diff --git a/nanobot/channels/websocket/tests/test_websocket_channel.py b/nanobot/channels/websocket/tests/test_websocket_channel.py
index 1a19b0e21..a0bdd8410 100644
--- a/nanobot/channels/websocket/tests/test_websocket_channel.py
+++ b/nanobot/channels/websocket/tests/test_websocket_channel.py
@@ -1060,7 +1060,6 @@ async def test_webui_sidebar_state_update_broadcasts_workbench_to_other_devices(
                                 "title": "Research",
                                 "paneKeys": ["websocket:a", "websocket:b"],
                                 "layoutPaneKeys": ["websocket:b", "websocket:a"],
-                                "activePaneKey": "websocket:a",
                                 "layout": "columns",
                                 "splitRatios": [0.35],
                             }
diff --git a/nanobot/webui/sidebar_state.py b/nanobot/webui/sidebar_state.py
index 1a0b80f53..c28b95610 100644
--- a/nanobot/webui/sidebar_state.py
+++ b/nanobot/webui/sidebar_state.py
@@ -10,6 +10,7 @@ from __future__ import annotations
 import json
 import math
 import os
+import threading
 import time
 from pathlib import Path
 from typing import Any, cast
@@ -29,6 +30,7 @@ _MAX_WORKBENCH_PANES = 4
 _ALLOWED_DENSITIES = {"comfortable", "compact"}
 _ALLOWED_SORTS = {"updated_desc", "created_desc", "title_asc", "manual"}
 _ALLOWED_WORKBENCH_LAYOUTS = {"columns", "rows", "grid", "bsp", "main-stack"}
+_SIDEBAR_STATE_WRITE_LOCK = threading.Lock()
 
 
 def webui_sidebar_state_path() -> Path:
@@ -152,7 +154,10 @@ def _clean_view(value: Any) -> dict[str, Any]:
 def _clean_workbench(value: Any) -> dict[str, Any]:
     if not isinstance(value, dict):
         return {"version": 1, "tabs": {}}
-    raw_tabs = cast(dict[str, Any], value).get("tabs")
+    workbench = cast(dict[str, Any], value)
+    if workbench.get("version") != 1:
+        return {"version": 1, "tabs": {}}
+    raw_tabs = workbench.get("tabs")
     if not isinstance(raw_tabs, dict):
         return {"version": 1, "tabs": {}}
 
@@ -170,6 +175,9 @@ def _clean_workbench(value: Any) -> dict[str, Any]:
         ][:_MAX_WORKBENCH_PANES]
         if not pane_keys:
             continue
+        explicit = tab.get("explicit") is True
+        if not explicit and len(pane_keys) == 1:
+            continue
         requested_layout_pane_keys = [
             key for key in _clean_string_list(tab.get("layoutPaneKeys")) if key in pane_keys
         ]
@@ -180,15 +188,11 @@ def _clean_workbench(value: Any) -> dict[str, Any]:
         raw_layout = tab.get("layout")
         layout = raw_layout if raw_layout in _ALLOWED_WORKBENCH_LAYOUTS else "columns"
         title = _clean_string(tab.get("title"), max_len=_MAX_TITLE_LEN)
-        active_pane_key = _clean_string(tab.get("activePaneKey"))
         tabs[tab_key] = {
-            "explicit": tab.get("explicit") is True,
+            "explicit": explicit,
             "title": title,
             "paneKeys": pane_keys,
             "layoutPaneKeys": layout_pane_keys,
-            "activePaneKey": (
-                active_pane_key if active_pane_key in pane_keys else pane_keys[0]
-            ),
             "layout": layout,
             "splitRatios": _clean_split_ratios(tab.get("splitRatios")),
         }
@@ -234,6 +238,11 @@ def read_webui_sidebar_state() -> dict[str, Any]:
 
 
 def write_webui_sidebar_state(raw: dict[str, Any]) -> dict[str, Any]:
+    with _SIDEBAR_STATE_WRITE_LOCK:
+        return _write_webui_sidebar_state(raw)
+
+
+def _write_webui_sidebar_state(raw: dict[str, Any]) -> dict[str, Any]:
     state = normalize_webui_sidebar_state(raw)
     state["updated_at"] = time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime())
     encoded = json.dumps(
diff --git a/tests/utils/test_webui_sidebar_state.py b/tests/utils/test_webui_sidebar_state.py
index 43779f157..c61ff52a2 100644
--- a/tests/utils/test_webui_sidebar_state.py
+++ b/tests/utils/test_webui_sidebar_state.py
@@ -1,5 +1,9 @@
 import json
+import threading
+import time
+from concurrent.futures import ThreadPoolExecutor
 
+import nanobot.webui.sidebar_state as sidebar_state
 from nanobot.webui.sidebar_state import (
     default_webui_sidebar_state,
     read_webui_sidebar_state,
@@ -39,13 +43,11 @@ def test_sidebar_state_normalizes_partial_payload(tmp_path, monkeypatch) -> None
                             "title": "  Research  ",
                             "paneKeys": ["websocket:a", "websocket:b", "websocket:a"],
                             "layoutPaneKeys": ["websocket:b", "missing", "websocket:a"],
-                            "activePaneKey": "missing",
                             "layout": "invalid-layout",
                             "splitRatios": [0.4, 2, "bad", float("nan")],
                         },
                         "tab:websocket:b": {
                             "paneKeys": ["websocket:b", "websocket:c"],
-                            "activePaneKey": "websocket:c",
                             "layout": "bsp",
                         },
                     },
@@ -74,19 +76,9 @@ def test_sidebar_state_normalizes_partial_payload(tmp_path, monkeypatch) -> None
                 "title": "Research",
                 "paneKeys": ["websocket:a", "websocket:b"],
                 "layoutPaneKeys": ["websocket:b", "websocket:a"],
-                "activePaneKey": "websocket:a",
                 "layout": "columns",
                 "splitRatios": [0.4, 0.95],
             },
-            "tab:websocket:b": {
-                "explicit": False,
-                "title": None,
-                "paneKeys": ["websocket:c"],
-                "layoutPaneKeys": ["websocket:c"],
-                "activePaneKey": "websocket:c",
-                "layout": "bsp",
-                "splitRatios": [],
-            },
         },
     }
     assert state["view"] == {
@@ -122,3 +114,67 @@ def test_sidebar_state_write_is_scoped_to_config_data_dir(tmp_path, monkeypatch)
     assert state["view"]["sort"] == "manual"
     assert webui_sidebar_state_path().is_file()
     assert read_webui_sidebar_state()["pinned_keys"] == ["websocket:a"]
+
+
+def test_sidebar_state_persists_only_visible_workbench_groups(tmp_path, monkeypatch) -> None:
+    monkeypatch.setattr("nanobot.config.paths.get_data_dir", lambda: tmp_path)
+    tabs = {
+        f"tab:websocket:{index}": {
+            "explicit": False,
+            "paneKeys": [f"websocket:{index}"],
+            "layoutPaneKeys": [f"websocket:{index}"],
+            "layout": "columns",
+            "splitRatios": [],
+        }
+        for index in range(2_000)
+    }
+
+    state = write_webui_sidebar_state({"workbench": {"version": 1, "tabs": tabs}})
+
+    assert state["workbench"] == {"version": 1, "tabs": {}}
+    assert webui_sidebar_state_path().stat().st_size < 2_048
+
+
+def test_sidebar_state_requires_supported_workbench_version(tmp_path, monkeypatch) -> None:
+    monkeypatch.setattr("nanobot.config.paths.get_data_dir", lambda: tmp_path)
+
+    state = write_webui_sidebar_state(
+        {
+            "workbench": {
+                "version": 2,
+                "tabs": {
+                    "tab:websocket:a": {
+                        "explicit": True,
+                        "paneKeys": ["websocket:a"],
+                    }
+                },
+            }
+        }
+    )
+
+    assert state["workbench"] == {"version": 1, "tabs": {}}
+
+
+def test_sidebar_state_serializes_concurrent_writes(monkeypatch) -> None:
+    counter_lock = threading.Lock()
+    active_writes = 0
+    peak_writes = 0
+
+    def fake_write(raw: dict[str, object]) -> dict[str, object]:
+        nonlocal active_writes, peak_writes
+        with counter_lock:
+            active_writes += 1
+            peak_writes = max(peak_writes, active_writes)
+        time.sleep(0.01)
+        with counter_lock:
+            active_writes -= 1
+        return raw
+
+    monkeypatch.setattr(sidebar_state, "_write_webui_sidebar_state", fake_write)
+    payloads = [{"write": index} for index in range(12)]
+
+    with ThreadPoolExecutor(max_workers=4) as executor:
+        results = list(executor.map(sidebar_state.write_webui_sidebar_state, payloads))
+
+    assert results == payloads
+    assert peak_writes == 1
diff --git a/webui/src/App.tsx b/webui/src/App.tsx
index 58c7db163..e2e7939c3 100644
--- a/webui/src/App.tsx
+++ b/webui/src/App.tsx
@@ -17,15 +17,12 @@ import type { SettingsSectionKey } from "@/components/settings/SettingsView";
 import { ThreadShell } from "@/components/thread/ThreadShell";
 import { PaneWorkbench } from "@/components/workbench/PaneWorkbench";
 import {
-  EMPTY_WORKBENCH_STATE,
   MAX_WORKBENCH_PANES,
   addWorkbenchPane,
   attachWorkbenchPane,
   createWorkbenchTab,
   detachWorkbenchPane,
   dissolveWorkbenchTab,
-  ensureWorkbenchPaneTab,
-  focusWorkbenchPane,
   orderWorkbenchTabs,
   reconcileWorkbench,
   renameWorkbenchTab,
@@ -1060,12 +1057,16 @@ function Shell({
   const [hostSidebarPreviewOpen, setHostSidebarPreviewOpen] = useState(false);
   const [mobileSidebarOpen, setMobileSidebarOpen] = useState(false);
   const [sessionSearchOpen, setSessionSearchOpen] = useState(false);
-  const [workbenchState, setWorkbenchState] = useState<WorkbenchState>(
-    EMPTY_WORKBENCH_STATE,
-  );
-  const workbenchServerHydratedRef = useRef(false);
-  const lastServerWorkbenchRef = useRef("");
-  const skipWorkbenchPersistenceRef = useRef(false);
+  const workbenchState = sidebarState.workbench;
+  const updateWorkbenchState = useCallback((
+    updater: (current: WorkbenchState) => WorkbenchState,
+  ) => {
+    void updateSidebarState((current) => {
+      const next = updater(current.workbench);
+      return next === current.workbench ? current : { ...current, workbench: next };
+    });
+  }, [updateSidebarState]);
+  const lastActivePaneByTabRef = useRef(new Map<string, string>());
   const [creatingPane, setCreatingPane] = useState(false);
   const topicSessions = sessions;
   const [pendingDelete, setPendingDelete] = useState<{
@@ -1183,19 +1184,6 @@ function Shell({
     };
   }, [getToken]);
 
-  useEffect(() => {
-    if (sidebarStateLoading) return;
-    const serialized = JSON.stringify(sidebarState.workbench);
-    if (
-      workbenchServerHydratedRef.current
-      && lastServerWorkbenchRef.current === serialized
-    ) return;
-    workbenchServerHydratedRef.current = true;
-    lastServerWorkbenchRef.current = serialized;
-    skipWorkbenchPersistenceRef.current = true;
-    setWorkbenchState(sidebarState.workbench);
-  }, [sidebarState.workbench, sidebarStateLoading]);
-
   useEffect(() => {
     try {
       window.localStorage.setItem(
@@ -1207,21 +1195,6 @@ function Shell({
     }
   }, [hostSidebarOpen]);
 
-  useEffect(() => {
-    if (!workbenchServerHydratedRef.current || sidebarStateLoading) return;
-    if (skipWorkbenchPersistenceRef.current) {
-      skipWorkbenchPersistenceRef.current = false;
-      return;
-    }
-    const serialized = JSON.stringify(workbenchState);
-    if (serialized === JSON.stringify(sidebarState.workbench)) return;
-    lastServerWorkbenchRef.current = serialized;
-    void updateSidebarState((current) => ({
-      ...current,
-      workbench: workbenchState,
-    }));
-  }, [sidebarState.workbench, sidebarStateLoading, updateSidebarState, workbenchState]);
-
   useEffect(() => {
     writeSessionUpdateChatIds(updatedChatIds);
   }, [updatedChatIds]);
@@ -1292,11 +1265,11 @@ function Shell({
   ), [activeKey, temporarySessions, workbenchState]);
   const activeTabKey = activeTabMatch?.tabKey ?? null;
   const activeTabState = activeTabMatch?.tab ?? null;
-  const activePaneSession = useMemo<ChatSummary | null>(() => {
-    if (!activeTabState) return activeSession;
-    return sessions.find((session) => session.key === activeTabState.activePaneKey)
-      ?? activeSession;
-  }, [activeSession, activeTabState, sessions]);
+  const activePaneSession = activeSession;
+  useEffect(() => {
+    if (!activeTabKey || !activeKey || !activeTabState?.paneKeys.includes(activeKey)) return;
+    lastActivePaneByTabRef.current.set(activeTabKey, activeKey);
+  }, [activeKey, activeTabKey, activeTabState]);
   const runningChatIdList = useMemo(() => Array.from(runningChatIds), [runningChatIds]);
   const updatedChatIdList = useMemo(() => Array.from(updatedChatIds), [updatedChatIds]);
   const activeChatId = activePaneSession?.chatId ?? null;
@@ -1364,20 +1337,14 @@ function Shell({
   useEffect(() => {
     if (loading || sidebarStateLoading) 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;
-      }
-      const match = workbenchTabForPane(reconciled, activeKey);
-      return focusWorkbenchPane(reconciled, match.tabKey, activeKey);
+    updateWorkbenchState((current) => {
+      return reconcileWorkbench(current, validKeys);
     });
   }, [
-    activeKey,
     loading,
     sidebarStateLoading,
     sessions,
-    temporarySessions,
+    updateWorkbenchState,
   ]);
 
   useEffect(() => {
@@ -1817,11 +1784,11 @@ function Shell({
 
   const onConfirmTabRename = useCallback((title: string) => {
     if (!pendingTabRename) return;
-    setWorkbenchState((current) => (
+    updateWorkbenchState((current) => (
       renameWorkbenchTab(current, pendingTabRename.key, title)
     ));
     setPendingTabRename(null);
-  }, [pendingTabRename]);
+  }, [pendingTabRename, updateWorkbenchState]);
 
   const onToggleGroup = useCallback(
     (groupId: string) => {
@@ -1940,11 +1907,7 @@ function Shell({
       const chatId = await createChat(scope);
       const paneKey = `websocket:${chatId}`;
       pendingCreatedSessionKeyRef.current = paneKey;
-      setWorkbenchState((current) => {
-        const withTab = ensureWorkbenchPaneTab(current, activeKey);
-        const target = workbenchTabForPane(withTab, activeKey);
-        return addWorkbenchPane(withTab, target.tabKey, paneKey);
-      });
+      updateWorkbenchState((current) => addWorkbenchPane(current, activeKey, paneKey));
       navigate({
         view: "chat",
         activeKey: paneKey,
@@ -1974,6 +1937,7 @@ function Shell({
     creatingPane,
     navigate,
     t,
+    updateWorkbenchState,
   ]);
 
   useEffect(() => {
@@ -2395,9 +2359,7 @@ function Shell({
     titleForSession,
     workbenchPaneSessions,
   ]);
-  const renderedActivePaneKey = paneChromeEnabled && activeTabState
-    ? activeTabState.activePaneKey
-    : renderedWorkbenchPanes[0].key;
+  const renderedActivePaneKey = activeKey ?? renderedWorkbenchPanes[0].key;
   const renderedWorkbenchLayout = paneChromeEnabled && activeTabState
     ? activeTabState.layout
     : "columns";
@@ -2419,12 +2381,15 @@ function Shell({
       return [presentation.rowKey, {
         tabKey: orderedTab.tabKey,
         title: presentation.title,
-        activePaneKey: orderedTab.tab.activePaneKey,
+        activePaneKey: activeKey && orderedTab.paneKeys.includes(activeKey)
+          ? activeKey
+          : orderedTab.paneKeys[0],
         visible: orderedTab.tab.explicit || orderedTab.paneKeys.length > 1,
         panes,
       }];
     }));
   }, [
+    activeKey,
     sessions,
     sidebarTabPresentations,
     titleForSession,
@@ -2434,14 +2399,18 @@ function Shell({
   );
 
   const onActivateWorkbenchPane = useCallback((paneKey: string) => {
-    if (!activeTabKey) return;
-    setWorkbenchState((current) => focusWorkbenchPane(current, activeTabKey, paneKey));
     onSelectChat(paneKey);
-  }, [activeTabKey, onSelectChat]);
+  }, [onSelectChat]);
 
   const onSelectSidebarTab = useCallback((tabKey: string) => {
     const tab = workbenchTab(workbenchState, tabKey);
-    if (tab) onSelectChat(tab.activePaneKey);
+    if (!tab) return;
+    const rememberedPaneKey = lastActivePaneByTabRef.current.get(tabKey);
+    onSelectChat(
+      rememberedPaneKey && tab.paneKeys.includes(rememberedPaneKey)
+        ? rememberedPaneKey
+        : tab.paneKeys[0],
+    );
   }, [onSelectChat, workbenchState]);
 
   const onSelectSidebarItem = useCallback((key: string) => {
@@ -2455,33 +2424,32 @@ function Shell({
     onSelectSidebarTab(key);
   }, [onSelectChat, onSelectSidebarTab, sessions]);
 
-  const onSelectSidebarPane = useCallback((tabKey: string, paneKey: string) => {
-    setWorkbenchState((current) => focusWorkbenchPane(current, tabKey, paneKey));
+  const onSelectSidebarPane = useCallback((_tabKey: string, paneKey: string) => {
     onSelectChat(paneKey);
   }, [onSelectChat]);
 
   const onDetachWorkbenchPane = useCallback((tabKey: string, paneKey: string) => {
-    setWorkbenchState((current) => detachWorkbenchPane(current, tabKey, paneKey));
-  }, []);
+    updateWorkbenchState((current) => detachWorkbenchPane(current, tabKey, paneKey));
+  }, [updateWorkbenchState]);
 
-  const onCreateWorkbenchTab = useCallback((tabKey: string) => {
-    setWorkbenchState((current) => createWorkbenchTab(current, tabKey));
-  }, []);
+  const onCreateWorkbenchTab = useCallback((paneKey: string) => {
+    updateWorkbenchState((current) => createWorkbenchTab(current, paneKey));
+  }, [updateWorkbenchState]);
 
   const onDissolveWorkbenchTab = useCallback((tabKey: string) => {
-    setWorkbenchState((current) => dissolveWorkbenchTab(current, tabKey));
-  }, []);
+    updateWorkbenchState((current) => dissolveWorkbenchTab(current, tabKey));
+  }, [updateWorkbenchState]);
 
   const onAttachWorkbenchPane = useCallback((
     paneKey: string,
     tabKey: string,
   ) => {
-    setWorkbenchState((current) => {
+    updateWorkbenchState((current) => {
       const target = workbenchTab(current, tabKey);
       if (!target || (!target.explicit && target.paneKeys.length < 2)) return current;
       return attachWorkbenchPane(current, tabKey, paneKey);
     });
-  }, []);
+  }, [updateWorkbenchState]);
 
   useEffect(() => {
     if (view === "settings") {
@@ -2746,19 +2714,19 @@ function Shell({
                 onAddPane={onAddPane}
                 onLayoutChange={(layout) => {
                   if (!activeTabKey) return;
-                  setWorkbenchState((current) => (
+                  updateWorkbenchState((current) => (
                     setWorkbenchLayout(current, activeTabKey, layout)
                   ));
                 }}
                 onPaneOrderChange={(paneKeys) => {
                   if (!activeTabKey) return;
-                  setWorkbenchState((current) => (
+                  updateWorkbenchState((current) => (
                     setWorkbenchPaneLayoutOrder(current, activeTabKey, paneKeys)
                   ));
                 }}
                 onSplitRatiosChange={(splitRatios) => {
                   if (!activeTabKey) return;
-                  setWorkbenchState((current) => (
+                  updateWorkbenchState((current) => (
                     setWorkbenchSplitRatios(current, activeTabKey, splitRatios)
                   ));
                 }}
diff --git a/webui/src/components/ChatList.tsx b/webui/src/components/ChatList.tsx
index 913a304f4..f87005a30 100644
--- a/webui/src/components/ChatList.tsx
+++ b/webui/src/components/ChatList.tsx
@@ -100,7 +100,7 @@ interface ChatListProps {
   onToggleArchive: (key: string) => void;
   paneGroups?: Record<string, SidebarPaneGroup>;
   onSelectPane?: (tabKey: string, paneKey: string) => void;
-  onCreateTab?: (tabKey: string) => void;
+  onCreateTab?: (paneKey: string) => void;
   onDetachPane?: (tabKey: string, paneKey: string) => void;
   onDissolveTab?: (tabKey: string) => void;
   onAttachPane?: (
@@ -710,7 +710,7 @@ export const ChatList = memo(function ChatList({
                                 {isArchived ? t("chat.unarchive") : t("chat.archive")}
                               </DropdownMenuItem>
                               {paneGroup && onCreateTab ? (
-                                <DropdownMenuItem onSelect={() => onCreateTab(paneGroup.tabKey)}>
+                                <DropdownMenuItem onSelect={() => onCreateTab(s.key)}>
                                   <PanelsTopLeft className="h-4 w-4 shrink-0" aria-hidden />
                                   {t("workbench.createGroup", {
                                     defaultValue: "Create group",
diff --git a/webui/src/components/Sidebar.tsx b/webui/src/components/Sidebar.tsx
index a300d3e21..28e5f5cd2 100644
--- a/webui/src/components/Sidebar.tsx
+++ b/webui/src/components/Sidebar.tsx
@@ -50,7 +50,7 @@ interface SidebarProps {
   onToggleArchive: (key: string) => void;
   paneGroups?: Record<string, SidebarPaneGroup>;
   onSelectPane?: (tabKey: string, paneKey: string) => void;
-  onCreateTab?: (tabKey: string) => void;
+  onCreateTab?: (paneKey: string) => void;
   onDetachPane?: (tabKey: string, paneKey: string) => void;
   onDissolveTab?: (tabKey: string) => void;
   onAttachPane?: (
diff --git a/webui/src/components/workbench/workbench-model.ts b/webui/src/components/workbench/workbench-model.ts
index 947938c77..6b8db2266 100644
--- a/webui/src/components/workbench/workbench-model.ts
+++ b/webui/src/components/workbench/workbench-model.ts
@@ -77,11 +77,6 @@ function normalizeTab(value: unknown): WorkbenchTabState {
     title: normalizeTitle(candidate.title),
     paneKeys,
     layoutPaneKeys,
-    activePaneKey:
-      typeof candidate.activePaneKey === "string"
-      && paneKeys.includes(candidate.activePaneKey)
-        ? candidate.activePaneKey
-        : paneKeys[0] ?? "",
     layout: isLayout(candidate.layout) ? candidate.layout : "columns",
     splitRatios: normalizeSplitRatios(candidate.splitRatios),
   };
@@ -111,7 +106,6 @@ function defaultWorkbenchTab(
     title: normalizeTitle(title),
     paneKeys: [paneKey],
     layoutPaneKeys: [paneKey],
-    activePaneKey: paneKey,
     layout: "columns",
     splitRatios: [],
   };
@@ -129,7 +123,9 @@ export function normalizeWorkbenchState(raw: unknown): WorkbenchState {
   return {
     version: 1,
     tabs: Object.fromEntries(
-      Object.entries(parsed.tabs).map(([tabKey, tab]) => [tabKey, normalizeTab(tab)]),
+      Object.entries(parsed.tabs)
+        .map(([tabKey, tab]) => [tabKey, normalizeTab(tab)] as const)
+        .filter(([, tab]) => tab.paneKeys.length > 1 || tab.explicit),
     ),
   };
 }
@@ -153,24 +149,6 @@ export function workbenchTabForPane(
   };
 }
 
-export function ensureWorkbenchPaneTab(
-  state: WorkbenchState,
-  paneKey: string,
-  title: string | null = null,
-): WorkbenchState {
-  if (!paneKey || Object.values(state.tabs).some((tab) => tab.paneKeys.includes(paneKey))) {
-    return state;
-  }
-  const tabKey = availableStandaloneTabKey(state.tabs, paneKey);
-  return {
-    version: 1,
-    tabs: {
-      ...state.tabs,
-      [tabKey]: defaultWorkbenchTab(paneKey, title),
-    },
-  };
-}
-
 function updateTab(
   state: WorkbenchState,
   tabKey: string,
@@ -191,31 +169,41 @@ function updateTab(
 
 export function addWorkbenchPane(
   state: WorkbenchState,
-  tabKey: string,
+  anchorPaneKey: string,
   paneKey: string,
 ): WorkbenchState {
-  return attachWorkbenchPane(state, tabKey, 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
-  ));
+  if (!anchorPaneKey || !paneKey || anchorPaneKey === paneKey) return state;
+  const target = workbenchTabForPane(state, anchorPaneKey);
+  if (state.tabs[target.tabKey]) return attachWorkbenchPane(state, target.tabKey, paneKey);
+  const withTarget = {
+    version: 1 as const,
+    tabs: {
+      ...state.tabs,
+      [target.tabKey]: target.tab,
+    },
+  };
+  return attachWorkbenchPane(withTarget, target.tabKey, paneKey);
 }
 
 export function createWorkbenchTab(
   state: WorkbenchState,
-  tabKey: string,
+  paneKey: string,
 ): WorkbenchState {
-  return updateTab(state, tabKey, (tab) => (
-    tab.explicit ? tab : { ...tab, explicit: true }
-  ));
+  if (!paneKey) return state;
+  const match = workbenchTabForPane(state, paneKey);
+  const persisted = state.tabs[match.tabKey];
+  if (persisted) {
+    return updateTab(state, match.tabKey, (tab) => (
+      tab.explicit ? tab : { ...tab, explicit: true }
+    ));
+  }
+  return {
+    version: 1,
+    tabs: {
+      ...state.tabs,
+      [match.tabKey]: { ...match.tab, explicit: true },
+    },
+  };
 }
 
 export function detachWorkbenchPane(
@@ -226,36 +214,25 @@ export function detachWorkbenchPane(
   const tab = state.tabs[tabKey];
   if (!tab || !tab.paneKeys.includes(paneKey)) return state;
   if (tab.paneKeys.length === 1) {
-    return tab.explicit
-      ? updateTab(state, tabKey, (current) => ({
-          ...current,
-          explicit: false,
-          title: null,
-          layout: "columns",
-          splitRatios: [],
-        }))
-      : state;
+    const tabs = { ...state.tabs };
+    delete tabs[tabKey];
+    return { version: 1, tabs };
   }
 
-  const index = tab.paneKeys.indexOf(paneKey);
   const paneKeys = tab.paneKeys.filter((key) => key !== paneKey);
   const layoutPaneKeys = tab.layoutPaneKeys.filter((key) => key !== paneKey);
-  const nextTabKey = availableStandaloneTabKey(state.tabs, paneKey);
+  const tabs = { ...state.tabs };
+  const nextTab = {
+    ...tab,
+    paneKeys,
+    layoutPaneKeys,
+    splitRatios: [],
+  };
+  if (nextTab.explicit || paneKeys.length > 1) tabs[tabKey] = nextTab;
+  else delete tabs[tabKey];
   return {
     version: 1,
-    tabs: {
-      ...state.tabs,
-      [tabKey]: {
-        ...tab,
-        paneKeys,
-        layoutPaneKeys,
-        activePaneKey: tab.activePaneKey === paneKey
-          ? paneKeys[Math.min(index, paneKeys.length - 1)]
-          : tab.activePaneKey,
-        splitRatios: [],
-      },
-      [nextTabKey]: defaultWorkbenchTab(paneKey),
-    },
+    tabs,
   };
 }
 
@@ -265,24 +242,8 @@ export function dissolveWorkbenchTab(
 ): WorkbenchState {
   const tab = state.tabs[tabKey];
   if (!tab) return state;
-  if (tab.paneKeys.length === 1) {
-    return tab.explicit
-      ? updateTab(state, tabKey, (current) => ({
-          ...current,
-          explicit: false,
-          title: null,
-          layout: "columns",
-          splitRatios: [],
-        }))
-      : state;
-  }
-
   const tabs = { ...state.tabs };
   delete tabs[tabKey];
-  for (const paneKey of tab.paneKeys) {
-    const standaloneTabKey = availableStandaloneTabKey(tabs, paneKey);
-    tabs[standaloneTabKey] = defaultWorkbenchTab(paneKey);
-  }
   return { version: 1, tabs };
 }
 
@@ -301,7 +262,7 @@ export function attachWorkbenchPane(
   const sourceTabKey = sourceEntry?.[0];
   const sourceTab = sourceEntry?.[1];
   if (sourceTabKey === targetTabKey) {
-    return focusWorkbenchPane(state, targetTabKey, paneKey);
+    return state;
   }
   if (!target.paneKeys.includes(paneKey) && target.paneKeys.length >= MAX_WORKBENCH_PANES) {
     return state;
@@ -309,19 +270,15 @@ export function attachWorkbenchPane(
 
   const tabs = { ...state.tabs };
   if (sourceTabKey && sourceTab) {
-    const index = sourceTab.paneKeys.indexOf(paneKey);
     const sourcePaneKeys = sourceTab.paneKeys.filter((key) => key !== paneKey);
     const sourceLayoutPaneKeys = sourceTab.layoutPaneKeys.filter((key) => key !== paneKey);
-    if (sourcePaneKeys.length === 0) {
+    if (sourcePaneKeys.length === 0 || (!sourceTab.explicit && sourcePaneKeys.length === 1)) {
       delete tabs[sourceTabKey];
     } else {
       tabs[sourceTabKey] = {
         ...sourceTab,
         paneKeys: sourcePaneKeys,
         layoutPaneKeys: sourceLayoutPaneKeys,
-        activePaneKey: sourceTab.activePaneKey === paneKey
-          ? sourcePaneKeys[Math.min(index, sourcePaneKeys.length - 1)]
-          : sourceTab.activePaneKey,
         splitRatios: [],
       };
     }
@@ -339,7 +296,6 @@ export function attachWorkbenchPane(
     ...nextTarget,
     paneKeys,
     layoutPaneKeys,
-    activePaneKey: paneKey,
     splitRatios: [],
   };
   return { version: 1, tabs };
@@ -410,28 +366,21 @@ export function reconcileWorkbench(
       .filter((key) => validKeys.has(key) && !claimedPaneKeys.has(key))
       .slice(0, MAX_WORKBENCH_PANES);
     if (paneKeys.length === 0) continue;
-    for (const paneKey of paneKeys) claimedPaneKeys.add(paneKey);
-    tabs[tabKey] = {
+    const nextTab = {
       ...tab,
       paneKeys,
       layoutPaneKeys: [
         ...tab.layoutPaneKeys.filter((key) => paneKeys.includes(key)),
         ...paneKeys.filter((key) => !tab.layoutPaneKeys.includes(key)),
       ],
-      activePaneKey: paneKeys.includes(tab.activePaneKey)
-        ? tab.activePaneKey
-        : paneKeys[0],
       splitRatios: paneKeys.length === tab.paneKeys.length
         && paneKeys.every((key, index) => key === tab.paneKeys[index])
         ? tab.splitRatios
         : [],
     };
-  }
-
-  for (const paneKey of validKeys) {
-    if (claimedPaneKeys.has(paneKey)) continue;
-    const tabKey = availableStandaloneTabKey(tabs, paneKey);
-    tabs[tabKey] = defaultWorkbenchTab(paneKey);
+    if (!nextTab.explicit && paneKeys.length === 1) continue;
+    for (const paneKey of paneKeys) claimedPaneKeys.add(paneKey);
+    tabs[tabKey] = nextTab;
   }
 
   return JSON.stringify(state.tabs) === JSON.stringify(tabs)
@@ -447,7 +396,16 @@ export function orderWorkbenchTabs(
   const rank = new Map(orderedSessionKeys.map((key, index) => [key, index]));
   const validKeys = new Set(orderedSessionKeys);
   const reconciled = reconcileWorkbench(state, validKeys);
-  const tabs = Object.entries(reconciled.tabs).map(([tabKey, tab]) => {
+  const projectedTabs = { ...reconciled.tabs };
+  const claimedPaneKeys = new Set(
+    Object.values(projectedTabs).flatMap((tab) => tab.paneKeys),
+  );
+  for (const paneKey of orderedSessionKeys) {
+    if (claimedPaneKeys.has(paneKey)) continue;
+    const tabKey = availableStandaloneTabKey(projectedTabs, paneKey);
+    projectedTabs[tabKey] = defaultWorkbenchTab(paneKey);
+  }
+  const tabs = Object.entries(projectedTabs).map(([tabKey, tab]) => {
     const paneKeys = tab.paneKeys
       .filter((key) => validKeys.has(key))
       .sort((left, right) => (rank.get(left) ?? Infinity) - (rank.get(right) ?? Infinity));
diff --git a/webui/src/hooks/useSidebarState.ts b/webui/src/hooks/useSidebarState.ts
index b937432de..47a8f54f0 100644
--- a/webui/src/hooks/useSidebarState.ts
+++ b/webui/src/hooks/useSidebarState.ts
@@ -230,6 +230,7 @@ export function useSidebarState(
   const update = useCallback(
     async (updater: (current: SidebarStatePayload) => SidebarStatePayload) => {
       const next = normalizeSidebarState(updater(stateRef.current));
+      if (sameState(next, stateRef.current)) return;
       stateRef.current = next;
       setState(next);
       persist(next);
diff --git a/webui/src/lib/types.ts b/webui/src/lib/types.ts
index 218b2dea5..9c056b29d 100644
--- a/webui/src/lib/types.ts
+++ b/webui/src/lib/types.ts
@@ -375,7 +375,6 @@ export interface WorkbenchTabState {
   title: string | null;
   paneKeys: string[];
   layoutPaneKeys: string[];
-  activePaneKey: string;
   layout: WorkbenchLayout;
   splitRatios: number[];
 }
diff --git a/webui/src/tests/app-layout.test.tsx b/webui/src/tests/app-layout.test.tsx
index dfc2e8007..f25daa1ca 100644
--- a/webui/src/tests/app-layout.test.tsx
+++ b/webui/src/tests/app-layout.test.tsx
@@ -3067,14 +3067,12 @@ describe("App layout", () => {
                   explicit: true,
                   title: "Alpha tab",
                   paneKeys: ["websocket:alpha", "websocket:alpha-child"],
-                  activePaneKey: "websocket:alpha-child",
                   layout: "columns",
                 },
                 "tab:websocket:beta": {
                   explicit: false,
                   title: null,
                   paneKeys: ["websocket:beta"],
-                  activePaneKey: "websocket:beta",
                   layout: "columns",
                 },
               },
diff --git a/webui/src/tests/chat-list.test.tsx b/webui/src/tests/chat-list.test.tsx
index ee67baca8..d24f0327f 100644
--- a/webui/src/tests/chat-list.test.tsx
+++ b/webui/src/tests/chat-list.test.tsx
@@ -122,8 +122,7 @@ describe("ChatList", () => {
       name: "Topic actions for Solo pane",
     }), { button: 0, ctrlKey: false });
     fireEvent.click(await screen.findByRole("menuitem", { name: "Create group" }));
-    expect(onCreateTab).toHaveBeenCalledWith("tab:solo");
-    expect(onAttachPane).not.toHaveBeenCalled();
+    expect(onCreateTab).toHaveBeenCalledWith("websocket:solo");
 
     fireEvent.pointerDown(screen.getByRole("button", {
       name: "Topic actions for Solo pane",
diff --git a/webui/src/tests/nanobot-client.test.ts b/webui/src/tests/nanobot-client.test.ts
index c90bc6dbc..48113a995 100644
--- a/webui/src/tests/nanobot-client.test.ts
+++ b/webui/src/tests/nanobot-client.test.ts
@@ -1267,7 +1267,6 @@ describe("NanobotClient", () => {
             explicit: true,
             title: "Research",
             paneKeys: ["websocket:a", "websocket:b"],
-            activePaneKey: "websocket:a",
             layout: "columns",
           },
         },
diff --git a/webui/src/tests/pane-workbench.test.tsx b/webui/src/tests/pane-workbench.test.tsx
index c889e7b9c..c2a4fce04 100644
--- a/webui/src/tests/pane-workbench.test.tsx
+++ b/webui/src/tests/pane-workbench.test.tsx
@@ -7,8 +7,6 @@ import { PaneWorkbench } from "@/components/workbench/PaneWorkbench";
 import {
   EMPTY_WORKBENCH_STATE,
   addWorkbenchPane,
-  ensureWorkbenchPaneTab,
-  focusWorkbenchPane,
   setWorkbenchLayout,
   setWorkbenchPaneLayoutOrder,
   workbenchTab,
@@ -38,14 +36,11 @@ function WorkbenchHarness({
   onPaneOrderChange?: (paneKeys: string[]) => void;
   onSplitRatiosChange?: (splitRatios: number[]) => void;
 } = {}) {
+  const [activePaneKey, setActivePaneKey] = useState("beta");
   const [state, setState] = useState(() => {
-    const initial = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "alpha");
+    const initial = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "alpha", "beta");
     const tabKey = workbenchTabForPane(initial, "alpha").tabKey;
-    return setWorkbenchLayout(
-      addWorkbenchPane(initial, tabKey, "beta"),
-      tabKey,
-      initialLayout,
-    );
+    return setWorkbenchLayout(initial, tabKey, initialLayout);
   });
   const tabKey = workbenchTabForPane(state, "alpha").tabKey;
   const tab = workbenchTab(state, tabKey);
@@ -55,13 +50,11 @@ function WorkbenchHarness({
   return (
     <PaneWorkbench
       panes={tab.layoutPaneKeys.map((key) => ({ key, title: titles[key] }))}
-      activePaneKey={tab.activePaneKey}
+      activePaneKey={activePaneKey}
       layout={tab.layout}
       splitRatios={tab.splitRatios}
       showLayoutControl
-      onActivatePane={(key) => setState((current) => (
-        focusWorkbenchPane(current, tabKey, key)
-      ))}
+      onActivatePane={setActivePaneKey}
       onAddPane={vi.fn()}
       onLayoutChange={(layout) => setState((current) => (
         setWorkbenchLayout(current, tabKey, layout)
diff --git a/webui/src/tests/workbench-model.test.ts b/webui/src/tests/workbench-model.test.ts
index 84ef9556d..6b1511b66 100644
--- a/webui/src/tests/workbench-model.test.ts
+++ b/webui/src/tests/workbench-model.test.ts
@@ -8,8 +8,6 @@ import {
   createWorkbenchTab,
   detachWorkbenchPane,
   dissolveWorkbenchTab,
-  ensureWorkbenchPaneTab,
-  focusWorkbenchPane,
   normalizeWorkbenchState,
   orderWorkbenchTabs,
   reconcileWorkbench,
@@ -19,152 +17,84 @@ import {
   setWorkbenchSplitRatios,
   workbenchTab,
   workbenchTabForPane,
-  type WorkbenchState,
 } from "@/components/workbench/workbench-model";
 
-function withPaneTab(
-  state: WorkbenchState,
-  paneKey: string,
-): [WorkbenchState, string] {
-  const next = ensureWorkbenchPaneTab(state, paneKey);
-  return [next, workbenchTabForPane(next, paneKey).tabKey];
-}
-
 describe("workbench model", () => {
-  it("creates a virtual tab whose identity is separate from its pane", () => {
-    const [state, tabKey] = withPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
+  it("derives standalone panes without persisting virtual tabs", () => {
+    const match = workbenchTabForPane(EMPTY_WORKBENCH_STATE, "pane-a");
 
-    expect(tabKey).not.toBe("pane-a");
-    expect(workbenchTab(state, tabKey)).toEqual({
+    expect(match.tabKey).not.toBe("pane-a");
+    expect(match.tab).toEqual({
       explicit: false,
       title: null,
       paneKeys: ["pane-a"],
       layoutPaneKeys: ["pane-a"],
-      activePaneKey: "pane-a",
       layout: "columns",
       splitRatios: [],
     });
+    expect(EMPTY_WORKBENCH_STATE.tabs).toEqual({});
   });
 
-  it("keeps pane membership, focus, title, and layout scoped to a tab", () => {
-    let state = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
-    const alphaTabKey = workbenchTabForPane(state, "pane-a").tabKey;
-    state = ensureWorkbenchPaneTab(state, "pane-b");
-    const betaTabKey = workbenchTabForPane(state, "pane-b").tabKey;
-    state = addWorkbenchPane(state, alphaTabKey, "pane-c");
-    state = setWorkbenchLayout(state, alphaTabKey, "main-stack");
-    state = renameWorkbenchTab(state, alphaTabKey, "Research");
+  it("persists only a visible singleton group", () => {
+    const state = createWorkbenchTab(EMPTY_WORKBENCH_STATE, "pane-a");
+    const match = workbenchTabForPane(state, "pane-a");
 
-    expect(workbenchTab(state, alphaTabKey)).toEqual({
+    expect(workbenchTab(state, match.tabKey)).toMatchObject({
+      explicit: true,
+      paneKeys: ["pane-a"],
+    });
+    expect(detachWorkbenchPane(state, match.tabKey, "pane-a").tabs).toEqual({});
+  });
+
+  it("materializes a group when a pane is added", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "pane-a", "pane-b");
+    const tabKey = workbenchTabForPane(state, "pane-a").tabKey;
+    state = setWorkbenchLayout(state, tabKey, "main-stack");
+    state = renameWorkbenchTab(state, tabKey, "Research");
+
+    expect(workbenchTab(state, tabKey)).toEqual({
       explicit: false,
       title: "Research",
-      paneKeys: ["pane-a", "pane-c"],
-      layoutPaneKeys: ["pane-a", "pane-c"],
-      activePaneKey: "pane-c",
+      paneKeys: ["pane-a", "pane-b"],
+      layoutPaneKeys: ["pane-a", "pane-b"],
       layout: "main-stack",
       splitRatios: [],
     });
-    expect(workbenchTab(state, betaTabKey)).toEqual({
-      explicit: false,
-      title: null,
-      paneKeys: ["pane-b"],
-      layoutPaneKeys: ["pane-b"],
-      activePaneKey: "pane-b",
-      layout: "columns",
-      splitRatios: [],
-    });
   });
 
-  it("focuses a pane without rewriting membership", () => {
-    let state = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
+  it("detaches a pane without persisting its standalone projection", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "pane-a", "pane-b");
     const tabKey = workbenchTabForPane(state, "pane-a").tabKey;
-    state = addWorkbenchPane(state, tabKey, "pane-b");
-    state = addWorkbenchPane(state, tabKey, "pane-c");
-    state = focusWorkbenchPane(state, tabKey, "pane-b");
-
-    expect(workbenchTab(state, tabKey)?.paneKeys).toEqual([
-      "pane-a",
-      "pane-b",
-      "pane-c",
-    ]);
-    expect(workbenchTab(state, tabKey)?.activePaneKey).toBe("pane-b");
-  });
-
-  it("detaches any pane into a new virtual tab", () => {
-    let state = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
-    const tabKey = workbenchTabForPane(state, "pane-a").tabKey;
-    state = addWorkbenchPane(state, tabKey, "pane-b");
-    state = addWorkbenchPane(state, tabKey, "pane-c");
-    state = focusWorkbenchPane(state, tabKey, "pane-a");
+    state = createWorkbenchTab(state, "pane-a");
+    state = addWorkbenchPane(state, "pane-a", "pane-c");
     state = detachWorkbenchPane(state, tabKey, "pane-a");
 
-    expect(workbenchTab(state, tabKey)).toMatchObject({
-      paneKeys: ["pane-b", "pane-c"],
-      activePaneKey: "pane-b",
-    });
-    const detached = workbenchTabForPane(state, "pane-a");
-    expect(detached.tabKey).not.toBe(tabKey);
-    expect(detached.tab.paneKeys).toEqual(["pane-a"]);
+    expect(workbenchTab(state, tabKey)?.paneKeys).toEqual(["pane-b", "pane-c"]);
+    expect(workbenchTabForPane(state, "pane-a").tab.paneKeys).toEqual(["pane-a"]);
+    expect(Object.values(state.tabs).some((tab) => tab.paneKeys.includes("pane-a"))).toBe(false);
   });
 
-  it("dissolves a tab into standalone panes without deleting them", () => {
-    let state = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
-    const tabKey = workbenchTabForPane(state, "pane-a").tabKey;
-    state = addWorkbenchPane(state, tabKey, "pane-b");
-    state = addWorkbenchPane(state, tabKey, "pane-c");
-    state = dissolveWorkbenchTab(state, tabKey);
+  it("dissolves a group into derived standalone panes", () => {
+    const grouped = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "pane-a", "pane-b");
+    const tabKey = workbenchTabForPane(grouped, "pane-a").tabKey;
+    const state = dissolveWorkbenchTab(grouped, tabKey);
 
-    expect(workbenchTab(state, tabKey)).toEqual({
-      explicit: false,
-      title: null,
-      paneKeys: ["pane-a"],
-      layoutPaneKeys: ["pane-a"],
-      activePaneKey: "pane-a",
-      layout: "columns",
-      splitRatios: [],
-    });
+    expect(state.tabs).toEqual({});
     expect(workbenchTabForPane(state, "pane-a").tab.paneKeys).toEqual(["pane-a"]);
     expect(workbenchTabForPane(state, "pane-b").tab.paneKeys).toEqual(["pane-b"]);
-    expect(workbenchTabForPane(state, "pane-c").tab.paneKeys).toEqual(["pane-c"]);
-    expect(Object.keys(state.tabs)).toHaveLength(3);
   });
 
-  it("makes a singleton tab visible without changing pane membership", () => {
-    let state = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
-    const tabKey = workbenchTabForPane(state, "pane-a").tabKey;
-
-    state = createWorkbenchTab(state, tabKey);
-    expect(workbenchTab(state, tabKey)).toMatchObject({
-      explicit: true,
-      paneKeys: ["pane-a"],
-      activePaneKey: "pane-a",
-    });
-
-    state = detachWorkbenchPane(state, tabKey, "pane-a");
-    expect(workbenchTab(state, tabKey)).toEqual({
-      explicit: false,
-      title: null,
-      paneKeys: ["pane-a"],
-      layoutPaneKeys: ["pane-a"],
-      activePaneKey: "pane-a",
-      layout: "columns",
-      splitRatios: [],
-    });
-  });
-
-  it("moves every pane symmetrically and removes an empty source tab", () => {
-    let state = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
-    const alphaTabKey = workbenchTabForPane(state, "pane-a").tabKey;
-    state = addWorkbenchPane(state, alphaTabKey, "pane-b");
-    state = ensureWorkbenchPaneTab(state, "pane-c");
+  it("moves panes symmetrically and removes an implicit singleton source", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "pane-a", "pane-b");
+    const sourceTabKey = workbenchTabForPane(state, "pane-a").tabKey;
+    state = createWorkbenchTab(state, "pane-c");
     const targetTabKey = workbenchTabForPane(state, "pane-c").tabKey;
 
     state = attachWorkbenchPane(state, targetTabKey, "pane-a");
-    expect(workbenchTab(state, alphaTabKey)?.paneKeys).toEqual(["pane-b"]);
+    expect(workbenchTab(state, sourceTabKey)).toBeNull();
     expect(workbenchTab(state, targetTabKey)?.paneKeys).toEqual(["pane-c", "pane-a"]);
 
     state = attachWorkbenchPane(state, targetTabKey, "pane-b");
-    expect(workbenchTab(state, alphaTabKey)).toBeNull();
     expect(workbenchTab(state, targetTabKey)?.paneKeys).toEqual([
       "pane-c",
       "pane-a",
@@ -172,11 +102,10 @@ describe("workbench model", () => {
     ]);
   });
 
-  it("keeps membership independent from projected display order", () => {
-    let state = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
+  it("keeps membership independent from workspace pane order", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "pane-a", "pane-b");
+    state = addWorkbenchPane(state, "pane-a", "pane-c");
     const tabKey = workbenchTabForPane(state, "pane-a").tabKey;
-    state = addWorkbenchPane(state, tabKey, "pane-b");
-    state = addWorkbenchPane(state, tabKey, "pane-c");
 
     const [ordered] = orderWorkbenchTabs(
       state,
@@ -187,73 +116,55 @@ describe("workbench model", () => {
     expect(ordered.paneKeys).toEqual(["pane-c", "pane-a", "pane-b"]);
 
     state = setWorkbenchPaneLayoutOrder(state, tabKey, ["pane-b", "pane-c", "pane-a"]);
-    expect(workbenchTab(state, tabKey)?.paneKeys).toEqual(["pane-a", "pane-b", "pane-c"]);
     expect(workbenchTab(state, tabKey)?.layoutPaneKeys).toEqual([
       "pane-b",
       "pane-c",
       "pane-a",
     ]);
-    expect(ordered.paneKeys).toEqual(["pane-c", "pane-a", "pane-b"]);
   });
 
-  it("stores resize ratios in the tab and resets them when its geometry changes", () => {
-    let state = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
+  it("stores resize ratios and resets them when geometry changes", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "pane-a", "pane-b");
     const tabKey = workbenchTabForPane(state, "pane-a").tabKey;
-    state = addWorkbenchPane(state, tabKey, "pane-b");
     state = setWorkbenchSplitRatios(state, tabKey, [0.35]);
 
     expect(workbenchTab(state, tabKey)?.splitRatios).toEqual([0.35]);
     state = setWorkbenchLayout(state, tabKey, "rows");
     expect(workbenchTab(state, tabKey)?.splitRatios).toEqual([]);
-
-    state = setWorkbenchSplitRatios(state, tabKey, [0.4]);
-    state = detachWorkbenchPane(state, tabKey, "pane-b");
-    expect(workbenchTab(state, tabKey)?.splitRatios).toEqual([]);
   });
 
-  it("keeps each tab contiguous and ranks it by its latest updated pane", () => {
-    let state = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
+  it("keeps groups contiguous and ranks them by their latest pane", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "pane-a", "pane-c");
+    state = addWorkbenchPane(state, "pane-b", "pane-d");
     const alphaTabKey = workbenchTabForPane(state, "pane-a").tabKey;
-    state = addWorkbenchPane(state, alphaTabKey, "pane-c");
-    state = ensureWorkbenchPaneTab(state, "pane-b");
     const betaTabKey = workbenchTabForPane(state, "pane-b").tabKey;
-    state = addWorkbenchPane(state, betaTabKey, "pane-d");
 
     const tabs = orderWorkbenchTabs(
       state,
-      ["pane-d", "pane-c", "pane-b", "pane-a"],
+      ["pane-d", "pane-c", "pane-b", "pane-a", "pane-e"],
       new Map([
         ["pane-a", "2026-08-01T10:00:00Z"],
         ["pane-b", "2026-08-03T10:00:00Z"],
         ["pane-c", "2026-08-05T10:00:00Z"],
         ["pane-d", "2026-08-04T10:00:00Z"],
+        ["pane-e", "2026-08-02T10:00:00Z"],
       ]),
     );
 
-    expect(tabs.map(({ tabKey, paneKeys, updatedAt }) => ({
-      tabKey,
-      paneKeys,
-      updatedAt,
-    }))).toEqual([
-      {
-        tabKey: alphaTabKey,
-        paneKeys: ["pane-c", "pane-a"],
-        updatedAt: "2026-08-05T10:00:00Z",
-      },
-      {
-        tabKey: betaTabKey,
-        paneKeys: ["pane-d", "pane-b"],
-        updatedAt: "2026-08-04T10:00:00Z",
-      },
+    expect(tabs.map(({ tabKey, paneKeys }) => ({ tabKey, paneKeys }))).toEqual([
+      { tabKey: alphaTabKey, paneKeys: ["pane-c", "pane-a"] },
+      { tabKey: betaTabKey, paneKeys: ["pane-d", "pane-b"] },
+      { tabKey: workbenchTabForPane(state, "pane-e").tabKey, paneKeys: ["pane-e"] },
     ]);
+    expect(Object.keys(state.tabs)).toHaveLength(2);
   });
 
-  it("caps every virtual tab at four panes", () => {
-    let state = ensureWorkbenchPaneTab(EMPTY_WORKBENCH_STATE, "pane-a");
-    const tabKey = workbenchTabForPane(state, "pane-a").tabKey;
-    for (let index = 1; index <= MAX_WORKBENCH_PANES; index += 1) {
-      state = addWorkbenchPane(state, tabKey, `pane-${index}`);
+  it("caps a group at four panes", () => {
+    let state = addWorkbenchPane(EMPTY_WORKBENCH_STATE, "pane-a", "pane-1");
+    for (let index = 2; index <= MAX_WORKBENCH_PANES; index += 1) {
+      state = addWorkbenchPane(state, "pane-a", `pane-${index}`);
     }
+    const tabKey = workbenchTabForPane(state, "pane-a").tabKey;
     expect(workbenchTab(state, tabKey)?.paneKeys).toEqual([
       "pane-a",
       "pane-1",
@@ -262,21 +173,23 @@ describe("workbench model", () => {
     ]);
   });
 
-  it("repairs duplicates, removes deleted panes, and creates missing tabs", () => {
+  it("repairs persisted groups without materializing missing sessions", () => {
     const state = normalizeWorkbenchState({
       version: 1,
       tabs: {
         alpha: {
           title: "Alpha",
           paneKeys: ["pane-a", "pane-b", "pane-b", 9],
-          activePaneKey: "missing",
           layout: "unknown",
         },
         duplicate: {
           paneKeys: ["pane-b", "deleted"],
-          activePaneKey: "pane-b",
           layout: "grid",
         },
+        invisible: {
+          paneKeys: ["pane-c"],
+          layout: "columns",
+        },
       },
     });
     const reconciled = reconcileWorkbench(
@@ -289,12 +202,19 @@ describe("workbench model", () => {
       title: "Alpha",
       paneKeys: ["pane-a", "pane-b"],
       layoutPaneKeys: ["pane-a", "pane-b"],
-      activePaneKey: "pane-a",
       layout: "columns",
       splitRatios: [],
     });
-    expect(workbenchTab(reconciled, "duplicate")).toBeNull();
+    expect(Object.keys(reconciled.tabs)).toEqual(["alpha"]);
     expect(workbenchTabForPane(reconciled, "pane-c").tab.paneKeys).toEqual(["pane-c"]);
   });
 
+  it("keeps persisted state sparse with thousands of standalone sessions", () => {
+    const sessionKeys = Array.from({ length: 2_000 }, (_, index) => `pane-${index}`);
+    const reconciled = reconcileWorkbench(EMPTY_WORKBENCH_STATE, new Set(sessionKeys));
+    const ordered = orderWorkbenchTabs(reconciled, sessionKeys, new Map());
+
+    expect(reconciled.tabs).toEqual({});
+    expect(ordered).toHaveLength(2_000);
+  });
 });