diff --git a/web/oss/src/components/AgentChatSlice/AgentConversation.tsx b/web/oss/src/components/AgentChatSlice/AgentConversation.tsx
index 67886275eb..c681d84c20 100644
--- a/web/oss/src/components/AgentChatSlice/AgentConversation.tsx
+++ b/web/oss/src/components/AgentChatSlice/AgentConversation.tsx
@@ -149,6 +149,7 @@ import {
     type SessionRunStatus,
     activeSessionIdAtomFamily,
     autoTitleSessionAtomFamily,
+    bumpSessionActivityAtomFamily,
     firstUserText,
     persistSessionMessagesAtom,
     sessionMessagesAtom,
@@ -791,6 +792,7 @@ const AgentConversation = ({
     // (cross-tab / cross-device) before it's opened. The atom no-ops once the session has a title,
     // so this fires at most once and never overwrites an explicit rename.
     const autoTitleSession = useSetAtom(autoTitleSessionAtomFamily(scopeKey))
+    const bumpSessionActivity = useSetAtom(bumpSessionActivityAtomFamily(scopeKey))
     const firstUserMessage = useMemo(() => firstUserText(messages), [messages])
     useEffect(() => {
         if (firstUserMessage) autoTitleSession({id: sessionId, text: firstUserMessage})
@@ -1257,6 +1259,17 @@ const AgentConversation = ({
         persistMessages({id: sessionId, messages})
     }, [messages, status, sessionId, persistMessages])
 
+    // Stamp last-message time when a live turn finishes streaming (issue #5553: order history by
+    // last message). Gated on the streaming→settled transition so hydration/restore — which sets
+    // messages while `status` stays "ready" — never back-dates an old session to "now".
+    const prevStatusRef = useRef(status)
+    useEffect(() => {
+        if (prevStatusRef.current === "streaming" && status !== "streaming") {
+            bumpSessionActivity(sessionId)
+        }
+        prevStatusRef.current = status
+    }, [status, sessionId, bumpSessionActivity])
+
     // Bound the in-message expand-state store: on settle, drop entries whose owning message is gone
     // (rewound / evicted / closed). Live = every open session's persisted messages ∪ this active one.
     // `store.get` reads without subscribing, so this never adds re-renders on the streaming hot path.
diff --git a/web/oss/src/components/AgentChatSlice/components/SessionHistoryMenu.tsx b/web/oss/src/components/AgentChatSlice/components/SessionHistoryMenu.tsx
index cb5466d530..cfd410cb53 100644
--- a/web/oss/src/components/AgentChatSlice/components/SessionHistoryMenu.tsx
+++ b/web/oss/src/components/AgentChatSlice/components/SessionHistoryMenu.tsx
@@ -108,7 +108,7 @@ const SessionHistoryRow = ({
                             Ended
                         </span>
                     )}
-                    {timeAgo(session.createdAt)}
+                    {timeAgo(session.lastMessageAt ?? session.createdAt)}
                 </Text>
             </div>
             {!archived && nest.isAlive && (
diff --git a/web/oss/src/components/AgentChatSlice/components/SessionRail.tsx b/web/oss/src/components/AgentChatSlice/components/SessionRail.tsx
index 1f68ff96a6..5c441d212a 100644
--- a/web/oss/src/components/AgentChatSlice/components/SessionRail.tsx
+++ b/web/oss/src/components/AgentChatSlice/components/SessionRail.tsx
@@ -173,14 +173,14 @@ const SessionRailRow = memo(function SessionRailRow({
                             active ? "text-colorText" : "text-colorTextSecondary",
                         )}
                     />
-                    {(session.ended || timeAgo(session.createdAt)) && (
+                    {(session.ended || timeAgo(session.lastMessageAt ?? session.createdAt)) && (
                         <span className="flex items-center gap-1.5 text-[11px] text-colorTextTertiary">
                             {session.ended && (
                                 <span className="rounded bg-colorFillTertiary px-1 text-[10px] leading-4">
                                     Ended
                                 </span>
                             )}
-                            {timeAgo(session.createdAt)}
+                            {timeAgo(session.lastMessageAt ?? session.createdAt)}
                         </span>
                     )}
                 </div>
diff --git a/web/oss/src/components/AgentChatSlice/state/projectSessions.ts b/web/oss/src/components/AgentChatSlice/state/projectSessions.ts
index 3ae0bc220b..c5ba06b222 100644
--- a/web/oss/src/components/AgentChatSlice/state/projectSessions.ts
+++ b/web/oss/src/components/AgentChatSlice/state/projectSessions.ts
@@ -74,6 +74,8 @@ const toSummary = (s: SessionStream): ServerSessionSummary => ({
     id: s.session_id,
     title: s.name?.trim() ? s.name : undefined,
     createdAt: s.created_at ? Date.parse(s.created_at) || undefined : undefined,
+    // Heartbeat `updated_at` (falls back to created_at) = last-activity time for ordering history.
+    lastMessageAt: activity(s) || undefined,
     // A soft-deleted stream row is a killed/ended session (still resumable) — the list includes it
     // via include_ended; the sidebar shows it muted.
     ended: Boolean(s.deleted_at),
diff --git a/web/oss/src/components/AgentChatSlice/state/sessions.ts b/web/oss/src/components/AgentChatSlice/state/sessions.ts
index c150e90384..ca270857ae 100644
--- a/web/oss/src/components/AgentChatSlice/state/sessions.ts
+++ b/web/oss/src/components/AgentChatSlice/state/sessions.ts
@@ -42,8 +42,11 @@ export interface AgentChatSession {
     id: string
     /** User-set title. When empty, the UI falls back to the first user message / "Chat N". */
     title?: string
-    /** Creation time (ms epoch). Orders the history picker; absent on pre-upgrade sessions. */
+    /** Creation time (ms epoch). Fallback ordering key; absent on pre-upgrade sessions. */
     createdAt?: number
+    /** Last-message time (ms epoch): server heartbeat `updated_at`, or a local turn settling.
+     * Primary ordering key for the history picker so recently-active chats float to the top. */
+    lastMessageAt?: number
     /** Set once the server list confirms this session exists. Distinguishes a remotely-deleted
      * session (was true, now absent from the server → drop) from a purely-local optimistic one. */
     serverKnown?: boolean
@@ -145,20 +148,24 @@ export const isSessionHusk = (
     messages: Record<string, UIMessage[]>,
 ): boolean => !session.serverKnown && !session.title?.trim() && !messages[session.id]?.length
 
-/** Active (non-archived) sessions for a scope, newest first. Backs the main history picker. */
+/** Ordering key: most-recent message first, falling back to creation time, then 0 (pre-upgrade
+ * sessions with neither sort last, preserving their order). */
+const sessionActivity = (s: AgentChatSession): number => s.lastMessageAt ?? s.createdAt ?? 0
+
+/** Active (non-archived) sessions for a scope, most-recently-active first. Backs the main history
+ * picker (see issue #5553: order by last message, not creation). */
 export const sessionHistoryAtomFamily = atomFamily((key: string) =>
     atom((get) => {
         const list = (get(sessionsByAppAtom)[key] ?? []).filter((s) => !s.archived)
-        // Newest first; pre-upgrade sessions (no createdAt) sort last, preserving their order.
-        return [...list].sort((a, b) => (b.createdAt ?? 0) - (a.createdAt ?? 0))
+        return [...list].sort((a, b) => sessionActivity(b) - sessionActivity(a))
     }),
 )
 
-/** Archived sessions for a scope, newest first. Backs the archived view. */
+/** Archived sessions for a scope, most-recently-active first. Backs the archived view. */
 export const archivedSessionHistoryAtomFamily = atomFamily((key: string) =>
     atom((get) => {
         const list = (get(sessionsByAppAtom)[key] ?? []).filter((s) => s.archived)
-        return [...list].sort((a, b) => (b.createdAt ?? 0) - (a.createdAt ?? 0))
+        return [...list].sort((a, b) => sessionActivity(b) - sessionActivity(a))
     }),
 )
 
@@ -374,6 +381,8 @@ export interface ServerSessionSummary {
     id: string
     title?: string
     createdAt?: number
+    /** Server heartbeat `updated_at` (ms epoch) — the last-activity time used to order history. */
+    lastMessageAt?: number
     ended?: boolean
     archived?: boolean
 }
@@ -405,6 +414,10 @@ export const reconcileServerSessionsAtomFamily = atomFamily((key: string) =>
                     serverKnown: true,
                     title: s.title?.trim() ? s.title : remote.title,
                     createdAt: s.createdAt ?? remote.createdAt,
+                    // Keep the freshest activity time: a local turn just settled may lead the
+                    // server heartbeat, and vice-versa across devices.
+                    lastMessageAt:
+                        Math.max(s.lastMessageAt ?? 0, remote.lastMessageAt ?? 0) || undefined,
                     ended: remote.ended,
                     archived: remote.archived,
                 })
@@ -420,6 +433,7 @@ export const reconcileServerSessionsAtomFamily = atomFamily((key: string) =>
                 id: s.id,
                 title: s.title,
                 createdAt: s.createdAt,
+                lastMessageAt: s.lastMessageAt,
                 serverKnown: true,
                 ended: s.ended,
                 archived: s.archived,
@@ -435,6 +449,7 @@ export const reconcileServerSessionsAtomFamily = atomFamily((key: string) =>
                     e.id !== m.id ||
                     e.title !== m.title ||
                     e.createdAt !== m.createdAt ||
+                    e.lastMessageAt !== m.lastMessageAt ||
                     e.serverKnown !== m.serverKnown ||
                     e.ended !== m.ended ||
                     e.archived !== m.archived
@@ -597,6 +612,27 @@ export const autoTitleSessionAtomFamily = atomFamily((key: string) =>
     }),
 )
 
+/** Stamp `now` as a session's last-message time (a local turn just settled), so the history picker
+ * floats it to the top immediately — ahead of the next server heartbeat/reconcile. No-op if the id
+ * isn't in this scope's history yet. */
+export const bumpSessionActivityAtomFamily = atomFamily((key: string) =>
+    atom(null, (get, set, id: string) => {
+        const all = get(sessionsByAppAtom)
+        const list = all[key] ?? []
+        if (!list.some((s) => s.id === id)) return
+        // Keep the freshest time: a server heartbeat may already lead the local clock (skew).
+        const now = Date.now()
+        set(sessionsByAppAtom, {
+            ...all,
+            [key]: list.map((s) =>
+                s.id === id
+                    ? {...s, lastMessageAt: Math.max(s.lastMessageAt ?? 0, now) || undefined}
+                    : s,
+            ),
+        })
+    }),
+)
+
 export const setActiveSessionAtomFamily = atomFamily((key: string) =>
     atom(null, (get, set, id: string) => {
         set(activeByAppAtom, {...get(activeByAppAtom), [key]: id})