diff --git a/refactor.md b/refactor.md
index 9f74241..6230f0e 100644
--- a/refactor.md
+++ b/refactor.md
@@ -132,3 +132,12 @@ This branch captures local refactors focused on frontend UX polish, IPC call con
 ### 17. External gateway shutdown compatibility
 - Added capability cache for externally managed Gateway shutdown RPC.
 - If `shutdown` is unsupported (`unknown method: shutdown`), mark it unsupported and skip future shutdown RPC attempts to avoid repeated warnings.
+
+### 18. Chat history sidebar grouping (ChatGPT-style buckets)
+- Updated chat session history display in sidebar to time buckets:
+  - Today / Yesterday / Within 1 Week / Within 2 Weeks / Within 1 Month / Older than 1 Month
+- Added `historyBuckets` locale keys in EN/ZH/JA (`chat` namespace).
+- Fixed i18n namespace usage for bucket labels in sidebar:
+  - explicitly resolves via `chat:historyBuckets.*` to avoid raw key fallback.
+- Removed forced uppercase rendering for bucket headers to preserve localized casing.
+- Grouping now applies to all sessions (including `:main`) for consistent bucket visibility and behavior.
diff --git a/src/components/layout/Sidebar.tsx b/src/components/layout/Sidebar.tsx
index 19d6f49..f857ccd 100644
--- a/src/components/layout/Sidebar.tsx
+++ b/src/components/layout/Sidebar.tsx
@@ -27,6 +27,14 @@ import { ConfirmDialog } from '@/components/ui/confirm-dialog';
 import { invokeIpc } from '@/lib/api-client';
 import { useTranslation } from 'react-i18next';
 
+type SessionBucketKey =
+  | 'today'
+  | 'yesterday'
+  | 'withinWeek'
+  | 'withinTwoWeeks'
+  | 'withinMonth'
+  | 'older';
+
 interface NavItemProps {
   to: string;
   icon: React.ReactNode;
@@ -67,6 +75,23 @@ function NavItem({ to, icon, label, badge, collapsed, onClick }: NavItemProps) {
   );
 }
 
+function getSessionBucket(activityMs: number, nowMs: number): SessionBucketKey {
+  if (!activityMs || activityMs <= 0) return 'older';
+
+  const now = new Date(nowMs);
+  const startOfToday = new Date(now.getFullYear(), now.getMonth(), now.getDate()).getTime();
+  const startOfYesterday = startOfToday - 24 * 60 * 60 * 1000;
+
+  if (activityMs >= startOfToday) return 'today';
+  if (activityMs >= startOfYesterday) return 'yesterday';
+
+  const daysAgo = (startOfToday - activityMs) / (24 * 60 * 60 * 1000);
+  if (daysAgo <= 7) return 'withinWeek';
+  if (daysAgo <= 14) return 'withinTwoWeeks';
+  if (daysAgo <= 30) return 'withinMonth';
+  return 'older';
+}
+
 export function Sidebar() {
   const sidebarCollapsed = useSettingsStore((state) => state.sidebarCollapsed);
   const setSidebarCollapsed = useSettingsStore((state) => state.setSidebarCollapsed);
@@ -83,9 +108,6 @@ export function Sidebar() {
   const navigate = useNavigate();
   const isOnChat = useLocation().pathname === '/';
 
-  const mainSessions = sessions.filter((s) => s.key.endsWith(':main'));
-  const otherSessions = sessions.filter((s) => !s.key.endsWith(':main'));
-
   const getSessionLabel = (key: string, displayName?: string, label?: string) =>
     sessionLabels[key] ?? label ?? displayName ?? key;
 
@@ -106,8 +128,28 @@ export function Sidebar() {
     }
   };
 
-  const { t } = useTranslation();
+  const { t } = useTranslation(['common', 'chat']);
   const [sessionToDelete, setSessionToDelete] = useState<{ key: string; label: string } | null>(null);
+  const nowMs = Date.now();
+  const sessionBuckets: Array<{ key: SessionBucketKey; label: string; sessions: typeof sessions }> = [
+    { key: 'today', label: t('chat:historyBuckets.today'), sessions: [] },
+    { key: 'yesterday', label: t('chat:historyBuckets.yesterday'), sessions: [] },
+    { key: 'withinWeek', label: t('chat:historyBuckets.withinWeek'), sessions: [] },
+    { key: 'withinTwoWeeks', label: t('chat:historyBuckets.withinTwoWeeks'), sessions: [] },
+    { key: 'withinMonth', label: t('chat:historyBuckets.withinMonth'), sessions: [] },
+    { key: 'older', label: t('chat:historyBuckets.older'), sessions: [] },
+  ];
+  const sessionBucketMap = Object.fromEntries(sessionBuckets.map((bucket) => [bucket.key, bucket])) as Record<
+    SessionBucketKey,
+    (typeof sessionBuckets)[number]
+  >;
+
+  for (const session of [...sessions].sort((a, b) =>
+    (sessionLastActivity[b.key] ?? 0) - (sessionLastActivity[a.key] ?? 0)
+  )) {
+    const bucketKey = getSessionBucket(sessionLastActivity[session.key] ?? 0, nowMs);
+    sessionBucketMap[bucketKey].sessions.push(session);
+  }
 
   const navItems = [
     { to: '/cron', icon: <Clock className="h-5 w-5" />, label: t('sidebar.cronTasks') },
@@ -154,43 +196,47 @@ export function Sidebar() {
         {/* Session list — below Settings, only when expanded */}
         {!sidebarCollapsed && sessions.length > 0 && (
           <div className="mt-1 overflow-y-auto max-h-72 space-y-0.5">
-            {[...mainSessions, ...[...otherSessions].sort((a, b) =>
-              (sessionLastActivity[b.key] ?? 0) - (sessionLastActivity[a.key] ?? 0)
-            )].map((s) => (
-              <div key={s.key} className="group relative flex items-center">
-                <button
-                  onClick={() => { switchSession(s.key); navigate('/'); }}
-                  className={cn(
-                    'w-full text-left rounded-md px-3 py-1.5 text-sm truncate transition-colors',
-                    !s.key.endsWith(':main') && 'pr-7',
-                    'hover:bg-accent hover:text-accent-foreground',
-                    isOnChat && currentSessionKey === s.key
-                      ? 'bg-accent/60 text-accent-foreground font-medium'
-                      : 'text-muted-foreground',
-                  )}
-                >
-                  {getSessionLabel(s.key, s.displayName, s.label)}
-                </button>
-                {!s.key.endsWith(':main') && (
-                  <button
-                    aria-label="Delete session"
-                    onClick={(e) => {
-                      e.stopPropagation();
-                      setSessionToDelete({
-                        key: s.key,
-                        label: getSessionLabel(s.key, s.displayName, s.label),
-                      });
-                    }}
-                    className={cn(
-                      'absolute right-1 flex items-center justify-center rounded p-0.5 transition-opacity',
-                      'opacity-0 group-hover:opacity-100',
-                      'text-muted-foreground hover:text-destructive hover:bg-destructive/10',
-                    )}
-                  >
-                    <Trash2 className="h-3.5 w-3.5" />
-                  </button>
-                )}
-              </div>
+            {sessionBuckets.map((bucket) => (
+              bucket.sessions.length > 0 ? (
+                <div key={bucket.key} className="pt-1">
+                  <div className="px-3 py-1 text-[11px] font-medium text-muted-foreground/80">
+                    {bucket.label}
+                  </div>
+                  {bucket.sessions.map((s) => (
+                    <div key={s.key} className="group relative flex items-center">
+                      <button
+                        onClick={() => { switchSession(s.key); navigate('/'); }}
+                        className={cn(
+                          'w-full text-left rounded-md px-3 py-1.5 text-sm truncate transition-colors pr-7',
+                          'hover:bg-accent hover:text-accent-foreground',
+                          isOnChat && currentSessionKey === s.key
+                            ? 'bg-accent/60 text-accent-foreground font-medium'
+                            : 'text-muted-foreground',
+                        )}
+                      >
+                        {getSessionLabel(s.key, s.displayName, s.label)}
+                      </button>
+                      <button
+                        aria-label="Delete session"
+                        onClick={(e) => {
+                          e.stopPropagation();
+                          setSessionToDelete({
+                            key: s.key,
+                            label: getSessionLabel(s.key, s.displayName, s.label),
+                          });
+                        }}
+                        className={cn(
+                          'absolute right-1 flex items-center justify-center rounded p-0.5 transition-opacity',
+                          'opacity-0 group-hover:opacity-100',
+                          'text-muted-foreground hover:text-destructive hover:bg-destructive/10',
+                        )}
+                      >
+                        <Trash2 className="h-3.5 w-3.5" />
+                      </button>
+                    </div>
+                  ))}
+                </div>
+              ) : null
             ))}
           </div>
         )}
diff --git a/src/i18n/locales/en/chat.json b/src/i18n/locales/en/chat.json
index af9d58a..8d82e0d 100644
--- a/src/i18n/locales/en/chat.json
+++ b/src/i18n/locales/en/chat.json
@@ -14,5 +14,13 @@
         "refresh": "Refresh chat",
         "showThinking": "Show thinking",
         "hideThinking": "Hide thinking"
+    },
+    "historyBuckets": {
+        "today": "Today",
+        "yesterday": "Yesterday",
+        "withinWeek": "Within 1 Week",
+        "withinTwoWeeks": "Within 2 Weeks",
+        "withinMonth": "Within 1 Month",
+        "older": "Older than 1 Month"
     }
-}
\ No newline at end of file
+}
diff --git a/src/i18n/locales/ja/chat.json b/src/i18n/locales/ja/chat.json
index 6904fa7..bdfd543 100644
--- a/src/i18n/locales/ja/chat.json
+++ b/src/i18n/locales/ja/chat.json
@@ -14,5 +14,13 @@
         "refresh": "チャットを更新",
         "showThinking": "思考を表示",
         "hideThinking": "思考を非表示"
+    },
+    "historyBuckets": {
+        "today": "今日",
+        "yesterday": "昨日",
+        "withinWeek": "1週間以内",
+        "withinTwoWeeks": "2週間以内",
+        "withinMonth": "1か月以内",
+        "older": "1か月より前"
     }
-}
\ No newline at end of file
+}
diff --git a/src/i18n/locales/zh/chat.json b/src/i18n/locales/zh/chat.json
index 21a7024..84031e1 100644
--- a/src/i18n/locales/zh/chat.json
+++ b/src/i18n/locales/zh/chat.json
@@ -14,5 +14,13 @@
         "refresh": "刷新聊天",
         "showThinking": "显示思考过程",
         "hideThinking": "隐藏思考过程"
+    },
+    "historyBuckets": {
+        "today": "今天",
+        "yesterday": "昨天",
+        "withinWeek": "一周内",
+        "withinTwoWeeks": "两周内",
+        "withinMonth": "一个月内",
+        "older": "一个月之前"
     }
-}
\ No newline at end of file
+}