChat"));
+                    const resolvedPaneGroup = paneGroup ?? {
+                      topicKey: s.key,
+                      activePaneKey: s.key,
+                      panes: [{ key: s.key, chatId: s.chatId, title }],
+                    };
+                    const active = topicActive && resolvedPaneGroup.activePaneKey === s.key;
+                    const paneCount = resolvedPaneGroup.panes.length;
+                    const tabDeleteKeys = resolvedPaneGroup.panes.map((pane) => pane.key);
+                    const tabSelected = tabDeleteKeys.every((key) => (
+                      selectedDeleteKeys.has(key)
+                    ));
+                    const tabPartiallySelected = !tabSelected && tabDeleteKeys.some((key) => (
+                      selectedDeleteKeys.has(key)
+                    ));
+                    const isAttachTarget = tabAttachTargetKey === s.key;
                     const tooltipTitle =
                       titleOverrides[s.key]?.trim() ||
                       generatedTitle ||
@@ -318,24 +545,83 @@ export const ChatList = memo(function ChatList({
                     const projectMode = group.kind === "project";
                     const activityState = running.has(s.chatId)
                       ? "running"
-                      : updated.has(s.chatId) && !active
+                      : updated.has(s.chatId) && !topicActive
                         ? "updated"
                         : null;
                     return (
                       <li
                         key={s.key}
-                        className="relative min-w-0"
+                        ref={(element) => {
+                          if (element) tabRowRefs.current.set(s.key, element);
+                          else tabRowRefs.current.delete(s.key);
+                        }}
+                        data-session-dragging={draggedSessionKey === s.key ? "true" : undefined}
+                        data-session-displaced={reorderOffsets.has(s.key) ? "true" : undefined}
+                        data-tab-attach-target={tabAttachTargetKey === s.key ? "true" : undefined}
+                        className={cn(
+                          "relative min-w-0 rounded-xl transition-[transform,opacity,background-color,box-shadow] duration-200 [transition-timing-function:cubic-bezier(0.2,0,0,1)] motion-reduce:transition-none",
+                          draggedSessionKey === s.key && "opacity-0",
+                          isAttachTarget
+                            && "bg-sidebar-accent/35 shadow-[inset_0_0_0_1px_hsl(var(--sidebar-border))]",
+                        )}
+                        style={{
+                          transform: reorderOffsets.has(s.key)
+                            ? `translateY(${reorderOffsets.get(s.key)}px)`
+                            : undefined,
+                        }}
                         onDragOver={(event) => {
+                          const rect = event.currentTarget.getBoundingClientRect();
+                          const relativeY = rect.height > 0
+                            ? (event.clientY - rect.top) / rect.height
+                            : 0.5;
+                          const paneCanAttach = Boolean(
+                            !deleteSelectionMode
+                            && draggedPane
+                            && draggedPane.sourceTabKey !== s.key
+                            && paneAcceptingTabs.has(s.key)
+                            && onAttachPane,
+                          );
+                          const tabCanAttach = Boolean(
+                            !deleteSelectionMode
+                            && draggedSessionKey
+                            && draggedSessionKey !== s.key
+                            && attachableTabs.has(draggedSessionKey)
+                            && paneAcceptingTabs.has(s.key)
+                            && relativeY >= 0.25
+                            && relativeY <= 0.75
+                            && onAttachPane,
+                          );
+                          if (paneCanAttach || tabCanAttach) {
+                            event.preventDefault();
+                            event.dataTransfer.dropEffect = "move";
+                            setSessionDropTarget(null);
+                            updateTabAttachTarget(s.key);
+                            return;
+                          }
+                          updateTabAttachTarget(null);
                           if (!canReorderSession(s.key)) return;
                           event.preventDefault();
                           event.dataTransfer.dropEffect = "move";
-                          const rect = event.currentTarget.getBoundingClientRect();
-                          setSessionDropTarget({
+                          const nextTarget = {
                             key: s.key,
                             edge: event.clientY < rect.top + rect.height / 2 ? "before" : "after",
-                          });
+                          } as const;
+                          setSessionDropTarget((current) => (
+                            current?.key === nextTarget.key && current.edge === nextTarget.edge
+                              ? current
+                              : nextTarget
+                          ));
                         }}
                         onDrop={(event) => {
+                          if (tabAttachTargetKey === s.key && onAttachPane) {
+                            const paneKey = draggedPane?.paneKey ?? draggedSessionKey;
+                            if (paneKey) {
+                              event.preventDefault();
+                              onAttachPane(paneKey, s.key);
+                            }
+                            resetDragState();
+                            return;
+                          }
                           if (!canReorderSession(s.key)) return;
                           event.preventDefault();
                           const rect = event.currentTarget.getBoundingClientRect();
@@ -343,23 +629,13 @@ export const ChatList = memo(function ChatList({
                             ? "before"
                             : "after";
                           reorderSession(s.key, edge);
-                          setDraggedSessionKey(null);
-                          setSessionDropTarget(null);
+                          resetDragState();
                         }}
                       >
-                        {sessionDropTarget?.key === s.key ? (
-                          <span
-                            aria-hidden
-                            data-session-drop-edge={sessionDropTarget.edge}
-                            className={cn(
-                              "pointer-events-none absolute inset-x-2 z-20 h-0.5 rounded-full bg-primary",
-                              sessionDropTarget.edge === "before" ? "-top-px" : "-bottom-px",
-                            )}
-                          />
-                        ) : null}
                         <div
                           ref={active ? activeRowRef : undefined}
                           data-chat-row={s.key}
+                          data-sidebar-tab={s.key}
                           className={cn(
                             "group flex min-w-0 max-w-full items-center gap-2 rounded-xl px-2 text-[13px]",
                             SIDEBAR_SELECTION_ITEM_CLASS,
@@ -367,36 +643,75 @@ export const ChatList = memo(function ChatList({
                             active
                               ? "text-sidebar-accent-foreground"
                               : "text-sidebar-foreground/82 hover:bg-sidebar-foreground/[0.035] hover:text-sidebar-foreground dark:hover:bg-white/[0.05]",
+                            isAttachTarget
+                              && "bg-sidebar-accent/65 text-sidebar-accent-foreground",
+                            deleteSelectionMode && (tabSelected || tabPartiallySelected)
+                              && "bg-sidebar-accent/55 text-sidebar-accent-foreground",
                           )}
                         >
                           <button
                             type="button"
-                            onClick={() => onSelect(s.key)}
-                            draggable
+                            onClick={() => {
+                              if (deleteSelectionMode) {
+                                toggleDeleteSelection(tabDeleteKeys);
+                                return;
+                              }
+                              if (topicActive && paneGroup && onSelectPane) {
+                                onSelectPane(s.key, s.key);
+                                return;
+                              }
+                              onSelect(s.key);
+                            }}
+                            draggable={!deleteSelectionMode}
                             onDragStart={(event) => {
                               setDraggedSessionKey(s.key);
+                              setDraggedPane(null);
                               setSessionDropTarget(null);
+                              updateTabAttachTarget(null);
+                              setDraggedSessionHeight(
+                                event.currentTarget.closest("li")?.getBoundingClientRect().height
+                                  ?? event.currentTarget.getBoundingClientRect().height,
+                              );
                               writeDraggedSession(event.dataTransfer, s.key);
                             }}
-                            onDragEnd={() => {
-                              clearDraggedSession();
-                              setDraggedSessionKey(null);
-                              setSessionDropTarget(null);
-                            }}
+                            onDragEnd={resetDragState}
                             aria-current={active ? "page" : undefined}
+                            aria-pressed={deleteSelectionMode ? tabSelected : undefined}
                             title={tooltipTitle}
                             className={cn(
-                              "min-w-0 flex-1 overflow-hidden text-left",
-                              "cursor-grab active:cursor-grabbing",
+                              "flex min-w-0 flex-1 items-center gap-2 overflow-hidden text-left",
+                              deleteSelectionMode
+                                ? "cursor-default"
+                                : "cursor-grab active:cursor-grabbing",
                               compact ? "py-1" : "py-1.5",
                               projectMode && "pl-7",
                             )}
                           >
+                            {deleteSelectionMode ? (
+                              <SelectionIndicator
+                                checked={tabSelected}
+                                partial={tabPartiallySelected}
+                              />
+                            ) : paneCount > 1 || isAttachTarget ? (
+                              <PanelsTopLeft
+                                aria-hidden
+                                className="h-3.5 w-3.5 shrink-0 text-muted-foreground/60"
+                              />
+                            ) : null}
+                            <span className="min-w-0 flex-1 overflow-hidden">
                             {projectMode ? (
                               <span className="flex w-full min-w-0 items-baseline gap-2">
                                 <span className="min-w-0 flex-1 truncate font-medium leading-5">
                                   {title}
                                 </span>
+                                {paneCount > 1 ? (
+                                  <span
+                                    aria-hidden
+                                    className="shrink-0 text-[10.5px] tabular-nums text-muted-foreground/55"
+                                  >
+                                    {paneCount}/{MAX_WORKBENCH_PANES}
+                                  </span>
+                                ) : null}
                                 {isPinned ? <PinnedChatIndicator label={labels.pinned} /> : null}
                                 {timestamp ? (
                                   <span className="shrink-0 text-[11.5px] font-medium text-muted-foreground/58">
@@ -409,6 +724,14 @@ export const ChatList = memo(function ChatList({
                                 <span className="min-w-0 flex-1 truncate font-medium leading-5">
                                   {title}
                                 </span>
+                                {paneCount > 1 ? (
+                                  <span
+                                    aria-hidden
+                                    className="shrink-0 text-[10.5px] tabular-nums text-muted-foreground/55"
+                                  >
+                                    {paneCount}/{MAX_WORKBENCH_PANES}
+                                  </span>
+                                ) : null}
                                 {isPinned ? <PinnedChatIndicator label={labels.pinned} /> : null}
                               </span>
                             )}
@@ -422,15 +745,16 @@ export const ChatList = memo(function ChatList({
                                 {timestamp}
                               </span>
                             ) : null}
+                            </span>
                           </button>
                           <SessionActivityIndicator state={activityState} />
-                          <DropdownMenu modal={false}>
+                          {!deleteSelectionMode ? <DropdownMenu modal={false}>
                             <DropdownMenuTrigger
                               className={cn(
                                 "inline-flex h-6 w-6 shrink-0 items-center justify-center rounded-md text-muted-foreground/75 opacity-40 transition-opacity",
                                 "hover:bg-sidebar-accent hover:text-sidebar-foreground group-hover:opacity-100",
                                 "focus-visible:opacity-100",
-                                active && "opacity-100",
+                                topicActive && "opacity-100",
                               )}
                               aria-label={t("chat.actions", { title })}
                             >
@@ -442,6 +766,17 @@ export const ChatList = memo(function ChatList({
                               portalContainer={actionMenuPortalContainer}
                               onCloseAutoFocus={(event) => event.preventDefault()}
                             >
+                              {paneGroup
+                              && paneGroup.panes.findIndex((pane) => pane.key === s.key) > 0
+                              && onPromotePane ? (
+                                <DropdownMenuItem onSelect={() => onPromotePane(s.key, s.key)}>
+                                  <BringToFront className="h-4 w-4 shrink-0" />
+                                  {t("workbench.promotePane", {
+                                    defaultValue: "Make {{title}} the primary pane",
+                                    title,
+                                  })}
+                                </DropdownMenuItem>
+                              ) : null}
                               <DropdownMenuItem
                                 onSelect={() => onTogglePin(s.key)}
                               >
@@ -468,18 +803,71 @@ export const ChatList = memo(function ChatList({
                                 )}
                                 {isArchived ? t("chat.unarchive") : t("chat.archive")}
                               </DropdownMenuItem>
+                              {attachableTabs.has(s.key) && onAttachPane ? (
+                                <MoveToTabSubmenu
+                                  targets={paneMoveTargets.filter((target) => target.key !== s.key)}
+                                  onMove={(targetKey) => onAttachPane(s.key, targetKey)}
+                                />
+                              ) : null}
+                              <DropdownMenuItem
+                                onSelect={() => beginDeleteSelection(tabDeleteKeys)}
+                              >
+                                <ListChecks className="h-4 w-4 shrink-0" />
+                                {t("chat.select", { defaultValue: "Select" })}
+                              </DropdownMenuItem>
                               <DropdownMenuItem
                                 tone="destructive"
                                 onSelect={() => {
-                                  window.setTimeout(() => onRequestDelete(s.key, title), 0);
+                                  window.setTimeout(() => requestDeleteKeys(tabDeleteKeys), 0);
                                 }}
                               >
                                 <Trash2 className="h-4 w-4 shrink-0" />
                                 {t("chat.delete")}
                               </DropdownMenuItem>
                             </DropdownMenuContent>
-                          </DropdownMenu>
+                          </DropdownMenu> : null}
                         </div>
+                        {paneCount > 1 || isAttachTarget ? (
+                          <ActivePaneRows
+                            group={resolvedPaneGroup}
+                            tabTitle={title}
+                            tabActive={topicActive}
+                            activeRowRef={activeRowRef}
+                            running={running}
+                            updated={updated}
+                            onSelectPane={onSelectPane}
+                            onRequestDelete={onRequestDelete}
+                            onRequestRename={onRequestRename}
+                            onDetachPane={onDetachPane}
+                            onPromotePane={onPromotePane}
+                            moveTargets={paneMoveTargets.filter((target) => (
+                              target.key !== resolvedPaneGroup.topicKey
+                            ))}
+                            onAttachPane={onAttachPane}
+                            deleteSelectionMode={deleteSelectionMode}
+                            selectedDeleteKeys={selectedDeleteKeys}
+                            onToggleDeleteSelection={toggleDeleteSelection}
+                            onBeginDeleteSelection={beginDeleteSelection}
+                            dropPreview={isAttachTarget && draggedItemTitle ? {
+                              paneTitle: draggedItemTitle,
+                              targetTitle: title,
+                            } : null}
+                            draggedPaneKey={draggedPane?.paneKey ?? null}
+                            onPaneDragStart={(event, pane) => {
+                              setDraggedPane(pane);
+                              setDraggedSessionKey(null);
+                              setSessionDropTarget(null);
+                              updateTabAttachTarget(null);
+                              setDraggedSessionHeight(
+                                event.currentTarget.closest("li")?.getBoundingClientRect().height
+                                  ?? event.currentTarget.getBoundingClientRect().height,
+                              );
+                              writeDraggedPane(event.dataTransfer, pane);
+                            }}
+                            onPaneDragEnd={resetDragState}
+                            actionMenuPortalContainer={actionMenuPortalContainer}
+                          />
+                        ) : null}
                       </li>
                     );
                   })}
@@ -510,11 +898,329 @@ export const ChatList = memo(function ChatList({
             </button>
           </div>
         ) : null}
+        {deleteSelectionMode ? (
+          <div
+            data-testid="delete-selection-bar"
+            className="sticky bottom-2 z-30 mx-1 mt-3 flex min-h-11 items-center gap-2 rounded-2xl border border-sidebar-border/80 bg-popover/95 p-1.5 pl-2 shadow-[0_10px_30px_rgba(15,23,42,0.14)] backdrop-blur-xl"
+          >
+            <button
+              type="button"
+              onClick={closeDeleteSelection}
+              aria-label={t("chat.cancelSelection", {
+                defaultValue: "Cancel selection",
+              })}
+              className="inline-flex h-8 w-8 shrink-0 items-center justify-center rounded-full text-muted-foreground transition-colors hover:bg-accent/60 hover:text-foreground"
+            >
+              <X className="h-4 w-4" aria-hidden />
+            </button>
+            <span className="min-w-0 flex-1 truncate px-1 text-[12.5px] font-medium text-foreground/85">
+              {t("chat.selectedCount", {
+                defaultValue: "{{count}} selected",
+                count: selectedDeleteKeys.size,
+              })}
+            </span>
+            <button
+              type="button"
+              disabled={selectedDeleteKeys.size === 0}
+              onClick={confirmDeleteSelection}
+              className="inline-flex h-8 shrink-0 items-center gap-1.5 rounded-full bg-destructive px-3 text-[12px] font-semibold text-destructive-foreground transition-colors hover:bg-destructive/90 disabled:pointer-events-none disabled:opacity-40"
+            >
+              <Trash2 className="h-3.5 w-3.5" aria-hidden />
+              {t("chat.deleteSelected", { defaultValue: "Delete" })}
+            </button>
+          </div>
+        ) : null}
       </SidebarSelectionHighlight>
     </div>
   );
 });
 
+function sessionReorderOffsets(
+  keys: string[],
+  draggedKey: string | null,
+  target: { edge: "before" | "after"; key: string } | null,
+  draggedHeight: number,
+): Map<string, number> {
+  const offsets = new Map<string, number>();
+  if (!draggedKey || !target || draggedHeight <= 0) return offsets;
+  const sourceIndex = keys.indexOf(draggedKey);
+  if (sourceIndex < 0 || target.key === draggedKey) return offsets;
+  const remaining = keys.filter((key) => key !== draggedKey);
+  const targetIndex = remaining.indexOf(target.key);
+  if (targetIndex < 0) return offsets;
+  const finalIndex = targetIndex + (target.edge === "after" ? 1 : 0);
+
+  if (sourceIndex < finalIndex) {
+    for (let index = sourceIndex + 1; index <= finalIndex; index += 1) {
+      offsets.set(keys[index], -draggedHeight);
+    }
+  } else if (sourceIndex > finalIndex) {
+    for (let index = finalIndex; index < sourceIndex; index += 1) {
+      offsets.set(keys[index], draggedHeight);
+    }
+  }
+  return offsets;
+}
+
+function ActivePaneRows({
+  group,
+  tabTitle,
+  tabActive,
+  activeRowRef,
+  running,
+  updated,
+  onSelectPane,
+  onRequestDelete,
+  onRequestRename,
+  onDetachPane,
+  onPromotePane,
+  moveTargets,
+  onAttachPane,
+  deleteSelectionMode,
+  selectedDeleteKeys,
+  onToggleDeleteSelection,
+  onBeginDeleteSelection,
+  dropPreview,
+  draggedPaneKey,
+  onPaneDragStart,
+  onPaneDragEnd,
+  actionMenuPortalContainer,
+}: {
+  group: SidebarPaneGroup;
+  tabTitle: string;
+  tabActive: boolean;
+  activeRowRef: RefObject<HTMLDivElement>;
+  running: ReadonlySet<string>;
+  updated: ReadonlySet<string>;
+  onSelectPane?: (tabKey: string, paneKey: string) => void;
+  onRequestDelete: (key: string, label: string) => void;
+  onRequestRename: (key: string, label: string) => void;
+  onDetachPane?: (tabKey: string, paneKey: string) => void;
+  onPromotePane?: (tabKey: string, paneKey: string) => void;
+  moveTargets: Array<{ key: string; title: string }>;
+  onAttachPane?: (paneKey: string, tabKey: string) => void;
+  deleteSelectionMode: boolean;
+  selectedDeleteKeys: ReadonlySet<string>;
+  onToggleDeleteSelection: (keys: string[]) => void;
+  onBeginDeleteSelection: (keys: string[]) => void;
+  dropPreview: { paneTitle: string; targetTitle: string } | null;
+  draggedPaneKey: string | null;
+  onPaneDragStart: (event: DragEvent<HTMLButtonElement>, pane: DraggedPane) => void;
+  onPaneDragEnd: () => void;
+  actionMenuPortalContainer?: HTMLElement | null;
+}) {
+  const { t } = useTranslation();
+  const childPanes = group.panes.filter((pane) => pane.key !== group.topicKey);
+
+  return (
+    <ul
+      aria-label={t("workbench.panesInTab", {
+        defaultValue: "Panes in {{title}}",
+        title: tabTitle,
+      })}
+      className={cn(
+        "relative ml-5 mr-1 mt-0.5 space-y-0.5 rounded-bl-lg border-l border-sidebar-border/60 py-0.5 pl-2 pr-0.5",
+        dropPreview && "pb-1",
+      )}
+    >
+      {childPanes.map((pane) => {
+        const index = group.panes.findIndex((candidate) => candidate.key === pane.key);
+        const active = tabActive && pane.key === group.activePaneKey;
+        const activityState = running.has(pane.chatId)
+          ? "running"
+          : updated.has(pane.chatId) && !active
+            ? "updated"
+            : null;
+        const paneActionsLabel = t("workbench.paneActions", {
+          defaultValue: "{{title}} pane actions",
+          title: pane.title,
+        });
+        const selected = selectedDeleteKeys.has(pane.key);
+
+        return (
+          <li
+            key={pane.key}
+            data-pane-dragging={draggedPaneKey === pane.key ? "true" : undefined}
+            className="relative min-w-0 before:absolute before:-left-2 before:top-1/2 before:h-px before:w-2 before:bg-sidebar-border/45"
+          >
+            <div
+              ref={active ? activeRowRef : undefined}
+              data-chat-row={pane.key}
+              data-sidebar-pane={pane.key}
+              className={cn(
+                "group/pane flex min-h-7 min-w-0 items-center gap-1 rounded-lg px-2 text-[12.5px]",
+                SIDEBAR_SELECTION_ITEM_CLASS,
+                active
+                  ? "text-sidebar-accent-foreground"
+                  : "text-sidebar-foreground/72 hover:bg-sidebar-foreground/[0.035] hover:text-sidebar-foreground dark:hover:bg-white/[0.05]",
+                deleteSelectionMode && selected
+                  && "bg-sidebar-accent/55 text-sidebar-accent-foreground",
+              )}
+            >
+              <button
+                type="button"
+                onClick={() => {
+                  if (deleteSelectionMode) {
+                    onToggleDeleteSelection([pane.key]);
+                    return;
+                  }
+                  onSelectPane?.(group.topicKey, pane.key);
+                }}
+                draggable={!deleteSelectionMode}
+                onDragStart={(event) => onPaneDragStart(event, {
+                  paneKey: pane.key,
+                  sourceTabKey: group.topicKey,
+                })}
+                onDragEnd={onPaneDragEnd}
+                aria-current={active ? "true" : undefined}
+                aria-pressed={deleteSelectionMode ? selected : undefined}
+                title={pane.title}
+                className={cn(
+                  "flex min-w-0 flex-1 items-center gap-2 py-1 text-left font-medium leading-5",
+                  deleteSelectionMode ? "cursor-default" : "cursor-grab active:cursor-grabbing",
+                )}
+              >
+                {deleteSelectionMode ? (
+                  <SelectionIndicator checked={selected} partial={false} />
+                ) : null}
+                <span className="min-w-0 flex-1 truncate">{pane.title}</span>
+              </button>
+              <SessionActivityIndicator state={activityState} />
+              {!deleteSelectionMode ? <DropdownMenu modal={false}>
+                <DropdownMenuTrigger
+                  className={cn(
+                    "inline-flex h-6 w-6 shrink-0 items-center justify-center rounded-md text-muted-foreground/70 opacity-0 transition-opacity",
+                    "hover:bg-sidebar-accent hover:text-sidebar-foreground group-hover/pane:opacity-100",
+                    "focus-visible:opacity-100",
+                    active && "opacity-100",
+                  )}
+                  aria-label={paneActionsLabel}
+                >
+                  <MoreHorizontal className="h-3.5 w-3.5" />
+                </DropdownMenuTrigger>
+                <DropdownMenuContent
+                  align="end"
+                  className={ACTION_MENU_CONTENT_CLASS}
+                  portalContainer={actionMenuPortalContainer}
+                  onCloseAutoFocus={(event) => event.preventDefault()}
+                >
+                  {index > 0 && onPromotePane ? (
+                    <DropdownMenuItem onSelect={() => onPromotePane(group.topicKey, pane.key)}>
+                      <BringToFront className="h-4 w-4 shrink-0" />
+                      {t("workbench.promotePane", {
+                        defaultValue: "Make {{title}} the primary pane",
+                        title: pane.title,
+                      })}
+                    </DropdownMenuItem>
+                  ) : null}
+                  <DropdownMenuItem
+                    onSelect={() => onRequestRename(pane.key, pane.title)}
+                  >
+                    <Pencil className="h-4 w-4 shrink-0" />
+                    {t("chat.rename")}
+                  </DropdownMenuItem>
+                  {onDetachPane ? (
+                    <DropdownMenuItem onSelect={() => onDetachPane(group.topicKey, pane.key)}>
+                      <Unplug className="h-4 w-4 shrink-0" />
+                      {t("workbench.detachPane", {
+                        defaultValue: "Move {{title}} to its own topic",
+                        title: pane.title,
+                      })}
+                    </DropdownMenuItem>
+                  ) : null}
+                  {onAttachPane ? (
+                    <MoveToTabSubmenu
+                      targets={moveTargets}
+                      onMove={(targetKey) => onAttachPane(pane.key, targetKey)}
+                    />
+                  ) : null}
+                  <DropdownMenuItem
+                    onSelect={() => onBeginDeleteSelection([pane.key])}
+                  >
+                    <ListChecks className="h-4 w-4 shrink-0" />
+                    {t("chat.select", { defaultValue: "Select" })}
+                  </DropdownMenuItem>
+                  <DropdownMenuItem
+                    tone="destructive"
+                    onSelect={() => {
+                      window.setTimeout(() => onRequestDelete(pane.key, pane.title), 0);
+                    }}
+                  >
+                    <Trash2 className="h-4 w-4 shrink-0" />
+                    {t("chat.delete")}
+                  </DropdownMenuItem>
+                </DropdownMenuContent>
+              </DropdownMenu> : null}
+            </div>
+          </li>
+        );
+      })}
+      {dropPreview ? (
+        <li
+          data-pane-drop-preview
+          role="status"
+          aria-label={t("workbench.dropPane", {
+            defaultValue: "Move {{pane}} into {{tab}}",
+            pane: dropPreview.paneTitle,
+            tab: dropPreview.targetTitle,
+          })}
+          className="relative min-w-0 before:absolute before:-left-2 before:top-1/2 before:h-px before:w-2 before:bg-primary/45"
+        >
+          <div className="flex min-h-7 items-center gap-2 rounded-lg border border-primary/30 bg-primary/[0.07] px-2 text-[12.5px] font-medium text-foreground/80 shadow-[inset_0_0_0_1px_hsl(var(--background)/0.5)] motion-safe:animate-in motion-safe:fade-in-0 motion-safe:slide-in-from-top-1 motion-safe:duration-150">
+            <CornerDownRight className="h-3.5 w-3.5 shrink-0 text-primary/75" aria-hidden />
+            <span className="min-w-0 flex-1 truncate">{dropPreview.paneTitle}</span>
+          </div>
+        </li>
+      ) : null}
+    </ul>
+  );
+}
+
+function SelectionIndicator({
+  checked,
+  partial,
+}: {
+  checked: boolean;
+  partial: boolean;
+}) {
+  const Icon = partial ? SquareMinus : checked ? SquareCheckBig : Square;
+  return (
+    <Icon
+      aria-hidden
+      className={cn(
+        "h-4 w-4 shrink-0",
+        checked || partial ? "text-primary" : "text-muted-foreground/55",
+      )}
+    />
+  );
+}
+
+function MoveToTabSubmenu({
+  targets,
+  onMove,
+}: {
+  targets: Array<{ key: string; title: string }>;
+  onMove: (targetKey: string) => void;
+}) {
+  const { t } = useTranslation();
+  if (targets.length === 0) return null;
+  return (
+    <DropdownMenuSub>
+      <DropdownMenuSubTrigger>
+        <PanelsTopLeft className="h-4 w-4 shrink-0" aria-hidden />
+        {t("workbench.moveToTab", { defaultValue: "Move to tab" })}
+      </DropdownMenuSubTrigger>
+      <DropdownMenuSubContent>
+        {targets.map((target) => (
+          <DropdownMenuItem key={target.key} onSelect={() => onMove(target.key)}>
+            <span className="max-w-56 truncate">{target.title}</span>
+          </DropdownMenuItem>
+        ))}
+      </DropdownMenuSubContent>
+    </DropdownMenuSub>
+  );
+}
+
 function TemporaryChatSection({
   sessions,
   activeKey,
diff --git a/webui/src/components/DeleteConfirm.tsx b/webui/src/components/DeleteConfirm.tsx
index f7ccb5c2a..d1717de4e 100644
--- a/webui/src/components/DeleteConfirm.tsx
+++ b/webui/src/components/DeleteConfirm.tsx
@@ -18,6 +18,7 @@ import type { SessionAutomationJob } from "@/lib/types";
 interface DeleteConfirmProps {
   open: boolean;
   title: string;
+  count?: number;
   automations?: SessionAutomationJob[];
   onCancel: () => void;
   onConfirm: () => void;
@@ -26,6 +27,7 @@ interface DeleteConfirmProps {
 export function DeleteConfirm({
   open,
   title,
+  count = 1,
   automations = [],
   onCancel,
   onConfirm,
@@ -33,6 +35,7 @@ export function DeleteConfirm({
   const { t } = useTranslation();
   const locale = currentLocale();
   const hasAutomations = automations.length > 0;
+  const multiple = count > 1;
   const visibleAutomations = automations.slice(0, 4);
   const hiddenCount = Math.max(0, automations.length - visibleAutomations.length);
   return (
@@ -47,12 +50,25 @@ export function DeleteConfirm({
             </div>
           </div>
           <AlertDialogTitle className="text-center text-[20px] font-semibold leading-tight tracking-[-0.02em] text-foreground">
-            {t("deleteConfirm.title", { title })}
+            {multiple
+              ? t("deleteConfirm.titleMany", {
+                  defaultValue: "Delete {{count}} topics and panes?",
+                  count,
+                })
+              : t("deleteConfirm.title", { title })}
           </AlertDialogTitle>
           <AlertDialogDescription className="mt-3 max-w-[17rem] text-center text-[14px] leading-6 text-muted-foreground">
             {hasAutomations
-              ? t("deleteConfirm.automationsDescription")
-              : t("deleteConfirm.description")}
+              ? multiple
+                ? t("deleteConfirm.automationsDescriptionMany", {
+                    defaultValue: "Linked automations will also be deleted.",
+                  })
+                : t("deleteConfirm.automationsDescription")
+              : multiple
+                ? t("deleteConfirm.descriptionMany", {
+                    defaultValue: "This action cannot be undone.",
+                  })
+                : t("deleteConfirm.description")}
           </AlertDialogDescription>
           {hasAutomations ? (
             <div className="mt-4 max-h-40 w-full overflow-y-auto rounded-2xl bg-muted/55 px-3 py-2 text-left">
diff --git a/webui/src/components/Sidebar.tsx b/webui/src/components/Sidebar.tsx
index f41ebcdf6..02fd3817e 100644
--- a/webui/src/components/Sidebar.tsx
+++ b/webui/src/components/Sidebar.tsx
@@ -16,7 +16,11 @@ import {
 } from "lucide-react";
 import { useTranslation } from "react-i18next";
 
-import { ChatList } from "@/components/ChatList";
+import {
+  ChatList,
+  type SidebarDeleteItem,
+  type SidebarPaneGroup,
+} from "@/components/ChatList";
 import { ConnectionBadge } from "@/components/ConnectionBadge";
 import {
   SIDEBAR_SELECTION_ACTION_ITEM_CLASS,
@@ -39,9 +43,17 @@ interface SidebarProps {
   onSelect: (key: string) => void;
   onCloseTemporaryChat?: (key: string) => void;
   onRequestDelete: (key: string, label: string) => void;
+  onRequestDeleteMany?: (items: SidebarDeleteItem[]) => void;
   onTogglePin: (key: string) => void;
   onRequestRename: (key: string, label: string) => void;
   onToggleArchive: (key: string) => void;
+  paneGroups?: Record<string, SidebarPaneGroup>;
+  onSelectPane?: (tabKey: string, paneKey: string) => void;
+  onDetachPane?: (tabKey: string, paneKey: string) => void;
+  onPromotePane?: (tabKey: string, paneKey: string) => void;
+  attachableTabKeys?: string[];
+  paneAcceptingTabKeys?: string[];
+  onAttachPane?: (paneKey: string, tabKey: string) => void;
   onReorderSessions: (keys: string[]) => void;
   onToggleGroup: (groupId: string) => void;
   onRequestRenameProject: (projectKey: string, label: string) => void;
@@ -230,9 +242,17 @@ export function Sidebar(props: SidebarProps) {
             onSelect={props.onSelect}
             onCloseTemporaryChat={props.onCloseTemporaryChat}
             onRequestDelete={props.onRequestDelete}
+            onRequestDeleteMany={props.onRequestDeleteMany}
             onTogglePin={props.onTogglePin}
             onRequestRename={props.onRequestRename}
             onToggleArchive={props.onToggleArchive}
+            paneGroups={props.paneGroups}
+            onSelectPane={props.onSelectPane}
+            onDetachPane={props.onDetachPane}
+            onPromotePane={props.onPromotePane}
+            attachableTabKeys={props.attachableTabKeys}
+            paneAcceptingTabKeys={props.paneAcceptingTabKeys}
+            onAttachPane={props.onAttachPane}
             onReorderSessions={props.onReorderSessions}
             onToggleGroup={props.onToggleGroup}
             onRequestRenameProject={props.onRequestRenameProject}
diff --git a/webui/src/components/thread/ThreadComposer.tsx b/webui/src/components/thread/ThreadComposer.tsx
index 50861de6d..439e9c385 100644
--- a/webui/src/components/thread/ThreadComposer.tsx
+++ b/webui/src/components/thread/ThreadComposer.tsx
@@ -186,6 +186,7 @@ interface ThreadComposerProps {
   ) => boolean | void | Promise<boolean | void>;
   disabled?: boolean;
   placeholder?: string;
+  inputAriaLabel?: string;
   isStreaming?: boolean;
   modelLabel?: string | null;
   modelDetail?: string | null;
@@ -940,6 +941,7 @@ export function ThreadComposer({
   onSend,
   disabled,
   placeholder,
+  inputAriaLabel,
   isStreaming = false,
   modelLabel = null,
   modelDetail = null,
@@ -2400,7 +2402,7 @@ export function ThreadComposer({
             rows={1}
             placeholder={sessionDragPreview ? "" : resolvedPlaceholder}
             disabled={interactionDisabled}
-            aria-label={t("thread.composer.inputAria")}
+            aria-label={inputAriaLabel ?? t("thread.composer.inputAria")}
             className={cn(
               inputTextClasses,
               "relative z-10 caret-foreground placeholder:text-muted-foreground/70",
diff --git a/webui/src/components/thread/ThreadHeader.tsx b/webui/src/components/thread/ThreadHeader.tsx
index ef43e96bb..f669f0f33 100644
--- a/webui/src/components/thread/ThreadHeader.tsx
+++ b/webui/src/components/thread/ThreadHeader.tsx
@@ -17,8 +17,11 @@ interface ThreadHeaderProps {
   theme: "light" | "dark";
   onToggleTheme: () => void;
   hideSidebarToggleForHostChrome?: boolean;
+  hideSidebarToggle?: boolean;
   hostChromeTitleInset?: boolean;
   hideThemeButton?: boolean;
+  hideTitle?: boolean;
+  actions?: ReactNode;
   minimal?: boolean;
   promptNavigatorAction?: ReactNode;
   sessionInfoAction?: ReactNode;
@@ -33,8 +36,11 @@ export function ThreadHeader({
   theme,
   onToggleTheme,
   hideSidebarToggleForHostChrome = false,
+  hideSidebarToggle = false,
   hostChromeTitleInset = false,
   hideThemeButton = false,
+  hideTitle = false,
+  actions,
   minimal = false,
   promptNavigatorAction,
   sessionInfoAction,
@@ -54,19 +60,21 @@ export function ThreadHeader({
       )}
     >
       <div className="relative flex min-w-0 items-center gap-2">
-        <Button
-          variant="ghost"
-          size="icon"
-          aria-label={t("thread.header.toggleSidebar")}
-          onClick={onToggleSidebar}
-          className={cn(
-            "h-7 w-7 rounded-md text-muted-foreground hover:bg-accent/35 hover:text-foreground",
-            hideSidebarToggleForHostChrome && "lg:hidden",
-          )}
-        >
-          <Menu className="h-3.5 w-3.5" />
-        </Button>
-        {!minimal ? (
+        {!hideSidebarToggle ? (
+          <Button
+            variant="ghost"
+            size="icon"
+            aria-label={t("thread.header.toggleSidebar")}
+            onClick={onToggleSidebar}
+            className={cn(
+              "h-7 w-7 rounded-md text-muted-foreground hover:bg-accent/35 hover:text-foreground",
+              hideSidebarToggleForHostChrome && "lg:hidden",
+            )}
+          >
+            <Menu className="h-3.5 w-3.5" />
+          </Button>
+        ) : null}
+        {!minimal && !hideTitle ? (
           <div className="flex min-w-0 items-center rounded-md px-1.5 py-1 text-[12px] font-medium text-muted-foreground">
             <span className="max-w-[min(60vw,32rem)] truncate">{title}</span>
           </div>
@@ -76,6 +84,7 @@ export function ThreadHeader({
       <div className="ml-auto flex shrink-0 items-center gap-1">
         {sessionInfoAction}
         {promptNavigatorAction}
+        {actions}
         {onTemporaryChatEnabledChange ? (
           <TooltipProvider delayDuration={700} skipDelayDuration={0}>
             <Tooltip>
diff --git a/webui/src/components/thread/ThreadShell.tsx b/webui/src/components/thread/ThreadShell.tsx
index f53463d1f..164e73387 100644
--- a/webui/src/components/thread/ThreadShell.tsx
+++ b/webui/src/components/thread/ThreadShell.tsx
@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useLayoutEffect, useMemo, useRef, useState } from "react";
-import type { PointerEvent as ReactPointerEvent } from "react";
+import type { PointerEvent as ReactPointerEvent, ReactNode } from "react";
+import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 
 import { FilePreviewAvailabilityProvider } from "@/components/FilePreviewAvailabilityContext";
@@ -311,9 +312,17 @@ interface ThreadShellProps {
   theme?: "light" | "dark";
   onToggleTheme?: () => void;
   hideSidebarToggleForHostChrome?: boolean;
+  hideSidebarToggle?: boolean;
   hostChromeTitleInset?: boolean;
   hideThemeButton?: boolean;
+  hideHeaderTitle?: boolean;
   hideHeader?: boolean;
+  headerActions?: ReactNode;
+  headerPortalTarget?: HTMLElement | null;
+  headerActive?: boolean;
+  composerPortalTarget?: HTMLElement | null;
+  composerActive?: boolean;
+  composerInputAriaLabel?: string;
   workspaceScope?: WorkspaceScopePayload | null;
   workspaceDefaultScope?: WorkspaceScopePayload | null;
   workspaceControls?: WorkspacesPayload["controls"] | null;
@@ -598,9 +607,17 @@ export function ThreadShell({
   theme = "light",
   onToggleTheme = () => {},
   hideSidebarToggleForHostChrome = false,
+  hideSidebarToggle = false,
   hostChromeTitleInset = false,
   hideThemeButton = false,
+  hideHeaderTitle = false,
   hideHeader = false,
+  headerActions,
+  headerPortalTarget,
+  headerActive = true,
+  composerPortalTarget,
+  composerActive = true,
+  composerInputAriaLabel,
   workspaceScope = null,
   workspaceDefaultScope = null,
   workspaceControls = null,
@@ -1405,6 +1422,7 @@ export function ThreadShell({
         <ThreadComposer
           onSend={handleThreadSend}
           disabled={!chatId}
+          inputAriaLabel={composerInputAriaLabel}
           isStreaming={turnActive}
           placeholder={
             showHeroComposer
@@ -1449,6 +1467,7 @@ export function ThreadShell({
         <ThreadComposer
           onSend={handleWelcomeSend}
           disabled={booting}
+          inputAriaLabel={composerInputAriaLabel}
           isStreaming={turnActive}
           placeholder={
             booting
@@ -1508,28 +1527,33 @@ export function ThreadShell({
     />
   ) : undefined;
 
+  const threadHeader = !hideHeader ? (
+    <ThreadHeader
+      title={title}
+      onToggleSidebar={onToggleSidebar}
+      theme={theme}
+      onToggleTheme={onToggleTheme}
+      hideSidebarToggleForHostChrome={hideSidebarToggleForHostChrome}
+      hideSidebarToggle={hideSidebarToggle}
+      hostChromeTitleInset={hostChromeTitleInset}
+      hideThemeButton={hideThemeButton}
+      hideTitle={hideHeaderTitle}
+      actions={headerActions}
+      minimal={!session && !loading}
+      promptNavigatorAction={promptNavigatorAction}
+      sessionInfoAction={sessionInfoAction}
+      temporaryChatEnabled={temporaryChatEnabled}
+      temporaryChatDisabled={booting || turnActive}
+      onTemporaryChatEnabledChange={
+        showTemporaryChatControl ? onTemporaryChatEnabledChange : undefined
+      }
+    />
+  ) : null;
+
   return (
     <section ref={shellRef} className="relative flex min-h-0 flex-1 overflow-hidden">
       <div className="relative flex min-w-0 flex-1 flex-col overflow-hidden">
-        {!hideHeader ? (
-          <ThreadHeader
-            title={title}
-            onToggleSidebar={onToggleSidebar}
-            theme={theme}
-            onToggleTheme={onToggleTheme}
-            hideSidebarToggleForHostChrome={hideSidebarToggleForHostChrome}
-            hostChromeTitleInset={hostChromeTitleInset}
-            hideThemeButton={hideThemeButton}
-            minimal={!session && !loading}
-            promptNavigatorAction={promptNavigatorAction}
-            sessionInfoAction={sessionInfoAction}
-            temporaryChatEnabled={temporaryChatEnabled}
-            temporaryChatDisabled={booting || turnActive}
-            onTemporaryChatEnabledChange={
-              showTemporaryChatControl ? onTemporaryChatEnabledChange : undefined
-            }
-          />
-        ) : null}
+        {headerPortalTarget === undefined ? threadHeader : null}
         <FilePreviewAvailabilityProvider
           resolve={historyKey ? resolveFilePreviewAvailability : undefined}
         >
@@ -1539,7 +1563,7 @@ export function ThreadShell({
             temporary={temporary}
             isStreaming={turnActive}
             emptyState={emptyState}
-            composer={composer}
+            composer={composerPortalTarget === undefined ? composer : null}
             activeTurnId={viewportTurnId}
             activeTurnStartedHere={activeTurnStartedHere}
             conversationKey={historyKey}
@@ -1559,6 +1583,19 @@ export function ThreadShell({
           />
         </FilePreviewAvailabilityProvider>
       </div>
+      {headerPortalTarget && headerActive
+        ? createPortal(threadHeader, headerPortalTarget)
+        : null}
+      {composerPortalTarget ? createPortal(
+        <div
+          hidden={!composerActive}
+          aria-hidden={!composerActive}
+          data-testid={composerActive ? "active-pane-composer" : undefined}
+        >
+          {composer}
+        </div>,
+        composerPortalTarget,
+      ) : null}
       {filePreviewPath && historyKey ? (
         <FilePreviewPanel
           sessionKey={historyKey}
diff --git a/webui/src/components/thread/ThreadViewport.tsx b/webui/src/components/thread/ThreadViewport.tsx
index abd94f234..919396dec 100644
--- a/webui/src/components/thread/ThreadViewport.tsx
+++ b/webui/src/components/thread/ThreadViewport.tsx
@@ -37,7 +37,7 @@ interface ThreadViewportProps {
   messages: UIMessage[];
   temporary?: boolean;
   isStreaming: boolean;
-  composer: ReactNode;
+  composer?: ReactNode;
   emptyState?: ReactNode;
   scrollToBottomSignal?: number;
   activeTurnId?: string | null;
@@ -61,6 +61,7 @@ interface ThreadViewportProps {
 const NEAR_BOTTOM_PX = 48;
 const NEAR_TOP_PX = 96;
 const DEFAULT_SCROLL_BUTTON_BOTTOM_PX = 192;
+const EXTERNAL_COMPOSER_SCROLL_BUTTON_BOTTOM_PX = 16;
 const SCROLL_BUTTON_COMPOSER_GAP_PX = 16;
 const SOFT_KEYBOARD_MIN_INSET_PX = 80;
 export const INITIAL_HISTORY_WINDOW = 160;
@@ -266,11 +267,14 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
     forkBoundaryMessageCount !== null && forkBoundaryMessageCount > hiddenMessageCount
       ? forkBoundaryMessageCount - hiddenMessageCount
       : null;
+  const hasComposer = composer !== null && composer !== undefined;
   const scrollButtonBottom =
     keyboardInsetBottom
     + (composerDockHeight > 0
       ? composerDockHeight + SCROLL_BUTTON_COMPOSER_GAP_PX
-      : DEFAULT_SCROLL_BUTTON_BOTTOM_PX);
+      : hasComposer
+        ? DEFAULT_SCROLL_BUTTON_BOTTOM_PX
+        : EXTERNAL_COMPOSER_SCROLL_BUTTON_BOTTOM_PX);
   const scrollViewportStyle =
     keyboardInsetBottom > 0 ? { bottom: keyboardInsetBottom } : undefined;
 
@@ -661,7 +665,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
         <div
           ref={contentRef}
           data-testid={!hasMessages ? "thread-welcome-layout" : undefined}
-          data-layout={hasMessages ? "thread" : "hero"}
+          data-layout={hasComposer ? (hasMessages ? "thread" : "hero") : "external"}
           className={cn(
             "thread-layout mx-auto grid min-h-full w-full",
             hasMessages
@@ -699,58 +703,67 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
               <div ref={bottomRef} aria-hidden className="h-px shrink-0" />
             </div>
           ) : (
-            <div className="row-start-1 flex min-h-0 min-w-0 w-full items-center justify-center sm:items-end sm:pb-8">
+            <div
+              className={cn(
+                "row-start-1 flex min-h-0 min-w-0 w-full items-center justify-center",
+                hasComposer && "sm:items-end sm:pb-8",
+              )}
+            >
               {emptyState}
             </div>
           )}
 
-          <div
-            ref={composerDockRef}
-            data-testid="thread-composer-dock"
-            onInputCapture={(event) => {
-              if (event.target instanceof HTMLTextAreaElement) {
-                composerInputScrollTopRef.current = scrollRef.current?.scrollTop ?? null;
-                threadMotionRef.current?.handleComposerInput();
-              }
-            }}
-            onInput={(event) => {
-              if (!(event.target instanceof HTMLTextAreaElement)) return;
-              const previousScrollTop = composerInputScrollTopRef.current;
-              composerInputScrollTopRef.current = null;
-              const scrollEl = scrollRef.current;
-              if (scrollEl && previousScrollTop !== null) {
-                // Textarea autosizing briefly collapses to `height: auto` while
-                // measuring. Chrome can clamp the sibling thread scrollport in
-                // that intermediate layout; restore it before paint, then let
-                // ResizeObserver handle any real final composer height change.
-                scrollEl.scrollTop = previousScrollTop;
-              }
-            }}
-            className={cn(
-              "row-start-2 z-10 w-full",
-              hasMessages ? "relative bg-background" : "relative self-center",
-            )}
-          >
+          {hasComposer ? (
             <div
+              ref={composerDockRef}
+              data-testid="thread-composer-dock"
+              onInputCapture={(event) => {
+                if (event.target instanceof HTMLTextAreaElement) {
+                  composerInputScrollTopRef.current = scrollRef.current?.scrollTop ?? null;
+                  threadMotionRef.current?.handleComposerInput();
+                }
+              }}
+              onInput={(event) => {
+                if (!(event.target instanceof HTMLTextAreaElement)) return;
+                const previousScrollTop = composerInputScrollTopRef.current;
+                composerInputScrollTopRef.current = null;
+                const scrollEl = scrollRef.current;
+                if (scrollEl && previousScrollTop !== null) {
+                  // Textarea autosizing briefly collapses to `height: auto` while
+                  // measuring. Chrome can clamp the sibling thread scrollport in
+                  // that intermediate layout; restore it before paint, then let
+                  // ResizeObserver handle any real final composer height change.
+                  scrollEl.scrollTop = previousScrollTop;
+                }
+              }}
               className={cn(
-                hasMessages
-                  ? "px-3 pb-[calc(0.75rem+env(safe-area-inset-bottom))] sm:px-4"
-                  : "",
+                "row-start-2 z-10 w-full",
+                hasMessages ? "relative bg-background" : "relative self-center",
               )}
             >
               <div
-                data-testid="thread-composer-motion"
-                className="mx-auto w-full max-w-[58rem]"
+                className={cn(
+                  hasMessages
+                    ? "px-3 pb-[calc(0.75rem+env(safe-area-inset-bottom))] sm:px-4"
+                    : "",
+                )}
               >
-                {composer}
+                <div
+                  data-testid="thread-composer-motion"
+                  className="mx-auto w-full max-w-[58rem]"
+                >
+                  {composer}
+                </div>
               </div>
             </div>
-          </div>
+          ) : null}
 
-          <div
-            aria-hidden
-            className="thread-layout-spacer row-start-3 min-h-0 overflow-hidden"
-          />
+          {hasComposer ? (
+            <div
+              aria-hidden
+              className="thread-layout-spacer row-start-3 min-h-0 overflow-hidden"
+            />
+          ) : null}
         </div>
         {!hasMessages ? <div ref={bottomRef} aria-hidden className="h-px" /> : null}
       </div>
diff --git a/webui/src/components/ui/dropdown-menu.tsx b/webui/src/components/ui/dropdown-menu.tsx
index f52b879d2..c6cbec579 100644
--- a/webui/src/components/ui/dropdown-menu.tsx
+++ b/webui/src/components/ui/dropdown-menu.tsx
@@ -1,6 +1,6 @@
 import * as React from "react";
 import * as DropdownMenuPrimitive from "@radix-ui/react-dropdown-menu";
-import { Circle } from "lucide-react";
+import { ChevronRight, Circle } from "lucide-react";
 
 import {
   floatingItemClassName,
@@ -13,6 +13,7 @@ import { cn } from "@/lib/utils";
 const DropdownMenu = DropdownMenuPrimitive.Root;
 const DropdownMenuTrigger = DropdownMenuPrimitive.Trigger;
 const DropdownMenuRadioGroup = DropdownMenuPrimitive.RadioGroup;
+const DropdownMenuSub = DropdownMenuPrimitive.Sub;
 
 const menuItemClassName =
   `${floatingItemClassName} ${floatingItemFocusClassName} cursor-default data-[disabled]:pointer-events-none data-[disabled]:opacity-50`;
@@ -115,6 +116,41 @@ const DropdownMenuSeparator = React.forwardRef<
 ));
 DropdownMenuSeparator.displayName = DropdownMenuPrimitive.Separator.displayName;
 
+const DropdownMenuSubTrigger = React.forwardRef<
+  React.ElementRef<typeof DropdownMenuPrimitive.SubTrigger>,
+  React.ComponentPropsWithoutRef<typeof DropdownMenuPrimitive.SubTrigger> & {
+    inset?: boolean;
+  }
+>(({ className, inset, children, ...props }, ref) => (
+  <DropdownMenuPrimitive.SubTrigger
+    ref={ref}
+    className={cn(menuItemClassName, inset && "pl-8", className)}
+    {...props}
+  >
+    {children}
+    <ChevronRight className="ml-auto h-3.5 w-3.5 text-muted-foreground" aria-hidden />
+  </DropdownMenuPrimitive.SubTrigger>
+));
+DropdownMenuSubTrigger.displayName = DropdownMenuPrimitive.SubTrigger.displayName;
+
+const DropdownMenuSubContent = React.forwardRef<
+  React.ElementRef<typeof DropdownMenuPrimitive.SubContent>,
+  React.ComponentPropsWithoutRef<typeof DropdownMenuPrimitive.SubContent>
+>(({ className, sideOffset = 6, ...props }, ref) => (
+  <DropdownMenuPrimitive.SubContent
+    ref={ref}
+    sideOffset={sideOffset}
+    className={cn(
+      floatingSurfaceClassName,
+      floatingSurfaceMotionClassName,
+      "max-h-[min(var(--radix-dropdown-menu-content-available-height),22rem)] min-w-[11rem] overflow-y-auto",
+      className,
+    )}
+    {...props}
+  />
+));
+DropdownMenuSubContent.displayName = DropdownMenuPrimitive.SubContent.displayName;
+
 export {
   DropdownMenu,
   DropdownMenuContent,
@@ -123,5 +159,8 @@ export {
   DropdownMenuRadioGroup,
   DropdownMenuRadioItem,
   DropdownMenuSeparator,
+  DropdownMenuSub,
+  DropdownMenuSubContent,
+  DropdownMenuSubTrigger,
   DropdownMenuTrigger,
 };
diff --git a/webui/src/components/workbench/PaneWorkbench.tsx b/webui/src/components/workbench/PaneWorkbench.tsx
new file mode 100644
index 000000000..d47734c44
--- /dev/null
+++ b/webui/src/components/workbench/PaneWorkbench.tsx
@@ -0,0 +1,428 @@
+import {
+  Columns2,
+  Grid2X2,
+  PanelLeft,
+  Plus,
+  Rows2,
+  Square,
+  type LucideIcon,
+} from "lucide-react";
+import {
+  type CSSProperties,
+  type FocusEvent,
+  type PointerEvent,
+  type ReactNode,
+  useCallback,
+  useEffect,
+  useLayoutEffect,
+  useMemo,
+  useRef,
+  useState,
+} from "react";
+import { useTranslation } from "react-i18next";
+
+import { Button } from "@/components/ui/button";
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuLabel,
+  DropdownMenuRadioGroup,
+  DropdownMenuRadioItem,
+  DropdownMenuSeparator,
+  DropdownMenuTrigger,
+} from "@/components/ui/dropdown-menu";
+import {
+  Tooltip,
+  TooltipContent,
+  TooltipProvider,
+  TooltipTrigger,
+} from "@/components/ui/tooltip";
+import type { WorkbenchLayout } from "@/components/workbench/workbench-model";
+import { useMediaQuery } from "@/hooks/useMediaQuery";
+import { cn } from "@/lib/utils";
+
+export interface WorkbenchPane {
+  key: string;
+  reactKey?: string;
+  title: string;
+}
+
+interface PaneRenderContext {
+  active: boolean;
+  headerPortalTarget: HTMLElement | null | undefined;
+  composerPortalTarget: HTMLElement | null | undefined;
+  headerActions: ReactNode;
+}
+
+interface PaneWorkbenchProps {
+  panes: WorkbenchPane[];
+  activePaneKey: string;
+  layout: WorkbenchLayout;
+  chrome?: boolean;
+  addPaneDisabled?: boolean;
+  onActivatePane: (key: string) => void;
+  onAddPane: () => void;
+  onLayoutChange: (layout: WorkbenchLayout) => void;
+  renderPane: (pane: WorkbenchPane, context: PaneRenderContext) => ReactNode;
+}
+
+const LAYOUT_MOTION_DURATION_MS = 260;
+const LAYOUT_MOTION_EASING = "cubic-bezier(0.2, 0, 0, 1)";
+
+const LAYOUT_CONTROLS: Array<{
+  icon: LucideIcon;
+  layout: WorkbenchLayout;
+  label: string;
+}> = [
+  { icon: Columns2, layout: "columns", label: "Columns" },
+  { icon: Rows2, layout: "rows", label: "Rows" },
+  { icon: Grid2X2, layout: "grid", label: "Grid" },
+  { icon: PanelLeft, layout: "main-stack", label: "Main and stack" },
+  { icon: Square, layout: "monocle", label: "Monocle" },
+];
+
+function paneGridStyle(layout: WorkbenchLayout, paneCount: number): CSSProperties {
+  const count = Math.max(1, paneCount);
+  switch (layout) {
+    case "columns":
+      return {
+        gridTemplateColumns: `repeat(${count}, minmax(0, 1fr))`,
+        gridTemplateRows: "minmax(0, 1fr)",
+      };
+    case "rows":
+      return {
+        gridTemplateColumns: "minmax(0, 1fr)",
+        gridTemplateRows: `repeat(${count}, minmax(0, 1fr))`,
+      };
+    case "grid": {
+      const columns = Math.ceil(Math.sqrt(count));
+      const rows = Math.ceil(count / columns);
+      return {
+        gridTemplateColumns: `repeat(${columns}, minmax(0, 1fr))`,
+        gridTemplateRows: `repeat(${rows}, minmax(0, 1fr))`,
+      };
+    }
+    case "main-stack":
+      return count === 1
+        ? {
+            gridTemplateColumns: "minmax(0, 1fr)",
+            gridTemplateRows: "minmax(0, 1fr)",
+          }
+        : {
+            gridTemplateColumns: "minmax(0, 1.65fr) minmax(0, 1fr)",
+            gridTemplateRows: `repeat(${count - 1}, minmax(0, 1fr))`,
+          };
+    case "monocle":
+      return {
+        gridTemplateColumns: "minmax(0, 1fr)",
+        gridTemplateRows: "minmax(0, 1fr)",
+      };
+  }
+}
+
+function paneCellStyle(
+  layout: WorkbenchLayout,
+  paneCount: number,
+  index: number,
+): CSSProperties | undefined {
+  if (layout !== "main-stack" || paneCount < 2) return undefined;
+  return index === 0
+    ? { gridColumn: 1, gridRow: `1 / span ${paneCount - 1}` }
+    : { gridColumn: 2, gridRow: index };
+}
+
+function isPaneAction(target: EventTarget | null): boolean {
+  return target instanceof Element
+    && target.closest("[data-workbench-pane-action]") !== null;
+}
+
+function HeaderIconButton({
+  disabled,
+  icon: Icon,
+  label,
+  onClick,
+}: {
+  disabled?: boolean;
+  icon: LucideIcon;
+  label: string;
+  onClick: () => void;
+}) {
+  return (
+    <Tooltip>
+      <TooltipTrigger asChild>
+        <Button
+          type="button"
+          variant="ghost"
+          size="icon"
+          disabled={disabled}
+          aria-label={label}
+          onClick={onClick}
+          className="host-no-drag h-8 w-8 shrink-0 rounded-full text-muted-foreground/85 hover:bg-accent/40 hover:text-foreground"
+        >
+          <Icon className="h-4 w-4" aria-hidden />
+        </Button>
+      </TooltipTrigger>
+      <TooltipContent side="bottom">{label}</TooltipContent>
+    </Tooltip>
+  );
+}
+
+export function PaneWorkbench({
+  panes,
+  activePaneKey,
+  layout,
+  chrome = true,
+  addPaneDisabled = false,
+  onActivatePane,
+  onAddPane,
+  onLayoutChange,
+  renderPane,
+}: PaneWorkbenchProps) {
+  const { t } = useTranslation();
+  const compact = useMediaQuery("(max-width: 767px)");
+  const effectiveLayout = compact ? "monocle" : layout;
+  const [headerPortalTarget, setHeaderPortalTarget] = useState<HTMLElement | null>(null);
+  const [composerPortalTarget, setComposerPortalTarget] = useState<HTMLElement | null>(null);
+  const paneRefs = useRef(new Map<string, HTMLElement>());
+  const lastRectsRef = useRef(new Map<string, DOMRect>());
+  const pendingRectsRef = useRef<Map<string, DOMRect> | null>(null);
+  const animationsRef = useRef(new Map<string, Animation>());
+  const paneOrder = useMemo(() => panes.map((pane) => pane.key).join("\u0000"), [panes]);
+
+  const measurePanes = useCallback(() => {
+    const rects = new Map<string, DOMRect>();
+    for (const [key, element] of paneRefs.current) {
+      if (!element.hidden) rects.set(key, element.getBoundingClientRect());
+    }
+    return rects;
+  }, []);
+
+  const captureLayout = useCallback(() => {
+    pendingRectsRef.current = measurePanes();
+    for (const animation of animationsRef.current.values()) animation.cancel();
+    animationsRef.current.clear();
+  }, [measurePanes]);
+
+  useLayoutEffect(() => {
+    const previousRects = pendingRectsRef.current ?? lastRectsRef.current;
+    pendingRectsRef.current = null;
+    const nextRects = measurePanes();
+    const reduceMotion = typeof window.matchMedia === "function"
+      && window.matchMedia("(prefers-reduced-motion: reduce)").matches;
+
+    if (!reduceMotion) {
+      for (const [key, nextRect] of nextRects) {
+        const previousRect = previousRects.get(key);
+        const element = paneRefs.current.get(key);
+        if (!element) continue;
+        if (!previousRect) {
+          if (previousRects.size === 0 || typeof element.animate !== "function") continue;
+          const animation = element.animate(
+            [
+              { opacity: 0, transform: "translateY(5px) scale(0.995)" },
+              { opacity: 1, transform: "translateY(0) scale(1)" },
+            ],
+            {
+              duration: 180,
+              easing: LAYOUT_MOTION_EASING,
+              fill: "backwards",
+            },
+          );
+          animationsRef.current.set(key, animation);
+          animation.addEventListener("finish", () => {
+            if (animationsRef.current.get(key) === animation) {
+              animationsRef.current.delete(key);
+            }
+          }, { once: true });
+          continue;
+        }
+        if (previousRect.width === 0 || previousRect.height === 0) continue;
+        const deltaX = previousRect.left - nextRect.left;
+        const deltaY = previousRect.top - nextRect.top;
+        const scaleX = previousRect.width / nextRect.width;
+        const scaleY = previousRect.height / nextRect.height;
+        if (
+          Math.abs(deltaX) < 0.5
+          && Math.abs(deltaY) < 0.5
+          && Math.abs(scaleX - 1) < 0.002
+          && Math.abs(scaleY - 1) < 0.002
+        ) {
+          continue;
+        }
+        if (typeof element.animate !== "function") continue;
+        const animation = element.animate(
+          [
+            { transform: `translate(${deltaX}px, ${deltaY}px) scale(${scaleX}, ${scaleY})` },
+            { transform: "translate(0, 0) scale(1, 1)" },
+          ],
+          {
+            duration: LAYOUT_MOTION_DURATION_MS,
+            easing: LAYOUT_MOTION_EASING,
+          },
+        );
+        animationsRef.current.set(key, animation);
+        animation.addEventListener("finish", () => {
+          if (animationsRef.current.get(key) === animation) {
+            animationsRef.current.delete(key);
+          }
+        }, { once: true });
+      }
+    }
+    lastRectsRef.current = nextRects;
+  }, [activePaneKey, effectiveLayout, measurePanes, paneOrder]);
+
+  useEffect(() => () => {
+    for (const animation of animationsRef.current.values()) animation.cancel();
+  }, []);
+
+  const activatePane = useCallback((key: string, target: EventTarget | null) => {
+    if (key === activePaneKey || isPaneAction(target)) return;
+    captureLayout();
+    onActivatePane(key);
+  }, [activePaneKey, captureLayout, onActivatePane]);
+
+  const handlePanePointerDown = useCallback((
+    key: string,
+    event: PointerEvent<HTMLElement>,
+  ) => {
+    activatePane(key, event.target);
+  }, [activatePane]);
+
+  const handlePaneFocus = useCallback((key: string, event: FocusEvent<HTMLElement>) => {
+    activatePane(key, event.target);
+  }, [activatePane]);
+
+  const gridStyle = paneGridStyle(effectiveLayout, panes.length);
+  const currentLayout = LAYOUT_CONTROLS.find((control) => control.layout === layout)
+    ?? LAYOUT_CONTROLS[0];
+  const headerActions = chrome ? (
+    <div
+      data-workbench-pane-action
+      className="host-no-drag flex items-center gap-0.5"
+    >
+      <DropdownMenu>
+        <DropdownMenuTrigger asChild>
+          <Button
+            type="button"
+            variant="ghost"
+            size="icon"
+            aria-label={t("workbench.layout", {
+              defaultValue: "Pane layout",
+            })}
+            className="host-no-drag h-8 w-8 rounded-full text-muted-foreground/85 hover:bg-accent/40 hover:text-foreground"
+          >
+            <currentLayout.icon className="h-4 w-4" aria-hidden />
+          </Button>
+        </DropdownMenuTrigger>
+        <DropdownMenuContent
+          align="end"
+          onCloseAutoFocus={(event) => event.preventDefault()}
+        >
+          <DropdownMenuLabel>
+            {t("workbench.layout", { defaultValue: "Pane layout" })}
+          </DropdownMenuLabel>
+          <DropdownMenuSeparator />
+          <DropdownMenuRadioGroup
+            value={layout}
+            onValueChange={(value) => {
+              const next = value as WorkbenchLayout;
+              if (next === layout) return;
+              captureLayout();
+              onLayoutChange(next);
+            }}
+          >
+            {LAYOUT_CONTROLS.map((control) => (
+              <DropdownMenuRadioItem
+                key={control.layout}
+                value={control.layout}
+              >
+                <control.icon aria-hidden />
+                {t(`workbench.layouts.${control.layout}`, {
+                  defaultValue: control.label,
+                })}
+              </DropdownMenuRadioItem>
+            ))}
+          </DropdownMenuRadioGroup>
+        </DropdownMenuContent>
+      </DropdownMenu>
+      <HeaderIconButton
+        disabled={addPaneDisabled}
+        icon={Plus}
+        label={t("workbench.addPane", { defaultValue: "Add pane" })}
+        onClick={() => {
+          captureLayout();
+          onAddPane();
+        }}
+      />
+    </div>
+  ) : null;
+
+  return (
+    <section
+      aria-label={t("workbench.aria", { defaultValue: "Conversation workbench" })}
+      className="flex h-full min-h-0 flex-col overflow-hidden bg-background"
+    >
+      <TooltipProvider delayDuration={500} skipDelayDuration={100}>
+        {chrome ? (
+          <header className="shrink-0 bg-background">
+            <div
+              ref={setHeaderPortalTarget}
+              data-testid="workbench-header-host"
+            />
+          </header>
+        ) : null}
+        <div className="min-h-0 flex-1 bg-background">
+          <div
+            data-testid="pane-grid"
+            data-layout={effectiveLayout}
+            className={cn(
+              "grid h-full min-h-0 min-w-0 overflow-hidden",
+              chrome && panes.length > 1 && "gap-px bg-border/55",
+            )}
+            style={gridStyle}
+          >
+            {panes.map((pane, index) => {
+              const active = pane.key === activePaneKey;
+              const hidden = effectiveLayout === "monocle" && !active;
+
+              return (
+                <section
+                  key={pane.reactKey ?? pane.key}
+                  ref={(element) => {
+                    if (element) paneRefs.current.set(pane.key, element);
+                    else paneRefs.current.delete(pane.key);
+                  }}
+                  hidden={hidden}
+                  aria-label={pane.title}
+                  data-active={active ? "true" : "false"}
+                  data-testid={`workbench-pane-${pane.key}`}
+                  onPointerDownCapture={(event) => handlePanePointerDown(pane.key, event)}
+                  onFocusCapture={(event) => handlePaneFocus(pane.key, event)}
+                  className="workbench-pane relative flex min-h-0 min-w-0 overflow-hidden bg-background"
+                  style={paneCellStyle(effectiveLayout, panes.length, index)}
+                >
+                  {renderPane(pane, {
+                    active,
+                    headerPortalTarget: chrome ? headerPortalTarget : undefined,
+                    composerPortalTarget: chrome ? composerPortalTarget : undefined,
+                    headerActions,
+                  })}
+                </section>
+              );
+            })}
+          </div>
+        </div>
+
+        {chrome ? (
+          <footer className="shrink-0 bg-background px-3 pb-[calc(0.75rem+env(safe-area-inset-bottom))] sm:px-4">
+            <div
+              ref={setComposerPortalTarget}
+              data-testid="workbench-composer-host"
+              className="mx-auto w-full max-w-[58rem]"
+            />
+          </footer>
+        ) : null}
+      </TooltipProvider>
+    </section>
+  );
+}
diff --git a/webui/src/components/workbench/workbench-model.ts b/webui/src/components/workbench/workbench-model.ts
new file mode 100644
index 000000000..fa486c43c
--- /dev/null
+++ b/webui/src/components/workbench/workbench-model.ts
@@ -0,0 +1,280 @@
+export const WORKBENCH_STORAGE_KEY = "nanobot.webui.workbench.v2";
+export const MAX_WORKBENCH_PANES = 4;
+
+export const WORKBENCH_LAYOUTS = [
+  "columns",
+  "rows",
+  "grid",
+  "main-stack",
+  "monocle",
+] as const;
+
+export type WorkbenchLayout = (typeof WORKBENCH_LAYOUTS)[number];
+
+export interface WorkbenchTabState {
+  paneKeys: string[];
+  activePaneKey: string;
+  layout: WorkbenchLayout;
+}
+
+export interface WorkbenchState {
+  version: 2;
+  tabs: Record<string, WorkbenchTabState>;
+}
+
+export const EMPTY_WORKBENCH_STATE: WorkbenchState = {
+  version: 2,
+  tabs: {},
+};
+
+function isLayout(value: unknown): value is WorkbenchLayout {
+  return typeof value === "string"
+    && (WORKBENCH_LAYOUTS as readonly string[]).includes(value);
+}
+
+function uniqueKeys(value: unknown): string[] {
+  if (!Array.isArray(value)) return [];
+  return Array.from(new Set(
+    value.filter((key): key is string => typeof key === "string" && key.length > 0),
+  ));
+}
+
+function normalizeTab(value: unknown, tabKey: string): WorkbenchTabState {
+  const candidate = value && typeof value === "object"
+    ? value as Partial<WorkbenchTabState>
+    : {};
+  const paneKeys = uniqueKeys(candidate.paneKeys);
+  const normalizedPaneKeys = (paneKeys.includes(tabKey)
+    ? paneKeys
+    : [tabKey, ...paneKeys]).slice(0, MAX_WORKBENCH_PANES);
+  return {
+    paneKeys: normalizedPaneKeys,
+    activePaneKey:
+      typeof candidate.activePaneKey === "string"
+      && normalizedPaneKeys.includes(candidate.activePaneKey)
+        ? candidate.activePaneKey
+        : normalizedPaneKeys[0],
+    layout: isLayout(candidate.layout) ? candidate.layout : "columns",
+  };
+}
+
+export function parseWorkbenchState(serialized: string | null): WorkbenchState {
+  if (!serialized) return EMPTY_WORKBENCH_STATE;
+  try {
+    const parsed = JSON.parse(serialized) as { version?: unknown; tabs?: unknown };
+    if (
+      parsed.version !== 2
+      || !parsed.tabs
+      || typeof parsed.tabs !== "object"
+      || Array.isArray(parsed.tabs)
+    ) {
+      return EMPTY_WORKBENCH_STATE;
+    }
+    const tabs = Object.fromEntries(
+      Object.entries(parsed.tabs).map(([tabKey, tab]) => [tabKey, normalizeTab(tab, tabKey)]),
+    );
+    return { version: 2, tabs };
+  } catch {
+    return EMPTY_WORKBENCH_STATE;
+  }
+}
+
+export function defaultWorkbenchTab(tabKey: string): WorkbenchTabState {
+  return {
+    paneKeys: [tabKey],
+    activePaneKey: tabKey,
+    layout: "columns",
+  };
+}
+
+export function workbenchTab(
+  state: WorkbenchState,
+  tabKey: string,
+): WorkbenchTabState {
+  return state.tabs[tabKey] ?? defaultWorkbenchTab(tabKey);
+}
+
+function updateTab(
+  state: WorkbenchState,
+  tabKey: string,
+  update: (tab: WorkbenchTabState) => WorkbenchTabState,
+): WorkbenchState {
+  const current = workbenchTab(state, tabKey);
+  const next = update(current);
+  if (state.tabs[tabKey] === next) return state;
+  return {
+    version: 2,
+    tabs: {
+      ...state.tabs,
+      [tabKey]: next,
+    },
+  };
+}
+
+export function ensureWorkbenchTab(
+  state: WorkbenchState,
+  tabKey: string,
+): WorkbenchState {
+  if (state.tabs[tabKey]) return state;
+  return updateTab(state, tabKey, (tab) => tab);
+}
+
+export function addWorkbenchPane(
+  state: WorkbenchState,
+  tabKey: string,
+  paneKey: string,
+): WorkbenchState {
+  return updateTab(state, tabKey, (tab) => {
+    if (tab.paneKeys.includes(paneKey)) {
+      if (tab.activePaneKey === paneKey) return tab;
+      return { ...tab, activePaneKey: paneKey };
+    }
+    if (tab.paneKeys.length >= MAX_WORKBENCH_PANES) return tab;
+    return {
+      ...tab,
+      paneKeys: [...tab.paneKeys, paneKey],
+      activePaneKey: paneKey,
+    };
+  });
+}
+
+export function focusWorkbenchPane(
+  state: WorkbenchState,
+  tabKey: string,
+  paneKey: string,
+): WorkbenchState {
+  return updateTab(state, tabKey, (tab) => (
+    tab.paneKeys.includes(paneKey) && tab.activePaneKey !== paneKey
+      ? { ...tab, activePaneKey: paneKey }
+      : tab
+  ));
+}
+
+export function detachWorkbenchPane(
+  state: WorkbenchState,
+  tabKey: string,
+  paneKey: string,
+): WorkbenchState {
+  return updateTab(state, tabKey, (tab) => {
+    const index = tab.paneKeys.indexOf(paneKey);
+    if (index < 0 || paneKey === tabKey || tab.paneKeys.length === 1) return tab;
+    const paneKeys = tab.paneKeys.filter((key) => key !== paneKey);
+    const activePaneKey = tab.activePaneKey === paneKey
+      ? paneKeys[Math.min(index, paneKeys.length - 1)]
+      : tab.activePaneKey;
+    return { ...tab, paneKeys, activePaneKey };
+  });
+}
+
+export function attachWorkbenchPane(
+  state: WorkbenchState,
+  targetTabKey: string,
+  paneKey: string,
+): WorkbenchState {
+  if (!targetTabKey || !paneKey || targetTabKey === paneKey) return state;
+
+  const sourceEntry = Object.entries(state.tabs).find(([, tab]) => (
+    tab.paneKeys.includes(paneKey)
+  ));
+  const sourceTabKey = sourceEntry?.[0];
+  const sourceTab = sourceEntry?.[1];
+  if (sourceTabKey === targetTabKey) {
+    return focusWorkbenchPane(state, targetTabKey, paneKey);
+  }
+  if (sourceTabKey === paneKey && sourceTab && sourceTab.paneKeys.length > 1) {
+    return state;
+  }
+  const targetBeforeMove = state.tabs[targetTabKey] ?? defaultWorkbenchTab(targetTabKey);
+  if (
+    !targetBeforeMove.paneKeys.includes(paneKey)
+    && targetBeforeMove.paneKeys.length >= MAX_WORKBENCH_PANES
+  ) {
+    return state;
+  }
+
+  const tabs = { ...state.tabs };
+  if (sourceTabKey && sourceTab) {
+    if (sourceTabKey === paneKey) {
+      delete tabs[sourceTabKey];
+    } else {
+      const index = sourceTab.paneKeys.indexOf(paneKey);
+      const paneKeys = sourceTab.paneKeys.filter((key) => key !== paneKey);
+      tabs[sourceTabKey] = {
+        ...sourceTab,
+        paneKeys,
+        activePaneKey: sourceTab.activePaneKey === paneKey
+          ? paneKeys[Math.min(index, paneKeys.length - 1)]
+          : sourceTab.activePaneKey,
+      };
+    }
+  }
+
+  const targetTab = tabs[targetTabKey] ?? defaultWorkbenchTab(targetTabKey);
+  tabs[targetTabKey] = targetTab.paneKeys.includes(paneKey)
+    ? { ...targetTab, activePaneKey: paneKey }
+    : {
+        ...targetTab,
+        paneKeys: [...targetTab.paneKeys, paneKey],
+        activePaneKey: paneKey,
+      };
+  return { version: 2, tabs };
+}
+
+export function promoteWorkbenchPane(
+  state: WorkbenchState,
+  tabKey: string,
+  paneKey: string,
+): WorkbenchState {
+  return updateTab(state, tabKey, (tab) => {
+    const index = tab.paneKeys.indexOf(paneKey);
+    if (index <= 0) return tab;
+    return {
+      ...tab,
+      paneKeys: [paneKey, ...tab.paneKeys.filter((key) => key !== paneKey)],
+    };
+  });
+}
+
+export function setWorkbenchLayout(
+  state: WorkbenchState,
+  tabKey: string,
+  layout: WorkbenchLayout,
+): WorkbenchState {
+  return updateTab(state, tabKey, (tab) => (
+    tab.layout === layout ? tab : { ...tab, layout }
+  ));
+}
+
+export function reconcileWorkbench(
+  state: WorkbenchState,
+  validKeys: ReadonlySet<string>,
+): WorkbenchState {
+  const tabs: Record<string, WorkbenchTabState> = {};
+  for (const [tabKey, tab] of Object.entries(state.tabs)) {
+    if (!validKeys.has(tabKey)) continue;
+    const paneKeys = tab.paneKeys.filter((key) => validKeys.has(key));
+    const normalizedPaneKeys = (paneKeys.includes(tabKey)
+      ? paneKeys
+      : [tabKey, ...paneKeys]).slice(0, MAX_WORKBENCH_PANES);
+    tabs[tabKey] = {
+      ...tab,
+      paneKeys: normalizedPaneKeys,
+      activePaneKey: normalizedPaneKeys.includes(tab.activePaneKey)
+        ? tab.activePaneKey
+        : normalizedPaneKeys[0],
+    };
+  }
+  const serializedCurrent = JSON.stringify(state.tabs);
+  const serializedNext = JSON.stringify(tabs);
+  return serializedCurrent === serializedNext ? state : { version: 2, tabs };
+}
+
+export function workbenchChildPaneKeys(state: WorkbenchState): Set<string> {
+  const childKeys = new Set<string>();
+  for (const [tabKey, tab] of Object.entries(state.tabs)) {
+    for (const paneKey of tab.paneKeys) {
+      if (paneKey !== tabKey) childKeys.add(paneKey);
+    }
+  }
+  return childKeys;
+}
diff --git a/webui/src/globals.css b/webui/src/globals.css
index 215ed1d51..b0cf216bd 100644
--- a/webui/src/globals.css
+++ b/webui/src/globals.css
@@ -360,6 +360,9 @@
   .thread-layout[data-layout="thread"] {
     grid-template-rows: minmax(0, 1fr) auto 0fr;
   }
+  .thread-layout[data-layout="external"] {
+    grid-template-rows: minmax(0, 1fr);
+  }
   @media (min-width: 640px) {
     .thread-layout[data-layout="hero"] {
       grid-template-rows: minmax(min-content, 1fr) auto 1fr;
@@ -564,6 +567,10 @@
     }
   }
 
+  .workbench-pane {
+    transform-origin: top left;
+  }
+
   /** Goal halo: pale sky blue (not ``--primary``, which often reads as neutral gray). */
   @keyframes goal-shell-glow-breathe {
     0%,
diff --git a/webui/src/i18n/locales/en/common.json b/webui/src/i18n/locales/en/common.json
index b0b5637a0..c77b2ca1f 100644
--- a/webui/src/i18n/locales/en/common.json
+++ b/webui/src/i18n/locales/en/common.json
@@ -965,6 +965,10 @@
     "unarchive": "Unarchive",
     "showArchived": "Show archived",
     "hideArchived": "Hide archived",
+    "select": "Select",
+    "cancelSelection": "Cancel selection",
+    "selectedCount": "{{count}} selected",
+    "deleteSelected": "Delete",
     "delete": "Delete",
     "newChat": "New topic",
     "groups": {
@@ -979,10 +983,13 @@
   },
   "deleteConfirm": {
     "title": "Delete this topic?",
+    "titleMany": "Delete {{count}} topics and panes?",
     "description": "This action cannot be undone.",
+    "descriptionMany": "This action cannot be undone.",
     "cancel": "Cancel",
     "confirm": "Delete",
     "automationsDescription": "This chat has scheduled automations. Deleting it will also delete them.",
+    "automationsDescriptionMany": "Linked automations will also be deleted.",
     "moreAutomations": "+ {{count}} more",
     "confirmWithAutomations": "Delete",
     "schedule": {
@@ -1378,6 +1385,26 @@
     "copy": "Copy",
     "copied": "Copied"
   },
+  "workbench": {
+    "aria": "Conversation workbench",
+    "panes": "Panes",
+    "panesInTab": "Panes in {{title}}",
+    "dropPane": "Move {{pane}} into {{tab}}",
+    "moveToTab": "Move to tab",
+    "layout": "Pane layout",
+    "addPane": "Add pane",
+    "promotePane": "Make {{title}} the primary pane",
+    "paneActions": "{{title}} pane actions",
+    "detachPane": "Move {{title}} to its own topic",
+    "composerAria": "Message {{title}}",
+    "layouts": {
+      "columns": "Columns",
+      "rows": "Rows",
+      "grid": "Grid",
+      "main-stack": "Main and stack",
+      "monocle": "Monocle"
+    }
+  },
   "common": {
     "dismiss": "Dismiss",
     "close": "Close",
diff --git a/webui/src/i18n/locales/es/common.json b/webui/src/i18n/locales/es/common.json
index ae2e9fb3e..21cb0fcb7 100644
--- a/webui/src/i18n/locales/es/common.json
+++ b/webui/src/i18n/locales/es/common.json
@@ -952,6 +952,10 @@
     "unarchive": "Desarchivar",
     "showArchived": "Mostrar archivados",
     "hideArchived": "Ocultar archivados",
+    "select": "Seleccionar",
+    "cancelSelection": "Cancelar selección",
+    "selectedCount": "{{count}} seleccionados",
+    "deleteSelected": "Eliminar",
     "delete": "Eliminar",
     "newChat": "Nuevo tema",
     "groups": {
@@ -966,10 +970,13 @@
   },
   "deleteConfirm": {
     "title": "¿Eliminar este chat?",
+    "titleMany": "¿Eliminar {{count}} chats y paneles?",
     "description": "Esta acción no se puede deshacer.",
+    "descriptionMany": "Esta acción no se puede deshacer.",
     "cancel": "Cancelar",
     "confirm": "Eliminar",
     "automationsDescription": "Este chat tiene automatizaciones programadas. Al eliminarlo también se eliminarán.",
+    "automationsDescriptionMany": "También se eliminarán las automatizaciones vinculadas.",
     "moreAutomations": "+ {{count}} más",
     "confirmWithAutomations": "Eliminar",
     "schedule": {
@@ -1365,6 +1372,26 @@
     "copy": "Copiar",
     "copied": "Copiado"
   },
+  "workbench": {
+    "aria": "Área de conversaciones",
+    "panes": "Paneles",
+    "panesInTab": "Paneles de {{title}}",
+    "dropPane": "Mover {{pane}} a {{tab}}",
+    "moveToTab": "Mover a una pestaña",
+    "layout": "Diseño de paneles",
+    "addPane": "Añadir panel",
+    "promotePane": "Convertir {{title}} en el panel principal",
+    "paneActions": "Acciones del panel {{title}}",
+    "detachPane": "Mover {{title}} a su propio tema",
+    "composerAria": "Mensaje para {{title}}",
+    "layouts": {
+      "columns": "Columnas",
+      "rows": "Filas",
+      "grid": "Cuadrícula",
+      "main-stack": "Principal y pila",
+      "monocle": "Monóculo"
+    }
+  },
   "common": {
     "dismiss": "Cerrar",
     "close": "Cerrar",
diff --git a/webui/src/i18n/locales/fr/common.json b/webui/src/i18n/locales/fr/common.json
index aa32089c6..18f8cec7f 100644
--- a/webui/src/i18n/locales/fr/common.json
+++ b/webui/src/i18n/locales/fr/common.json
@@ -951,6 +951,10 @@
     "unarchive": "Désarchiver",
     "showArchived": "Afficher les archives",
     "hideArchived": "Masquer les archives",
+    "select": "Sélectionner",
+    "cancelSelection": "Annuler la sélection",
+    "selectedCount": "{{count}} sélectionnés",
+    "deleteSelected": "Supprimer",
     "delete": "Supprimer",
     "newChat": "Nouveau sujet",
     "groups": {
@@ -965,10 +969,13 @@
   },
   "deleteConfirm": {
     "title": "Supprimer cette discussion ?",
+    "titleMany": "Supprimer {{count}} discussions et volets ?",
     "description": "Cette action est irréversible.",
+    "descriptionMany": "Cette action est irréversible.",
     "cancel": "Annuler",
     "confirm": "Supprimer",
     "automationsDescription": "Cette discussion contient des automatisations planifiées. La supprimer les supprimera aussi.",
+    "automationsDescriptionMany": "Les automatisations liées seront également supprimées.",
     "moreAutomations": "+ {{count}} autres",
     "confirmWithAutomations": "Supprimer",
     "schedule": {
@@ -1364,6 +1371,26 @@
     "copy": "Copier",
     "copied": "Copié"
   },
+  "workbench": {
+    "aria": "Espace de conversations",
+    "panes": "Volets",
+    "panesInTab": "Volets dans {{title}}",
+    "dropPane": "Déplacer {{pane}} dans {{tab}}",
+    "moveToTab": "Déplacer vers un onglet",
+    "layout": "Disposition des volets",
+    "addPane": "Ajouter un volet",
+    "promotePane": "Définir {{title}} comme volet principal",
+    "paneActions": "Actions du volet {{title}}",
+    "detachPane": "Déplacer {{title}} vers son propre sujet",
+    "composerAria": "Message à {{title}}",
+    "layouts": {
+      "columns": "Colonnes",
+      "rows": "Lignes",
+      "grid": "Grille",
+      "main-stack": "Principal et pile",
+      "monocle": "Monocle"
+    }
+  },
   "common": {
     "dismiss": "Fermer",
     "close": "Fermer",
diff --git a/webui/src/i18n/locales/id/common.json b/webui/src/i18n/locales/id/common.json
index 81326edf9..8e6681d08 100644
--- a/webui/src/i18n/locales/id/common.json
+++ b/webui/src/i18n/locales/id/common.json
@@ -951,6 +951,10 @@
     "unarchive": "Batalkan arsip",
     "showArchived": "Tampilkan yang diarsipkan",
     "hideArchived": "Sembunyikan yang diarsipkan",
+    "select": "Pilih",
+    "cancelSelection": "Batalkan pilihan",
+    "selectedCount": "{{count}} dipilih",
+    "deleteSelected": "Hapus",
     "delete": "Hapus",
     "newChat": "Topik baru",
     "groups": {
@@ -965,10 +969,13 @@
   },
   "deleteConfirm": {
     "title": "Hapus obrolan ini?",
+    "titleMany": "Hapus {{count}} obrolan dan panel?",
     "description": "Tindakan ini tidak dapat dibatalkan.",
+    "descriptionMany": "Tindakan ini tidak dapat dibatalkan.",
     "cancel": "Batal",
     "confirm": "Hapus",
     "automationsDescription": "Obrolan ini memiliki automasi terjadwal. Menghapusnya juga akan menghapus automasi tersebut.",
+    "automationsDescriptionMany": "Automasi terkait juga akan dihapus.",
     "moreAutomations": "+ {{count}} lagi",
     "confirmWithAutomations": "Hapus",
     "schedule": {
@@ -1364,6 +1371,26 @@
     "copy": "Salin",
     "copied": "Tersalin"
   },
+  "workbench": {
+    "aria": "Ruang kerja percakapan",
+    "panes": "Panel",
+    "panesInTab": "Panel di {{title}}",
+    "dropPane": "Pindahkan {{pane}} ke {{tab}}",
+    "moveToTab": "Pindahkan ke tab",
+    "layout": "Tata letak panel",
+    "addPane": "Tambah panel",
+    "promotePane": "Jadikan {{title}} panel utama",
+    "paneActions": "Tindakan panel {{title}}",
+    "detachPane": "Pindahkan {{title}} ke topik tersendiri",
+    "composerAria": "Pesan untuk {{title}}",
+    "layouts": {
+      "columns": "Kolom",
+      "rows": "Baris",
+      "grid": "Kisi",
+      "main-stack": "Utama dan tumpukan",
+      "monocle": "Panel tunggal"
+    }
+  },
   "common": {
     "dismiss": "Tutup",
     "close": "Tutup",
diff --git a/webui/src/i18n/locales/ja/common.json b/webui/src/i18n/locales/ja/common.json
index 08fe99bd7..137857c24 100644
--- a/webui/src/i18n/locales/ja/common.json
+++ b/webui/src/i18n/locales/ja/common.json
@@ -951,6 +951,10 @@
     "unarchive": "アーカイブを解除",
     "showArchived": "アーカイブ済みを表示",
     "hideArchived": "アーカイブ済みを隠す",
+    "select": "選択",
+    "cancelSelection": "選択を解除",
+    "selectedCount": "{{count}} 件を選択中",
+    "deleteSelected": "削除",
     "delete": "削除",
     "newChat": "新しいトピック",
     "groups": {
@@ -965,10 +969,13 @@
   },
   "deleteConfirm": {
     "title": "このチャットを削除しますか？",
+    "titleMany": "{{count}} 件のチャットとペインを削除しますか？",
     "description": "この操作は元に戻せません。",
+    "descriptionMany": "この操作は元に戻せません。",
     "cancel": "キャンセル",
     "confirm": "削除",
     "automationsDescription": "このチャットにはスケジュール済みの自動タスクがあります。削除するとそれらも削除されます。",
+    "automationsDescriptionMany": "関連する自動タスクも削除されます。",
     "moreAutomations": "他 {{count}} 件",
     "confirmWithAutomations": "削除",
     "schedule": {
@@ -1364,6 +1371,26 @@
     "copy": "コピー",
     "copied": "コピーしました"
   },
+  "workbench": {
+    "aria": "会話ワークベンチ",
+    "panes": "ペイン",
+    "panesInTab": "{{title}} のペイン",
+    "dropPane": "{{pane}} を {{tab}} に移動",
+    "moveToTab": "タブへ移動",
+    "layout": "ペインレイアウト",
+    "addPane": "ペインを追加",
+    "promotePane": "{{title}} をメインペインにする",
+    "paneActions": "{{title}} ペインの操作",
+    "detachPane": "{{title}} を独立したトピックに移動",
+    "composerAria": "{{title}} へのメッセージ",
+    "layouts": {
+      "columns": "列",
+      "rows": "行",
+      "grid": "グリッド",
+      "main-stack": "メインとスタック",
+      "monocle": "モノクル"
+    }
+  },
   "common": {
     "dismiss": "閉じる",
     "close": "閉じる",
diff --git a/webui/src/i18n/locales/ko/common.json b/webui/src/i18n/locales/ko/common.json
index dd3bfdf60..94b1c46e4 100644
--- a/webui/src/i18n/locales/ko/common.json
+++ b/webui/src/i18n/locales/ko/common.json
@@ -951,6 +951,10 @@
     "unarchive": "보관 해제",
     "showArchived": "보관된 항목 표시",
     "hideArchived": "보관된 항목 숨기기",
+    "select": "선택",
+    "cancelSelection": "선택 취소",
+    "selectedCount": "{{count}}개 선택됨",
+    "deleteSelected": "삭제",
     "delete": "삭제",
     "newChat": "새 주제",
     "groups": {
@@ -965,10 +969,13 @@
   },
   "deleteConfirm": {
     "title": "이 채팅을 삭제할까요?",
+    "titleMany": "채팅과 창 {{count}}개를 삭제할까요?",
     "description": "이 작업은 되돌릴 수 없습니다.",
+    "descriptionMany": "이 작업은 되돌릴 수 없습니다.",
     "cancel": "취소",
     "confirm": "삭제",
     "automationsDescription": "이 채팅에는 예약된 자동화가 있습니다. 채팅을 삭제하면 자동화도 함께 삭제됩니다.",
+    "automationsDescriptionMany": "연결된 자동화도 함께 삭제됩니다.",
     "moreAutomations": "+ {{count}}개 더",
     "confirmWithAutomations": "삭제",
     "schedule": {
@@ -1364,6 +1371,26 @@
     "copy": "복사",
     "copied": "복사됨"
   },
+  "workbench": {
+    "aria": "대화 워크벤치",
+    "panes": "창",
+    "panesInTab": "{{title}}의 창",
+    "dropPane": "{{pane}}을(를) {{tab}}으로 이동",
+    "moveToTab": "탭으로 이동",
+    "layout": "창 레이아웃",
+    "addPane": "창 추가",
+    "promotePane": "{{title}}을(를) 기본 창으로 설정",
+    "paneActions": "{{title}} 창 작업",
+    "detachPane": "{{title}}을(를) 별도 주제로 이동",
+    "composerAria": "{{title}}에 메시지 보내기",
+    "layouts": {
+      "columns": "열",
+      "rows": "행",
+      "grid": "그리드",
+      "main-stack": "기본 창과 스택",
+      "monocle": "단일 창"
+    }
+  },
   "common": {
     "dismiss": "닫기",
     "close": "닫기",
diff --git a/webui/src/i18n/locales/pt-BR/common.json b/webui/src/i18n/locales/pt-BR/common.json
index eef0bea72..b9b8f3bee 100644
--- a/webui/src/i18n/locales/pt-BR/common.json
+++ b/webui/src/i18n/locales/pt-BR/common.json
@@ -965,6 +965,10 @@
     "unarchive": "Desarquivar",
     "showArchived": "Mostrar arquivadas",
     "hideArchived": "Ocultar arquivadas",
+    "select": "Selecionar",
+    "cancelSelection": "Cancelar seleção",
+    "selectedCount": "{{count}} selecionados",
+    "deleteSelected": "Excluir",
     "delete": "Excluir",
     "newChat": "Novo tópico",
     "groups": {
@@ -979,10 +983,13 @@
   },
   "deleteConfirm": {
     "title": "Excluir esta conversa?",
+    "titleMany": "Excluir {{count}} conversas e painéis?",
     "description": "Esta ação não pode ser desfeita.",
+    "descriptionMany": "Esta ação não pode ser desfeita.",
     "cancel": "Cancelar",
     "confirm": "Excluir",
     "automationsDescription": "Esta conversa possui automações agendadas. Excluí-la também excluirá essas automações.",
+    "automationsDescriptionMany": "As automações vinculadas também serão excluídas.",
     "moreAutomations": "+ {{count}} a mais",
     "confirmWithAutomations": "Excluir",
     "schedule": {
@@ -1378,6 +1385,26 @@
     "copy": "Copiar",
     "copied": "Copiado"
   },
+  "workbench": {
+    "aria": "Área de conversas",
+    "panes": "Painéis",
+    "panesInTab": "Painéis em {{title}}",
+    "dropPane": "Mover {{pane}} para {{tab}}",
+    "moveToTab": "Mover para uma aba",
+    "layout": "Layout de painéis",
+    "addPane": "Adicionar painel",
+    "promotePane": "Tornar {{title}} o painel principal",
+    "paneActions": "Ações do painel {{title}}",
+    "detachPane": "Mover {{title}} para seu próprio tópico",
+    "composerAria": "Mensagem para {{title}}",
+    "layouts": {
+      "columns": "Colunas",
+      "rows": "Linhas",
+      "grid": "Grade",
+      "main-stack": "Principal e pilha",
+      "monocle": "Monóculo"
+    }
+  },
   "common": {
     "dismiss": "Descartar",
     "close": "Fechar",
diff --git a/webui/src/i18n/locales/vi/common.json b/webui/src/i18n/locales/vi/common.json
index 98ca6c1b3..15843fd6d 100644
--- a/webui/src/i18n/locales/vi/common.json
+++ b/webui/src/i18n/locales/vi/common.json
@@ -951,6 +951,10 @@
     "unarchive": "Bỏ lưu trữ",
     "showArchived": "Hiện mục đã lưu trữ",
     "hideArchived": "Ẩn mục đã lưu trữ",
+    "select": "Chọn",
+    "cancelSelection": "Hủy chọn",
+    "selectedCount": "Đã chọn {{count}} mục",
+    "deleteSelected": "Xóa",
     "delete": "Xóa",
     "newChat": "Chủ đề mới",
     "groups": {
@@ -965,10 +969,13 @@
   },
   "deleteConfirm": {
     "title": "Xóa cuộc trò chuyện này?",
+    "titleMany": "Xóa {{count}} cuộc trò chuyện và khung?",
     "description": "Không thể hoàn tác thao tác này.",
+    "descriptionMany": "Không thể hoàn tác thao tác này.",
     "cancel": "Hủy",
     "confirm": "Xóa",
     "automationsDescription": "Cuộc trò chuyện này có các tự động hóa đã lên lịch. Xóa cuộc trò chuyện cũng sẽ xóa chúng.",
+    "automationsDescriptionMany": "Các tự động hóa liên kết cũng sẽ bị xóa.",
     "moreAutomations": "+ {{count}} mục nữa",
     "confirmWithAutomations": "Xóa",
     "schedule": {
@@ -1364,6 +1371,26 @@
     "copy": "Sao chép",
     "copied": "Đã sao chép"
   },
+  "workbench": {
+    "aria": "Không gian hội thoại",
+    "panes": "Khung",
+    "panesInTab": "Các khung trong {{title}}",
+    "dropPane": "Di chuyển {{pane}} vào {{tab}}",
+    "moveToTab": "Di chuyển vào thẻ",
+    "layout": "Bố cục khung",
+    "addPane": "Thêm khung",
+    "promotePane": "Đặt {{title}} làm khung chính",
+    "paneActions": "Thao tác cho khung {{title}}",
+    "detachPane": "Chuyển {{title}} thành chủ đề riêng",
+    "composerAria": "Nhắn tin cho {{title}}",
+    "layouts": {
+      "columns": "Cột",
+      "rows": "Hàng",
+      "grid": "Lưới",
+      "main-stack": "Khung chính và ngăn xếp",
+      "monocle": "Một khung"
+    }
+  },
   "common": {
     "dismiss": "Đóng",
     "close": "Đóng",
diff --git a/webui/src/i18n/locales/zh-CN/common.json b/webui/src/i18n/locales/zh-CN/common.json
index 265f81797..d59e341d4 100644
--- a/webui/src/i18n/locales/zh-CN/common.json
+++ b/webui/src/i18n/locales/zh-CN/common.json
@@ -965,6 +965,10 @@
     "unarchive": "取消归档",
     "showArchived": "显示归档",
     "hideArchived": "隐藏归档",
+    "select": "选择",
+    "cancelSelection": "取消选择",
+    "selectedCount": "已选择 {{count}} 项",
+    "deleteSelected": "删除",
     "delete": "删除",
     "newChat": "新建话题",
     "groups": {
@@ -979,10 +983,13 @@
   },
   "deleteConfirm": {
     "title": "删除这个话题？",
+    "titleMany": "删除这 {{count}} 个话题和窗格？",
     "description": "此操作无法撤销。",
+    "descriptionMany": "此操作无法撤销。",
     "cancel": "取消",
     "confirm": "删除",
     "automationsDescription": "这个话题有关联的自动任务。删除话题也会删除这些自动任务。",
+    "automationsDescriptionMany": "关联的自动任务也会一并删除。",
     "moreAutomations": "另有 {{count}} 个",
     "confirmWithAutomations": "删除",
     "schedule": {
@@ -1378,6 +1385,26 @@
     "copy": "复制",
     "copied": "已复制"
   },
+  "workbench": {
+    "aria": "会话工作台",
+    "panes": "窗格",
+    "panesInTab": "{{title}} 中的窗格",
+    "dropPane": "将 {{pane}} 移入 {{tab}}",
+    "moveToTab": "移动到标签页",
+    "layout": "窗格布局",
+    "addPane": "添加窗格",
+    "promotePane": "将 {{title}} 设为主窗格",
+    "paneActions": "{{title}} 窗格操作",
+    "detachPane": "将 {{title}} 移至独立主题",
+    "composerAria": "向 {{title}} 发送消息",
+    "layouts": {
+      "columns": "列布局",
+      "rows": "行布局",
+      "grid": "网格",
+      "main-stack": "主窗格与堆栈",
+      "monocle": "单窗格"
+    }
+  },
   "common": {
     "dismiss": "关闭",
     "close": "关闭",
diff --git a/webui/src/i18n/locales/zh-TW/common.json b/webui/src/i18n/locales/zh-TW/common.json
index a2eef59f5..882b08ec7 100644
--- a/webui/src/i18n/locales/zh-TW/common.json
+++ b/webui/src/i18n/locales/zh-TW/common.json
@@ -951,6 +951,10 @@
     "unarchive": "取消封存",
     "showArchived": "顯示封存",
     "hideArchived": "隱藏封存",
+    "select": "選取",
+    "cancelSelection": "取消選取",
+    "selectedCount": "已選取 {{count}} 項",
+    "deleteSelected": "刪除",
     "delete": "刪除",
     "newChat": "新增話題",
     "groups": {
@@ -965,10 +969,13 @@
   },
   "deleteConfirm": {
     "title": "刪除這個話題？",
+    "titleMany": "刪除這 {{count}} 個話題和窗格？",
     "description": "此操作無法復原。",
+    "descriptionMany": "此操作無法復原。",
     "cancel": "取消",
     "confirm": "刪除",
     "automationsDescription": "這個話題含有已排程的自動任務。刪除話題時也會一併刪除這些任務。",
+    "automationsDescriptionMany": "關聯的自動任務也會一併刪除。",
     "moreAutomations": "另有 {{count}} 個",
     "confirmWithAutomations": "刪除",
     "schedule": {
@@ -1364,6 +1371,26 @@
     "copy": "複製",
     "copied": "已複製"
   },
+  "workbench": {
+    "aria": "對話工作台",
+    "panes": "窗格",
+    "panesInTab": "{{title}} 中的窗格",
+    "dropPane": "將 {{pane}} 移入 {{tab}}",
+    "moveToTab": "移動到分頁",
+    "layout": "窗格佈局",
+    "addPane": "新增窗格",
+    "promotePane": "將 {{title}} 設為主窗格",
+    "paneActions": "{{title}} 窗格操作",
+    "detachPane": "將 {{title}} 移至獨立主題",
+    "composerAria": "傳送訊息給 {{title}}",
+    "layouts": {
+      "columns": "欄佈局",
+      "rows": "列佈局",
+      "grid": "網格",
+      "main-stack": "主窗格與堆疊",
+      "monocle": "單窗格"
+    }
+  },
   "common": {
     "dismiss": "關閉",
     "close": "關閉",
diff --git a/webui/src/lib/session-drag.ts b/webui/src/lib/session-drag.ts
index 0a4339cc9..c089beaea 100644
--- a/webui/src/lib/session-drag.ts
+++ b/webui/src/lib/session-drag.ts
@@ -1,6 +1,13 @@
 export const SESSION_DRAG_TYPE = "application/x-nanobot-session-key";
+export const PANE_DRAG_TYPE = "application/x-nanobot-pane";
+
+export interface DraggedPane {
+  paneKey: string;
+  sourceTabKey: string;
+}
 
 let activeSessionKey: string | null = null;
+let activePane: DraggedPane | null = null;
 
 export function hasDraggedSession(dataTransfer: DataTransfer): boolean {
   return Array.from(dataTransfer.types).includes(SESSION_DRAG_TYPE);
@@ -13,6 +20,7 @@ export function readDraggedSession(dataTransfer: DataTransfer): string | null {
 
 export function clearDraggedSession(): void {
   activeSessionKey = null;
+  activePane = null;
 }
 
 export function writeDraggedSession(
@@ -23,3 +31,27 @@ export function writeDraggedSession(
   dataTransfer.effectAllowed = "copyMove";
   dataTransfer.setData(SESSION_DRAG_TYPE, sessionKey);
 }
+
+export function readDraggedPane(dataTransfer: DataTransfer): DraggedPane | null {
+  const serialized = dataTransfer.getData(PANE_DRAG_TYPE).trim();
+  if (serialized) {
+    try {
+      const parsed = JSON.parse(serialized) as Partial<DraggedPane>;
+      if (parsed.paneKey && parsed.sourceTabKey) {
+        return { paneKey: parsed.paneKey, sourceTabKey: parsed.sourceTabKey };
+      }
+    } catch {
+      // Fall through to the in-memory payload used while the native drag is active.
+    }
+  }
+  return activePane;
+}
+
+export function writeDraggedPane(
+  dataTransfer: DataTransfer,
+  pane: DraggedPane,
+): void {
+  activePane = pane;
+  writeDraggedSession(dataTransfer, pane.paneKey);
+  dataTransfer.setData(PANE_DRAG_TYPE, JSON.stringify(pane));
+}
diff --git a/webui/src/tests/app-layout.test.tsx b/webui/src/tests/app-layout.test.tsx
index 32abc7017..f6b25f568 100644
--- a/webui/src/tests/app-layout.test.tsx
+++ b/webui/src/tests/app-layout.test.tsx
@@ -164,7 +164,24 @@ vi.mock("@/hooks/useSessions", async (importOriginal) => {
         loading: false,
         error: null,
         refresh: refreshSpy,
-        createChat: createChatSpy,
+        createChat: async (scope?: WorkspaceScopePayload | null) => {
+          const chatId = await createChatSpy(scope);
+          const now = new Date().toISOString();
+          setSessions((prev: ChatSummary[]) => [
+            {
+              key: `websocket:${chatId}`,
+              channel: "websocket",
+              chatId,
+              createdAt: now,
+              updatedAt: now,
+              title: "",
+              preview: "",
+              workspaceScope: scope ?? null,
+            },
+            ...prev.filter((session) => session.chatId !== chatId),
+          ]);
+          return chatId;
+        },
         forkChat: async () => "fork-chat",
         getSessionAutomations: getSessionAutomationsSpy,
         deleteChat: async (key: string, options?: { deleteAutomations?: boolean }) => {
@@ -290,6 +307,8 @@ describe("App layout", () => {
     localStorage.removeItem("nanobot-webui.sidebar.session-updates.v1");
     localStorage.removeItem("nanobot-webui.restartStartedAt");
     localStorage.removeItem("nanobot-webui.restartRoute");
+    localStorage.removeItem("nanobot.webui.workbench.v1");
+    localStorage.removeItem("nanobot.webui.workbench.v2");
     vi.mocked(fetchBootstrap).mockReset().mockResolvedValue({
       token: "tok",
       api_token: "api-tok",
@@ -485,8 +504,9 @@ describe("App layout", () => {
     render(<App />);
 
     await waitFor(() => expect(connectSpy).toHaveBeenCalled());
+    const firstMessage = "keep this first turn visible";
     fireEvent.change(screen.getByRole("textbox", { name: "Message input" }), {
-      target: { value: "/model" },
+      target: { value: firstMessage },
     });
     fireEvent.click(screen.getByRole("button", { name: "Send message" }));
 
@@ -496,6 +516,7 @@ describe("App layout", () => {
         `#/chat/${encodeURIComponent("websocket:chat-1")}`,
       ),
     );
+    expect(await screen.findByText(firstMessage)).toBeInTheDocument();
   });
 
   it("creates a new temporary chat from the hero each time", async () => {
@@ -1649,6 +1670,60 @@ describe("App layout", () => {
     expect(document.body.style.pointerEvents).not.toBe("none");
   }, 15_000);
 
+  it("deletes multiple selected topics through one confirmation", async () => {
+    mockSessions = [
+      {
+        key: "websocket:chat-a",
+        channel: "websocket",
+        chatId: "chat-a",
+        createdAt: "2026-04-16T10:00:00Z",
+        updatedAt: "2026-04-16T10:00:00Z",
+        preview: "First chat",
+      },
+      {
+        key: "websocket:chat-b",
+        channel: "websocket",
+        chatId: "chat-b",
+        createdAt: "2026-04-16T11:00:00Z",
+        updatedAt: "2026-04-16T11:00:00Z",
+        preview: "Second chat",
+      },
+      {
+        key: "websocket:chat-c",
+        channel: "websocket",
+        chatId: "chat-c",
+        createdAt: "2026-04-16T12:00:00Z",
+        updatedAt: "2026-04-16T12:00:00Z",
+        preview: "Third chat",
+      },
+    ];
+
+    render(<App />);
+
+    await waitFor(() => expect(connectSpy).toHaveBeenCalled());
+    const sidebar = screen.getByRole("navigation", { name: "Sidebar navigation" });
+    fireEvent.pointerDown(within(sidebar).getByLabelText(
+      "Topic actions for First chat",
+    ), { button: 0 });
+    fireEvent.click(await screen.findByRole("menuitem", { name: "Select" }));
+    fireEvent.click(within(sidebar).getByRole("button", { name: "Second chat" }));
+    expect(within(sidebar).getByText("2 selected")).toBeInTheDocument();
+
+    fireEvent.click(within(sidebar).getByRole("button", { name: "Delete" }));
+    expect(await screen.findByText("Delete 2 topics and panes?")).toBeInTheDocument();
+    fireEvent.click(screen.getByRole("button", { name: "Delete" }));
+
+    await waitFor(() => expect(deleteChatSpy).toHaveBeenCalledTimes(2));
+    expect(deleteChatSpy.mock.calls.map(([key]) => key)).toEqual([
+      "websocket:chat-a",
+      "websocket:chat-b",
+    ]);
+    expect(getSessionAutomationsSpy).toHaveBeenCalledWith("websocket:chat-a");
+    expect(getSessionAutomationsSpy).toHaveBeenCalledWith("websocket:chat-b");
+    expect(within(sidebar).getByRole("button", { name: "Third chat" }))
+      .toBeInTheDocument();
+  }, 15_000);
+
   it("shows localized bound automations in the first delete confirmation", async () => {
     mockSessions = [
       {
@@ -2943,6 +3018,109 @@ describe("App layout", () => {
     );
   });
 
+  it("keeps panes and layout scoped to the current topic tab", async () => {
+    vi.stubGlobal("matchMedia", vi.fn((query: string) => ({
+      matches: false,
+      media: query,
+      onchange: null,
+      addListener: vi.fn(),
+      removeListener: vi.fn(),
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+      dispatchEvent: vi.fn(),
+    })));
+    createChatSpy.mockResolvedValueOnce("chat-pane");
+    mockSessions = [
+      {
+        key: "websocket:chat-alpha",
+        channel: "websocket",
+        chatId: "chat-alpha",
+        createdAt: "2026-04-16T10:00:00Z",
+        updatedAt: "2026-04-16T10:00:00Z",
+        title: "Alpha",
+        preview: "Alpha notes",
+      },
+      {
+        key: "websocket:chat-beta",
+        channel: "websocket",
+        chatId: "chat-beta",
+        createdAt: "2026-04-16T11:00:00Z",
+        updatedAt: "2026-04-16T11:00:00Z",
+        title: "Beta",
+        preview: "Beta notes",
+      },
+    ];
+    window.history.replaceState(
+      null,
+      "",
+      "/#/chat/websocket%3Achat-alpha",
+    );
+
+    render(<App />);
+
+    await waitFor(() => expect(connectSpy).toHaveBeenCalled());
+    const grid = await screen.findByTestId("pane-grid");
+    expect(Array.from(grid.children).map((pane) => pane.getAttribute("aria-label")))
+      .toEqual(["Alpha"]);
+
+    fireEvent.click(screen.getByRole("button", { name: "Add pane" }));
+    expect(screen.queryByRole("dialog", { name: "Search" })).not.toBeInTheDocument();
+    await waitFor(() => expect(createChatSpy).toHaveBeenCalledTimes(1));
+
+    await waitFor(() => expect(grid.children).toHaveLength(2));
+    expect(window.location.hash).toBe("#/chat/websocket%3Achat-alpha");
+    expect(Array.from(grid.children).map((pane) => pane.getAttribute("aria-label")))
+      .toEqual(["Alpha", "New topic"]);
+
+    const activeComposer = screen.getByTestId("active-pane-composer");
+    const paneInput = within(activeComposer).getByRole("textbox", {
+      name: "Message New topic",
+    });
+    fireEvent.change(paneInput, { target: { value: "route this to the new pane" } });
+    fireEvent.keyDown(paneInput, { key: "Enter" });
+    await waitFor(() => expect(sendMessageSpy).toHaveBeenCalled());
+    expect(sendMessageSpy.mock.calls.at(-1)?.[0]).toBe("chat-pane");
+
+    fireEvent.pointerDown(screen.getByRole("button", { name: "Pane layout" }), {
+      button: 0,
+      ctrlKey: false,
+    });
+    fireEvent.click(screen.getByRole("menuitemradio", { name: "Rows" }));
+    expect(grid).toHaveAttribute("data-layout", "rows");
+
+    const sidebar = screen.getByRole("navigation", { name: "Sidebar navigation" });
+    const paneTopicButton = within(sidebar)
+      .getAllByRole("button", { name: "New topic" })
+      .find((button) => button.closest("[data-sidebar-pane]"));
+    expect(paneTopicButton).toBeDefined();
+    expect(paneTopicButton?.closest("[data-sidebar-pane]"))
+      .toHaveAttribute("data-sidebar-pane", "websocket:chat-pane");
+    fireEvent.click(within(sidebar).getByRole("button", { name: "Beta" }));
+    await waitFor(() => {
+      const nextGrid = screen.getByTestId("pane-grid");
+      expect(Array.from(nextGrid.children).map((pane) => pane.getAttribute("aria-label")))
+        .toEqual(["Beta"]);
+      expect(nextGrid).toHaveAttribute("data-layout", "columns");
+    });
+
+    fireEvent.click(within(sidebar).getByRole("button", { name: "Alpha" }));
+    await waitFor(() => {
+      const restoredGrid = screen.getByTestId("pane-grid");
+      expect(Array.from(restoredGrid.children).map((pane) => pane.getAttribute("aria-label")))
+        .toEqual(["Alpha", "New topic"]);
+      expect(restoredGrid).toHaveAttribute("data-layout", "rows");
+    });
+
+    fireEvent.pointerDown(within(sidebar).getByRole("button", {
+      name: "New topic pane actions",
+    }), { button: 0, ctrlKey: false });
+    fireEvent.click(screen.getByRole("menuitem", {
+      name: "Move New topic to its own topic",
+    }));
+    await waitFor(() => expect(screen.getByTestId("pane-grid").children).toHaveLength(1));
+    expect(within(sidebar).getAllByRole("button", { name: "New topic" })).toHaveLength(2);
+  });
+
   it("opens search from the keyboard shortcut", async () => {
     mockSessions = [
       {
diff --git a/webui/src/tests/chat-list.test.tsx b/webui/src/tests/chat-list.test.tsx
index fa2e3cb79..cef3424b6 100644
--- a/webui/src/tests/chat-list.test.tsx
+++ b/webui/src/tests/chat-list.test.tsx
@@ -1,8 +1,8 @@
-import { fireEvent, render, screen, within } from "@testing-library/react";
+import { createEvent, fireEvent, render, screen, within } from "@testing-library/react";
 import { afterEach, describe, expect, it, vi } from "vitest";
 
 import { ChatList } from "@/components/ChatList";
-import { SESSION_DRAG_TYPE } from "@/lib/session-drag";
+import { PANE_DRAG_TYPE, SESSION_DRAG_TYPE } from "@/lib/session-drag";
 import type { ChatSummary } from "@/lib/types";
 
 function session(overrides: Partial<ChatSummary>): ChatSummary {
@@ -42,6 +42,26 @@ function rect({
   } as DOMRect;
 }
 
+function dragOverAt(
+  element: Element,
+  clientY: number,
+  dataTransfer: Record<string, unknown>,
+): void {
+  const event = createEvent.dragOver(element, { dataTransfer });
+  Object.defineProperty(event, "clientY", { value: clientY });
+  fireEvent(element, event);
+}
+
+function dropAt(
+  element: Element,
+  clientY: number,
+  dataTransfer: Record<string, unknown>,
+): void {
+  const event = createEvent.drop(element, { dataTransfer });
+  Object.defineProperty(event, "clientY", { value: clientY });
+  fireEvent(element, event);
+}
+
 describe("ChatList", () => {
   afterEach(() => {
     vi.restoreAllMocks();
@@ -82,7 +102,13 @@ describe("ChatList", () => {
     fireEvent.dragEnd(reference, { dataTransfer });
   });
 
-  it("reorders chats around a Codex-style insertion line", () => {
+  it("reorders topics with a displaced-neighbor preview instead of an insertion line", () => {
+    vi.spyOn(HTMLElement.prototype, "getBoundingClientRect").mockReturnValue(rect({
+      left: 0,
+      top: 0,
+      width: 284,
+      height: 32,
+    }));
     const onReorderSessions = vi.fn();
     const sessions = [
       session({ chatId: "alpha", title: "Alpha" }),
@@ -112,10 +138,14 @@ describe("ChatList", () => {
     };
     fireEvent.dragStart(screen.getByRole("button", { name: "Alpha" }), { dataTransfer });
     const charlieRow = screen.getByRole("button", { name: "Charlie" }).closest("li")!;
-    fireEvent.dragOver(charlieRow, { clientY: 1, dataTransfer });
-    expect(charlieRow.querySelector("[data-session-drop-edge='after']"))
-      .toBeInTheDocument();
-    fireEvent.drop(charlieRow, { clientY: 1, dataTransfer });
+    dragOverAt(charlieRow, 24, dataTransfer);
+    expect(document.querySelector("[data-session-drop-edge]")).not.toBeInTheDocument();
+    expect(screen.getByRole("button", { name: "Bravo" }).closest("li"))
+      .toHaveAttribute("data-session-displaced", "true");
+    expect(charlieRow).toHaveStyle({ transform: "translateY(-32px)" });
+    expect(screen.getByRole("button", { name: "Alpha" }).closest("li"))
+      .toHaveAttribute("data-session-dragging", "true");
+    dropAt(charlieRow, 24, dataTransfer);
 
     expect(onReorderSessions).toHaveBeenCalledWith([
       "websocket:bravo",
@@ -152,6 +182,228 @@ describe("ChatList", () => {
     expect(text.indexOf("Charlie")).toBeLessThan(text.indexOf("Alpha"));
   });
 
+  it("shows every tab's pane membership in the sidebar tree", async () => {
+    const onSelect = vi.fn();
+    const onSelectPane = vi.fn();
+    const onDetachPane = vi.fn();
+    const onPromotePane = vi.fn();
+    const onRequestRename = vi.fn();
+    const onAttachPane = vi.fn();
+
+    render(
+      <ChatList
+        sessions={[
+          session({ chatId: "root", title: "Root topic" }),
+          session({ chatId: "target", title: "Target tab" }),
+        ]}
+        activeKey="websocket:root"
+        paneGroups={{
+          "websocket:root": {
+            topicKey: "websocket:root",
+            activePaneKey: "websocket:child",
+            panes: [
+              { key: "websocket:root", chatId: "root", title: "Root topic" },
+              { key: "websocket:child", chatId: "child", title: "Research pane" },
+            ],
+          },
+          "websocket:target": {
+            topicKey: "websocket:target",
+            activePaneKey: "websocket:target-child",
+            panes: [
+              { key: "websocket:target", chatId: "target", title: "Target tab" },
+              {
+                key: "websocket:target-child",
+                chatId: "target-child",
+                title: "Target research",
+              },
+            ],
+          },
+        }}
+        onSelect={onSelect}
+        onSelectPane={onSelectPane}
+        onDetachPane={onDetachPane}
+        onPromotePane={onPromotePane}
+        paneAcceptingTabKeys={["websocket:target"]}
+        onAttachPane={onAttachPane}
+        onRequestDelete={vi.fn()}
+        onTogglePin={vi.fn()}
+        onRequestRename={onRequestRename}
+        onToggleArchive={vi.fn()}
+      />,
+    );
+
+    const child = screen.getByRole("button", { name: "Research pane" });
+    expect(child.closest("[data-sidebar-pane]"))
+      .toHaveAttribute("data-sidebar-pane", "websocket:child");
+    expect(child).toHaveAttribute("aria-current", "true");
+    const targetTabRow = screen.getByRole("button", { name: "Target tab" }).closest("li")!;
+    const targetChild = within(targetTabRow).getByRole("button", {
+      name: "Target research",
+    });
+    expect(targetChild.closest("[data-sidebar-pane]"))
+      .toHaveAttribute("data-sidebar-pane", "websocket:target-child");
+    expect(targetChild).not.toHaveAttribute("aria-current");
+    fireEvent.click(targetChild);
+    expect(onSelectPane).toHaveBeenCalledWith(
+      "websocket:target",
+      "websocket:target-child",
+    );
+    fireEvent.click(child);
+    expect(onSelectPane).toHaveBeenCalledWith("websocket:root", "websocket:child");
+
+    fireEvent.click(screen.getByRole("button", { name: "Root topic" }));
+    expect(onSelectPane).toHaveBeenCalledWith("websocket:root", "websocket:root");
+    expect(onSelect).not.toHaveBeenCalled();
+
+    fireEvent.pointerDown(screen.getByRole("button", {
+      name: "Research pane pane actions",
+    }), { button: 0, ctrlKey: false });
+    const moveToTab = await screen.findByRole("menuitem", { name: "Move to tab" });
+    fireEvent.pointerMove(moveToTab, { pointerType: "mouse" });
+    fireEvent.click(await screen.findByRole("menuitem", { name: "Target tab" }));
+    expect(onAttachPane).toHaveBeenCalledWith("websocket:child", "websocket:target");
+
+    fireEvent.pointerDown(screen.getByRole("button", {
+      name: "Research pane pane actions",
+    }), { button: 0, ctrlKey: false });
+    fireEvent.click(await screen.findByRole("menuitem", {
+      name: "Move Research pane to its own topic",
+    }));
+    expect(onDetachPane).toHaveBeenCalledWith("websocket:root", "websocket:child");
+
+    const dataTransfer = {
+      effectAllowed: "",
+      dropEffect: "",
+      setData: vi.fn(),
+    };
+    fireEvent.dragStart(child, { dataTransfer });
+    expect(child.closest("li")).toHaveAttribute("data-pane-dragging", "true");
+    expect(child.closest("li")).not.toHaveClass("opacity-0");
+    const targetTab = screen.getByRole("button", { name: "Target tab" });
+    dragOverAt(targetTab.closest("li")!, 0, dataTransfer);
+    expect(targetTab.closest("li"))
+      .toHaveAttribute("data-tab-attach-target", "true");
+    expect(within(targetTab.closest("li")!).getByRole("status", {
+      name: "Move Research pane into Target tab",
+    })).toHaveTextContent("Research pane");
+    dropAt(targetTab.closest("li")!, 0, dataTransfer);
+    expect(dataTransfer.setData).toHaveBeenCalledWith(
+      PANE_DRAG_TYPE,
+      JSON.stringify({
+        paneKey: "websocket:child",
+        sourceTabKey: "websocket:root",
+      }),
+    );
+    expect(onAttachPane).toHaveBeenCalledWith("websocket:child", "websocket:target");
+  });
+
+  it("selects a whole tab or individual panes for one bulk delete", async () => {
+    const onRequestDeleteMany = vi.fn();
+    render(
+      <ChatList
+        sessions={[
+          session({ chatId: "root", title: "Root topic" }),
+          session({ chatId: "target", title: "Target tab" }),
+        ]}
+        activeKey="websocket:root"
+        paneGroups={{
+          "websocket:root": {
+            topicKey: "websocket:root",
+            activePaneKey: "websocket:root",
+            panes: [
+              { key: "websocket:root", chatId: "root", title: "Root topic" },
+              { key: "websocket:child", chatId: "child", title: "Research pane" },
+            ],
+          },
+        }}
+        onSelect={vi.fn()}
+        onRequestDelete={vi.fn()}
+        onRequestDeleteMany={onRequestDeleteMany}
+        onTogglePin={vi.fn()}
+        onRequestRename={vi.fn()}
+        onToggleArchive={vi.fn()}
+      />,
+    );
+
+    fireEvent.pointerDown(screen.getByRole("button", {
+      name: "Topic actions for Root topic",
+    }), { button: 0, ctrlKey: false });
+    fireEvent.click(await screen.findByRole("menuitem", { name: "Select" }));
+
+    expect(screen.getByRole("button", { name: "Root topic" }))
+      .toHaveAttribute("aria-pressed", "true");
+    expect(screen.getByRole("button", { name: "Research pane" }))
+      .toHaveAttribute("aria-pressed", "true");
+    expect(screen.getByText("2 selected")).toBeInTheDocument();
+
+    fireEvent.click(screen.getByRole("button", { name: "Target tab" }));
+    expect(screen.getByText("3 selected")).toBeInTheDocument();
+    fireEvent.click(within(screen.getByTestId("delete-selection-bar")).getByRole("button", {
+      name: "Delete",
+    }));
+
+    expect(onRequestDeleteMany).toHaveBeenCalledWith([
+      { key: "websocket:root", label: "Root topic" },
+      { key: "websocket:child", label: "Research pane" },
+      { key: "websocket:target", label: "Target tab" },
+    ]);
+    expect(screen.queryByTestId("delete-selection-bar")).not.toBeInTheDocument();
+  });
+
+  it("reattaches a one-pane tab through the center of another tab", () => {
+    vi.spyOn(HTMLElement.prototype, "getBoundingClientRect").mockReturnValue(rect({
+      left: 0,
+      top: 0,
+      width: 284,
+      height: 32,
+    }));
+    const onAttachPane = vi.fn();
+    const onReorderSessions = vi.fn();
+    const dataTransfer = {
+      effectAllowed: "",
+      dropEffect: "",
+      setData: vi.fn(),
+    };
+
+    render(
+      <ChatList
+        sessions={[
+          session({ chatId: "detached", title: "Detached pane" }),
+          session({ chatId: "target", title: "Target tab" }),
+        ]}
+        activeKey={null}
+        attachableTabKeys={["websocket:detached", "websocket:target"]}
+        paneAcceptingTabKeys={["websocket:detached", "websocket:target"]}
+        onAttachPane={onAttachPane}
+        onSelect={vi.fn()}
+        onRequestDelete={vi.fn()}
+        onTogglePin={vi.fn()}
+        onRequestRename={vi.fn()}
+        onToggleArchive={vi.fn()}
+        onReorderSessions={onReorderSessions}
+      />,
+    );
+
+    const detached = screen.getByRole("button", { name: "Detached pane" });
+    fireEvent.dragStart(detached, {
+      dataTransfer,
+    });
+    expect(detached.closest("li"))
+      .toHaveAttribute("data-session-dragging", "true");
+    const target = screen.getByRole("button", { name: "Target tab" }).closest("li")!;
+    dragOverAt(target, 16, dataTransfer);
+    expect(target).toHaveAttribute("data-tab-attach-target", "true");
+    expect(document.querySelector("[data-session-displaced='true']"))
+      .not.toBeInTheDocument();
+    dropAt(target, 16, dataTransfer);
+
+    expect(onAttachPane).toHaveBeenCalledWith(
+      "websocket:detached",
+      "websocket:target",
+    );
+    expect(onReorderSessions).not.toHaveBeenCalled();
+  });
+
   it("shows temporary chats separately and lets the user reopen or close them", async () => {
     const temporarySession = session({
       key: "temporary:temporary-one",
diff --git a/webui/src/tests/pane-workbench.test.tsx b/webui/src/tests/pane-workbench.test.tsx
new file mode 100644
index 000000000..1f4988b67
--- /dev/null
+++ b/webui/src/tests/pane-workbench.test.tsx
@@ -0,0 +1,139 @@
+import { createPortal } from "react-dom";
+import { useState } from "react";
+import { fireEvent, render, screen, waitFor, within } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+import { PaneWorkbench } from "@/components/workbench/PaneWorkbench";
+import {
+  EMPTY_WORKBENCH_STATE,
+  addWorkbenchPane,
+  focusWorkbenchPane,
+  setWorkbenchLayout,
+  workbenchTab,
+} from "@/components/workbench/workbench-model";
+
+function rect(left: number, top: number, width: number, height: number): DOMRect {
+  return {
+    x: left,
+    y: top,
+    left,
+    top,
+    width,
+    height,
+    right: left + width,
+    bottom: top + height,
+    toJSON: () => ({}),
+  };
+}
+
+function WorkbenchHarness() {
+  const [state, setState] = useState(() => (
+    addWorkbenchPane(EMPTY_WORKBENCH_STATE, "alpha", "beta")
+  ));
+  const tab = workbenchTab(state, "alpha");
+  const titles: Record<string, string> = { alpha: "Alpha", beta: "Beta" };
+
+  return (
+    <PaneWorkbench
+      panes={tab.paneKeys.map((key) => ({ key, title: titles[key] }))}
+      activePaneKey={tab.activePaneKey}
+      layout={tab.layout}
+      onActivatePane={(key) => setState((current) => (
+        focusWorkbenchPane(current, "alpha", key)
+      ))}
+      onAddPane={vi.fn()}
+      onLayoutChange={(layout) => setState((current) => (
+        setWorkbenchLayout(current, "alpha", layout)
+      ))}
+      renderPane={(pane, context) => (
+        <>
+          <button type="button">Focus {pane.title}</button>
+          {context.headerPortalTarget && context.active ? createPortal(
+            context.headerActions,
+            context.headerPortalTarget,
+          ) : null}
+          {context.composerPortalTarget ? createPortal(
+            <div hidden={!context.active}>
+              <textarea aria-label={`Composer ${pane.title}`} />
+            </div>,
+            context.composerPortalTarget,
+          ) : null}
+        </>
+      )}
+    />
+  );
+}
+
+describe("PaneWorkbench", () => {
+  const originalGetBoundingClientRect = HTMLElement.prototype.getBoundingClientRect;
+  const originalAnimate = HTMLElement.prototype.animate;
+  const animate = vi.fn(() => ({
+    addEventListener: vi.fn(),
+    cancel: vi.fn(),
+  }) as unknown as Animation);
+
+  beforeEach(() => {
+    vi.stubGlobal("matchMedia", vi.fn((query: string) => ({
+      matches: false,
+      media: query,
+      onchange: null,
+      addListener: vi.fn(),
+      removeListener: vi.fn(),
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+      dispatchEvent: vi.fn(),
+    })));
+    HTMLElement.prototype.animate = animate;
+    HTMLElement.prototype.getBoundingClientRect = function getBoundingClientRect() {
+      if (!this.classList.contains("workbench-pane")) {
+        return originalGetBoundingClientRect.call(this);
+      }
+      const layout = this.parentElement?.dataset.layout;
+      const index = Array.from(this.parentElement?.children ?? []).indexOf(this);
+      return layout === "rows"
+        ? rect(0, index * 500, 1000, 500)
+        : rect(index * 500, 0, 500, 1000);
+    };
+  });
+
+  afterEach(() => {
+    HTMLElement.prototype.animate = originalAnimate;
+    HTMLElement.prototype.getBoundingClientRect = originalGetBoundingClientRect;
+    vi.unstubAllGlobals();
+    vi.clearAllMocks();
+  });
+
+  it("focuses without reordering and keeps only the focused composer visible", () => {
+    render(<WorkbenchHarness />);
+
+    const grid = screen.getByTestId("pane-grid");
+    expect(Array.from(grid.children).map((pane) => pane.getAttribute("aria-label")))
+      .toEqual(["Alpha", "Beta"]);
+    expect(screen.getByLabelText("Composer Beta")).toBeVisible();
+    expect(screen.getByLabelText("Composer Alpha")).not.toBeVisible();
+
+    fireEvent.pointerDown(
+      within(screen.getByRole("region", { name: "Alpha" }))
+        .getByRole("button", { name: "Focus Alpha" }),
+    );
+
+    expect(Array.from(grid.children).map((pane) => pane.getAttribute("aria-label")))
+      .toEqual(["Alpha", "Beta"]);
+    expect(screen.getByLabelText("Composer Alpha")).toBeVisible();
+    expect(screen.getByLabelText("Composer Beta")).not.toBeVisible();
+  });
+
+  it("keeps one shared layout control and animates geometry changes", async () => {
+    render(<WorkbenchHarness />);
+
+    const header = screen.getByTestId("workbench-header-host");
+    expect(within(header).getAllByRole("button", { name: "Pane layout" })).toHaveLength(1);
+    fireEvent.pointerDown(within(header).getByRole("button", { name: "Pane layout" }), {
+      button: 0,
+      ctrlKey: false,
+    });
+    fireEvent.click(screen.getByRole("menuitemradio", { name: "Rows" }));
+    expect(screen.getByTestId("pane-grid")).toHaveAttribute("data-layout", "rows");
+    await waitFor(() => expect(animate).toHaveBeenCalledTimes(2));
+  });
+});
diff --git a/webui/src/tests/workbench-model.test.ts b/webui/src/tests/workbench-model.test.ts
new file mode 100644
index 000000000..74b3894f4
--- /dev/null
+++ b/webui/src/tests/workbench-model.test.ts
@@ -0,0 +1,173 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  EMPTY_WORKBENCH_STATE,
+  MAX_WORKBENCH_PANES,
+  addWorkbenchPane,
+  attachWorkbenchPane,
+  detachWorkbenchPane,
+  ensureWorkbenchTab,
+  focusWorkbenchPane,
+  parseWorkbenchState,
+  promoteWorkbenchPane,
+  reconcileWorkbench,
+  setWorkbenchLayout,
+  workbenchChildPaneKeys,
+  workbenchTab,
+} from "@/components/workbench/workbench-model";
+
+describe("workbench model", () => {
+  it("gives every topic its own one-pane tab by default", () => {
+    const state = ensureWorkbenchTab(EMPTY_WORKBENCH_STATE, "topic-a");
+
+    expect(workbenchTab(state, "topic-a")).toEqual({
+      paneKeys: ["topic-a"],
+      activePaneKey: "topic-a",
+      layout: "columns",
+    });
+    expect(state.tabs["topic-b"]).toBeUndefined();
+  });
+
+  it("keeps pane membership, focus, and layout scoped to a tab", () => {
+    let state = ensureWorkbenchTab(EMPTY_WORKBENCH_STATE, "topic-a");
+    state = ensureWorkbenchTab(state, "topic-b");
+    state = addWorkbenchPane(state, "topic-a", "topic-c");
+    state = setWorkbenchLayout(state, "topic-a", "main-stack");
+
+    expect(workbenchTab(state, "topic-a")).toEqual({
+      paneKeys: ["topic-a", "topic-c"],
+      activePaneKey: "topic-c",
+      layout: "main-stack",
+    });
+    expect(workbenchTab(state, "topic-b")).toEqual({
+      paneKeys: ["topic-b"],
+      activePaneKey: "topic-b",
+      layout: "columns",
+    });
+  });
+
+  it("focuses without reordering and promotes only when asked", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "topic-a", "topic-b");
+    state = addWorkbenchPane(state, "topic-a", "topic-c");
+    state = focusWorkbenchPane(state, "topic-a", "topic-b");
+
+    expect(workbenchTab(state, "topic-a").paneKeys).toEqual([
+      "topic-a",
+      "topic-b",
+      "topic-c",
+    ]);
+
+    state = promoteWorkbenchPane(state, "topic-a", "topic-b");
+    expect(workbenchTab(state, "topic-a")).toMatchObject({
+      paneKeys: ["topic-b", "topic-a", "topic-c"],
+      activePaneKey: "topic-b",
+    });
+  });
+
+  it("detaches child panes, keeps the root, and chooses the adjacent focus", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "topic-a", "topic-b");
+    state = addWorkbenchPane(state, "topic-a", "topic-c");
+    state = focusWorkbenchPane(state, "topic-a", "topic-b");
+    state = detachWorkbenchPane(state, "topic-a", "topic-b");
+
+    expect(workbenchTab(state, "topic-a")).toMatchObject({
+      paneKeys: ["topic-a", "topic-c"],
+      activePaneKey: "topic-c",
+    });
+
+    state = detachWorkbenchPane(state, "topic-a", "topic-c");
+    state = detachWorkbenchPane(state, "topic-a", "topic-a");
+    expect(workbenchTab(state, "topic-a").paneKeys).toEqual(["topic-a"]);
+  });
+
+  it("moves a pane between tabs and can reattach a one-pane tab", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "topic-a", "pane-a");
+    state = ensureWorkbenchTab(state, "topic-b");
+    state = attachWorkbenchPane(state, "topic-b", "pane-a");
+
+    expect(workbenchTab(state, "topic-a")).toMatchObject({
+      paneKeys: ["topic-a"],
+      activePaneKey: "topic-a",
+    });
+    expect(workbenchTab(state, "topic-b")).toMatchObject({
+      paneKeys: ["topic-b", "pane-a"],
+      activePaneKey: "pane-a",
+    });
+
+    state = ensureWorkbenchTab(state, "topic-c");
+    state = attachWorkbenchPane(state, "topic-b", "topic-c");
+    expect(state.tabs["topic-c"]).toBeUndefined();
+    expect(workbenchTab(state, "topic-b").paneKeys).toEqual([
+      "topic-b",
+      "pane-a",
+      "topic-c",
+    ]);
+  });
+
+  it("does not collapse a multi-pane tab into another tab", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "topic-a", "pane-a");
+    state = ensureWorkbenchTab(state, "topic-b");
+
+    expect(attachWorkbenchPane(state, "topic-b", "topic-a")).toBe(state);
+  });
+
+  it("caps every tab at four panes", () => {
+    let state = EMPTY_WORKBENCH_STATE;
+    for (let index = 1; index <= MAX_WORKBENCH_PANES; index += 1) {
+      state = addWorkbenchPane(state, "topic-a", `pane-${index}`);
+    }
+    expect(workbenchTab(state, "topic-a").paneKeys).toEqual([
+      "topic-a",
+      "pane-1",
+      "pane-2",
+      "pane-3",
+    ]);
+
+    const beforeAttach = state;
+    state = attachWorkbenchPane(state, "topic-a", "standalone");
+    expect(state).toBe(beforeAttach);
+  });
+
+  it("identifies only sessions attached beneath another topic", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "topic-a", "pane-a");
+    state = addWorkbenchPane(state, "topic-b", "pane-b");
+
+    expect(workbenchChildPaneKeys(state)).toEqual(new Set(["pane-a", "pane-b"]));
+
+    state = detachWorkbenchPane(state, "topic-a", "pane-a");
+    expect(workbenchChildPaneKeys(state)).toEqual(new Set(["pane-b"]));
+  });
+
+  it("repairs persisted state and removes deleted sessions", () => {
+    const parsed = parseWorkbenchState(JSON.stringify({
+      version: 2,
+      tabs: {
+        "topic-a": {
+          paneKeys: ["topic-a", "topic-b", "topic-b", 9],
+          activePaneKey: "missing",
+          layout: "unknown",
+        },
+        deleted: {
+          paneKeys: ["deleted"],
+          activePaneKey: "deleted",
+          layout: "grid",
+        },
+      },
+    }));
+    const reconciled = reconcileWorkbench(parsed, new Set(["topic-a"]));
+
+    expect(reconciled).toEqual({
+      version: 2,
+      tabs: {
+        "topic-a": {
+          paneKeys: ["topic-a"],
+          activePaneKey: "topic-a",
+          layout: "columns",
+        },
+      },
+    });
+    expect(parseWorkbenchState(JSON.stringify({ version: 1, tabs: {} })))
+      .toEqual(EMPTY_WORKBENCH_STATE);
+    expect(parseWorkbenchState("not-json")).toEqual(EMPTY_WORKBENCH_STATE);
+  });
+});