From 26a58282d4ff2440512aada1759ac91634328f3e Mon Sep 17 00:00:00 2001
From: Xubin Ren <52506698+Re-bin@users.noreply.github.com>
Date: Wed, 10 Jun 2026 02:07:47 +0800
Subject: [PATCH] feat(webui): show forked history boundary

---
 webui/src/App.tsx                             |  13 +-
 webui/src/components/MessageBubble.tsx        |   4 +-
 .../src/components/thread/ThreadMessages.tsx  | 111 +++++++++++-------
 webui/src/components/thread/ThreadShell.tsx   |   2 +
 .../src/components/thread/ThreadViewport.tsx  |   7 ++
 webui/src/hooks/useSessions.ts                |  22 +++-
 webui/src/i18n/locales/en/common.json         |  14 ++-
 webui/src/i18n/locales/es/common.json         |  14 ++-
 webui/src/i18n/locales/fr/common.json         |  14 ++-
 webui/src/i18n/locales/id/common.json         |  14 ++-
 webui/src/i18n/locales/ja/common.json         |  14 ++-
 webui/src/i18n/locales/ko/common.json         |  14 ++-
 webui/src/i18n/locales/vi/common.json         |  14 ++-
 webui/src/i18n/locales/zh-CN/common.json      |  14 ++-
 webui/src/i18n/locales/zh-TW/common.json      |  14 ++-
 webui/src/lib/nanobot-client.ts               |   2 +
 webui/src/lib/types.ts                        |   3 +-
 webui/src/tests/message-bubble.test.tsx       |  26 ++--
 webui/src/tests/thread-messages.test.tsx      |  21 +++-
 webui/src/tests/thread-shell.test.tsx         |   8 +-
 webui/src/tests/useSessions.test.tsx          |  18 +++
 21 files changed, 242 insertions(+), 121 deletions(-)

diff --git a/webui/src/App.tsx b/webui/src/App.tsx
index 33c24ccc8..70c6ef6cf 100644
--- a/webui/src/App.tsx
+++ b/webui/src/App.tsx
@@ -29,6 +29,7 @@ import {
   loadSavedSecret,
   saveSecret,
 } from "@/lib/bootstrap";
+import { displayTitle } from "@/lib/chat-groups";
 import { deriveTitle } from "@/lib/format";
 import { NanobotClient } from "@/lib/nanobot-client";
 import { ClientProvider, useClient } from "@/providers/ClientProvider";
@@ -890,7 +891,15 @@ function Shell({
     beforeUserIndex: number,
   ) => {
     try {
-      const chatId = await forkChat(sourceChatId, beforeUserIndex);
+      const sourceSession = sessions.find((session) => session.chatId === sourceChatId);
+      const sourceTitle = sourceSession
+        ? displayTitle(sourceSession, sidebarState.title_overrides, t("chat.newChat"))
+        : t("chat.newChat");
+      const chatId = await forkChat(
+        sourceChatId,
+        beforeUserIndex,
+        t("chat.forkTitle", { title: sourceTitle }),
+      );
       navigate({
         view: "chat",
         activeKey: `websocket:${chatId}`,
@@ -902,7 +911,7 @@ function Shell({
       console.error("Failed to fork chat", e);
       return null;
     }
-  }, [forkChat, navigate]);
+  }, [forkChat, navigate, sessions, sidebarState.title_overrides, t]);
 
   const onNewChat = useCallback(() => {
     navigate(defaultShellRoute());
diff --git a/webui/src/components/MessageBubble.tsx b/webui/src/components/MessageBubble.tsx
index 39b61911e..9449a7199 100644
--- a/webui/src/components/MessageBubble.tsx
+++ b/webui/src/components/MessageBubble.tsx
@@ -117,8 +117,8 @@ export function MessageBubble({
     const showUserActions = hasText;
     const timeLabel = formatMessageClock(message.createdAt);
     const copyLabel = copied
-      ? t("message.copiedMessage", { defaultValue: "Copied message" })
-      : t("message.copyMessage", { defaultValue: "Copy message" });
+      ? t("message.copiedMessage", { defaultValue: "Copied" })
+      : t("message.copyMessage", { defaultValue: "Copy" });
     return (
       <div
         className={cn(
diff --git a/webui/src/components/thread/ThreadMessages.tsx b/webui/src/components/thread/ThreadMessages.tsx
index f7f481ede..d1fdba0be 100644
--- a/webui/src/components/thread/ThreadMessages.tsx
+++ b/webui/src/components/thread/ThreadMessages.tsx
@@ -1,4 +1,4 @@
-import { useMemo } from "react";
+import { Fragment, useMemo } from "react";
 import { useTranslation } from "react-i18next";
 
 import { MessageBubble } from "@/components/MessageBubble";
@@ -15,6 +15,7 @@ interface ThreadMessagesProps {
   onLoadEarlier?: () => void;
   cliApps?: CliAppInfo[];
   mcpPresets?: McpPresetInfo[];
+  forkBoundaryMessageCount?: number | null;
   onOpenFilePreview?: (path: string) => void;
   onForkFromMessage?: (beforeUserIndex: number) => void;
 }
@@ -70,11 +71,16 @@ export function ThreadMessages({
   onLoadEarlier,
   cliApps = [],
   mcpPresets = [],
+  forkBoundaryMessageCount = null,
   onOpenFilePreview,
   onForkFromMessage,
 }: ThreadMessagesProps) {
   const { t } = useTranslation();
   const units = useMemo(() => buildDisplayUnits(messages, isStreaming), [isStreaming, messages]);
+  const forkBoundaryAfterUnitIndex = useMemo(
+    () => unitIndexAfterMessageCount(units, forkBoundaryMessageCount),
+    [forkBoundaryMessageCount, units],
+  );
   const assistantForkIndexById = useMemo(
     () => assistantForkIndexByMessageId(allMessages ?? messages),
     [allMessages, messages],
@@ -119,51 +125,76 @@ export function ThreadMessages({
             : undefined;
 
         return (
-          <div
-            key={unitKey(unit, index)}
-            className={marginTop}
-            data-user-prompt-id={userPromptId}
-          >
-            {unit.type === "activity" ? (
-              <AgentActivityCluster
-                messages={unit.messages}
-                isTurnStreaming={liveActivityClusterIndices.has(index)}
-                hasBodyBelow={hasBodyBelow}
-                turnLatencyMs={unit.turnLatencyMs}
-                cliApps={cliApps}
-                mcpPresets={mcpPresets}
-                onOpenFilePreview={onOpenFilePreview}
-              />
-            ) : (
-              <MessageBubble
-                message={unit.message}
-                showAssistantCopyAction={
-                  unit.message.role === "assistant"
-                    ? copyFlags[index]
-                    : true
-                }
-                cliApps={cliApps}
-                mcpPresets={mcpPresets}
-                onOpenFilePreview={onOpenFilePreview}
-                onForkFromHere={
-                  onForkFromMessage
-                    ? forkHandlerForAssistantMessage(
-                        unit.message,
-                        copyFlags[index],
-                        assistantForkIndexById,
-                        onForkFromMessage,
-                      )
-                    : undefined
-                }
-              />
-            )}
-          </div>
+          <Fragment key={unitKey(unit, index)}>
+            <div className={marginTop} data-user-prompt-id={userPromptId}>
+              {unit.type === "activity" ? (
+                <AgentActivityCluster
+                  messages={unit.messages}
+                  isTurnStreaming={liveActivityClusterIndices.has(index)}
+                  hasBodyBelow={hasBodyBelow}
+                  turnLatencyMs={unit.turnLatencyMs}
+                  cliApps={cliApps}
+                  mcpPresets={mcpPresets}
+                  onOpenFilePreview={onOpenFilePreview}
+                />
+              ) : (
+                <MessageBubble
+                  message={unit.message}
+                  showAssistantCopyAction={
+                    unit.message.role === "assistant"
+                      ? copyFlags[index]
+                      : true
+                  }
+                  cliApps={cliApps}
+                  mcpPresets={mcpPresets}
+                  onOpenFilePreview={onOpenFilePreview}
+                  onForkFromHere={
+                    onForkFromMessage
+                      ? forkHandlerForAssistantMessage(
+                          unit.message,
+                          copyFlags[index],
+                          assistantForkIndexById,
+                          onForkFromMessage,
+                        )
+                      : undefined
+                  }
+                />
+              )}
+            </div>
+            {index === forkBoundaryAfterUnitIndex ? (
+              <ForkBoundaryDivider label={t("thread.fork.fromHistory")} />
+            ) : null}
+          </Fragment>
         );
       })}
     </div>
   );
 }
 
+function unitIndexAfterMessageCount(
+  units: DisplayUnit[],
+  messageCount: number | null | undefined,
+): number | null {
+  if (messageCount == null || messageCount <= 0) return null;
+  let seen = 0;
+  for (let i = 0; i < units.length; i += 1) {
+    const unit = units[i];
+    seen += unit.type === "activity" ? unit.messages.length : 1;
+    if (seen >= messageCount) return i;
+  }
+  return null;
+}
+
+function ForkBoundaryDivider({ label }: { label: string }) {
+  return (
+    <div className="my-5 flex items-center gap-3 text-[11px] text-muted-foreground/80">
+      <span aria-hidden className="h-px flex-1 bg-border/70" />
+      <span className="shrink-0">{label}</span>
+      <span aria-hidden className="h-px flex-1 bg-border/70" />
+    </div>
+  );
+}
+
 function assistantForkIndexByMessageId(messages: UIMessage[]): Map<string, number> {
   const out = new Map<string, number>();
   let nextUserIndex = 0;
diff --git a/webui/src/components/thread/ThreadShell.tsx b/webui/src/components/thread/ThreadShell.tsx
index b22cc7fd2..46c0ce58e 100644
--- a/webui/src/components/thread/ThreadShell.tsx
+++ b/webui/src/components/thread/ThreadShell.tsx
@@ -253,6 +253,7 @@ export function ThreadShell({
     hasPendingToolCalls,
     refresh: refreshHistory,
     version: historyVersion,
+    forkBoundaryMessageCount,
   } = useSessionHistory(historyKey);
   const { client, modelName, token } = useClient();
   const [booting, setBooting] = useState(false);
@@ -776,6 +777,7 @@ export function ThreadShell({
           cliApps={cliApps}
           mcpPresets={mcpPresets}
           allMessages={displayMessages}
+          forkBoundaryMessageCount={forkBoundaryMessageCount}
           onOpenFilePreview={historyKey ? handleOpenFilePreview : undefined}
           onForkFromMessage={onForkChat ? handleForkFromMessage : undefined}
         />
diff --git a/webui/src/components/thread/ThreadViewport.tsx b/webui/src/components/thread/ThreadViewport.tsx
index 37de373b0..bdfe2dbf2 100644
--- a/webui/src/components/thread/ThreadViewport.tsx
+++ b/webui/src/components/thread/ThreadViewport.tsx
@@ -38,6 +38,7 @@ interface ThreadViewportProps {
   showScrollToBottomButton?: boolean;
   cliApps?: CliAppInfo[];
   mcpPresets?: McpPresetInfo[];
+  forkBoundaryMessageCount?: number | null;
   onOpenFilePreview?: (path: string) => void;
   onForkFromMessage?: (beforeUserIndex: number) => void;
 }
@@ -72,6 +73,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
   showScrollToBottomButton = true,
   cliApps = [],
   mcpPresets = [],
+  forkBoundaryMessageCount = null,
   onOpenFilePreview,
   onForkFromMessage,
 }, ref) {
@@ -98,6 +100,10 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
     [messages, visibleMessageCount],
   );
   const hiddenMessageCount = messages.length - visibleMessages.length;
+  const visibleForkBoundaryMessageCount =
+    forkBoundaryMessageCount !== null && forkBoundaryMessageCount > hiddenMessageCount
+      ? forkBoundaryMessageCount - hiddenMessageCount
+      : null;
   const scrollButtonBottom = composerDockHeight > 0
     ? composerDockHeight + SCROLL_BUTTON_COMPOSER_GAP_PX
     : DEFAULT_SCROLL_BUTTON_BOTTOM_PX;
@@ -299,6 +305,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
                   onLoadEarlier={loadEarlierMessages}
                   cliApps={cliApps}
                   mcpPresets={mcpPresets}
+                  forkBoundaryMessageCount={visibleForkBoundaryMessageCount}
                   onOpenFilePreview={onOpenFilePreview}
                   onForkFromMessage={onForkFromMessage}
                 />
diff --git a/webui/src/hooks/useSessions.ts b/webui/src/hooks/useSessions.ts
index b361565b1..a493a816f 100644
--- a/webui/src/hooks/useSessions.ts
+++ b/webui/src/hooks/useSessions.ts
@@ -20,7 +20,7 @@ export function useSessions(): {
   error: string | null;
   refresh: () => Promise<void>;
   createChat: (workspaceScope?: WorkspaceScopePayload | null) => Promise<string>;
-  forkChat: (sourceChatId: string, beforeUserIndex: number) => Promise<string>;
+  forkChat: (sourceChatId: string, beforeUserIndex: number, title?: string) => Promise<string>;
   deleteChat: (key: string) => Promise<void>;
 } {
   const { client, token } = useClient();
@@ -92,8 +92,9 @@ export function useSessions(): {
   const forkChat = useCallback(async (
     sourceChatId: string,
     beforeUserIndex: number,
+    title?: string,
   ): Promise<string> => {
-    const chatId = await client.forkChat(sourceChatId, beforeUserIndex);
+    const chatId = await client.forkChat(sourceChatId, beforeUserIndex, title);
     const key = `websocket:${chatId}`;
     optimisticKeysRef.current.add(key);
     setSessions((prev) => [
@@ -103,7 +104,7 @@ export function useSessions(): {
         chatId,
         createdAt: new Date().toISOString(),
         updatedAt: new Date().toISOString(),
-        title: "",
+        title: title ?? "",
         preview: "",
         workspaceScope: null,
       },
@@ -131,6 +132,7 @@ export function useSessionHistory(key: string | null): {
   error: string | null;
   refresh: () => void;
   version: number;
+  forkBoundaryMessageCount: number | null;
   /** ``true`` when the replayed transcript ends with a trace row (turn still in flight). */
   hasPendingToolCalls: boolean;
 } {
@@ -145,6 +147,7 @@ export function useSessionHistory(key: string | null): {
     loading: boolean;
     error: string | null;
     hasPendingToolCalls: boolean;
+    forkBoundaryMessageCount: number | null;
     version: number;
   }>({
     key: null,
@@ -152,6 +155,7 @@ export function useSessionHistory(key: string | null): {
     loading: false,
     error: null,
     hasPendingToolCalls: false,
+    forkBoundaryMessageCount: null,
     version: 0,
   });
 
@@ -163,6 +167,7 @@ export function useSessionHistory(key: string | null): {
         loading: false,
         error: null,
         hasPendingToolCalls: false,
+        forkBoundaryMessageCount: null,
         version: 0,
       });
       return;
@@ -178,6 +183,7 @@ export function useSessionHistory(key: string | null): {
           loading: true,
           error: null,
           hasPendingToolCalls: false,
+          forkBoundaryMessageCount: null,
           version: 0,
         });
     (async () => {
@@ -191,6 +197,7 @@ export function useSessionHistory(key: string | null): {
             loading: false,
             error: null,
             hasPendingToolCalls: false,
+            forkBoundaryMessageCount: null,
             version: prev.key === key ? prev.version + 1 : 1,
           }));
           return;
@@ -202,12 +209,16 @@ export function useSessionHistory(key: string | null): {
         }));
         const last = ui[ui.length - 1];
         const hasPending = last?.kind === "trace";
+        const forkBoundary = typeof body.fork_boundary_message_count === "number"
+          ? Math.max(0, Math.min(body.fork_boundary_message_count, ui.length))
+          : null;
         setState((prev) => ({
           key,
           messages: ui,
           loading: false,
           error: null,
           hasPendingToolCalls: hasPending,
+          forkBoundaryMessageCount: forkBoundary,
           version: prev.key === key ? prev.version + 1 : 1,
         }));
       } catch (e) {
@@ -219,6 +230,7 @@ export function useSessionHistory(key: string | null): {
             loading: false,
             error: null,
             hasPendingToolCalls: false,
+            forkBoundaryMessageCount: null,
             version: prev.key === key ? prev.version + 1 : 1,
           }));
         } else {
@@ -228,6 +240,7 @@ export function useSessionHistory(key: string | null): {
             loading: false,
             error: (e as Error).message,
             hasPendingToolCalls: false,
+            forkBoundaryMessageCount: null,
             version: prev.key === key ? prev.version : 0,
           }));
         }
@@ -245,6 +258,7 @@ export function useSessionHistory(key: string | null): {
       error: null,
       refresh,
       version: 0,
+      forkBoundaryMessageCount: null,
       hasPendingToolCalls: false,
     };
   }
@@ -258,6 +272,7 @@ export function useSessionHistory(key: string | null): {
       error: null,
       refresh,
       version: 0,
+      forkBoundaryMessageCount: null,
       hasPendingToolCalls: false,
     };
   }
@@ -268,6 +283,7 @@ export function useSessionHistory(key: string | null): {
     error: state.error,
     refresh,
     version: state.version,
+    forkBoundaryMessageCount: state.forkBoundaryMessageCount,
     hasPendingToolCalls: state.hasPendingToolCalls,
   };
 }
diff --git a/webui/src/i18n/locales/en/common.json b/webui/src/i18n/locales/en/common.json
index 2ca281576..06444e662 100644
--- a/webui/src/i18n/locales/en/common.json
+++ b/webui/src/i18n/locales/en/common.json
@@ -509,6 +509,7 @@
   },
   "chat": {
     "fallbackTitle": "Chat {{id}}",
+    "forkTitle": "Fork: {{title}}",
     "loading": "Loading…",
     "noSessions": "No sessions yet.",
     "showMore": "Show {{count}} more",
@@ -811,7 +812,8 @@
     "scrollToBottom": "Scroll to bottom",
     "loadEarlier": "Load earlier messages",
     "fork": {
-      "failed": "Could not fork this chat. Try again."
+      "failed": "Could not fork this chat. Try again.",
+      "fromHistory": "Forked from history"
     },
     "promptNavigator": {
       "open": "Open prompt navigator",
@@ -852,11 +854,11 @@
     "imageAttachment": "Image attachment",
     "automationSourceFallback": "Automation",
     "automationTriggered": "Triggered automatically",
-    "copyMessage": "Copy message",
-    "copiedMessage": "Copied message",
-    "forkFromHere": "Fork from here",
-    "copyReply": "Copy reply",
-    "copiedReply": "Copied reply",
+    "copyMessage": "Copy",
+    "copiedMessage": "Copied",
+    "forkFromHere": "Fork",
+    "copyReply": "Copy",
+    "copiedReply": "Copied",
     "turnLatencyTitle": "Response time (end-to-end)"
   },
   "lightbox": {
diff --git a/webui/src/i18n/locales/es/common.json b/webui/src/i18n/locales/es/common.json
index 8070cdc60..c0461da39 100644
--- a/webui/src/i18n/locales/es/common.json
+++ b/webui/src/i18n/locales/es/common.json
@@ -509,6 +509,7 @@
   },
   "chat": {
     "fallbackTitle": "Chat {{id}}",
+    "forkTitle": "Bifurcación: {{title}}",
     "loading": "Cargando…",
     "noSessions": "Todavía no hay sesiones.",
     "showMore": "Mostrar {{count}} más",
@@ -811,7 +812,8 @@
     "scrollToBottom": "Desplazarse al final",
     "loadEarlier": "Cargar mensajes anteriores",
     "fork": {
-      "failed": "No se pudo bifurcar este chat. Inténtalo de nuevo."
+      "failed": "No se pudo bifurcar este chat. Inténtalo de nuevo.",
+      "fromHistory": "Bifurcado desde el historial"
     },
     "promptNavigator": {
       "open": "Abrir navegador de prompts",
@@ -838,11 +840,11 @@
     "agentActivityLiveSummary": "En curso… · {{reasoning}} pasos · {{tools}} llamadas a herramientas",
     "agentActivityLiveToolsOnly": "En curso… · {{tools}} llamadas a herramientas",
     "imageAttachment": "Imagen adjunta",
-    "copyMessage": "Copiar mensaje",
-    "copiedMessage": "Mensaje copiado",
-    "forkFromHere": "Bifurcar desde aquí",
-    "copyReply": "Copiar respuesta",
-    "copiedReply": "Respuesta copiada",
+    "copyMessage": "Copiar",
+    "copiedMessage": "Copiado",
+    "forkFromHere": "Bifurcar",
+    "copyReply": "Copiar",
+    "copiedReply": "Copiado",
     "turnLatencyTitle": "Tiempo de respuesta (extremo a extremo)",
     "activityThinkingFor": "Pensando durante {{duration}}",
     "activityThought": "Pensamiento completado",
diff --git a/webui/src/i18n/locales/fr/common.json b/webui/src/i18n/locales/fr/common.json
index d4d7ce769..aa809e081 100644
--- a/webui/src/i18n/locales/fr/common.json
+++ b/webui/src/i18n/locales/fr/common.json
@@ -509,6 +509,7 @@
   },
   "chat": {
     "fallbackTitle": "Discussion {{id}}",
+    "forkTitle": "Branche : {{title}}",
     "loading": "Chargement…",
     "noSessions": "Aucune session pour le moment.",
     "showMore": "Afficher {{count}} de plus",
@@ -811,7 +812,8 @@
     "scrollToBottom": "Faire défiler vers le bas",
     "loadEarlier": "Charger les messages précédents",
     "fork": {
-      "failed": "Impossible de bifurquer cette conversation. Réessayez."
+      "failed": "Impossible de bifurquer cette conversation. Réessayez.",
+      "fromHistory": "Bifurqué depuis l'historique"
     },
     "promptNavigator": {
       "open": "Ouvrir le navigateur de prompts",
@@ -838,11 +840,11 @@
     "agentActivityLiveSummary": "En cours… · {{reasoning}} étapes · {{tools}} appels d’outils",
     "agentActivityLiveToolsOnly": "En cours… · {{tools}} appels d’outils",
     "imageAttachment": "Pièce jointe image",
-    "copyMessage": "Copier le message",
-    "copiedMessage": "Message copié",
-    "forkFromHere": "Bifurquer depuis ici",
-    "copyReply": "Copier la réponse",
-    "copiedReply": "Réponse copiée",
+    "copyMessage": "Copier",
+    "copiedMessage": "Copié",
+    "forkFromHere": "Bifurquer",
+    "copyReply": "Copier",
+    "copiedReply": "Copié",
     "turnLatencyTitle": "Temps de réponse (de bout en bout)",
     "activityThinkingFor": "Réflexion pendant {{duration}}",
     "activityThought": "Réflexion terminée",
diff --git a/webui/src/i18n/locales/id/common.json b/webui/src/i18n/locales/id/common.json
index 5d7101e5c..13cc84e65 100644
--- a/webui/src/i18n/locales/id/common.json
+++ b/webui/src/i18n/locales/id/common.json
@@ -509,6 +509,7 @@
   },
   "chat": {
     "fallbackTitle": "Obrolan {{id}}",
+    "forkTitle": "Cabang: {{title}}",
     "loading": "Memuat…",
     "noSessions": "Belum ada sesi.",
     "showMore": "Tampilkan {{count}} lagi",
@@ -811,7 +812,8 @@
     "scrollToBottom": "Gulir ke bawah",
     "loadEarlier": "Muat pesan sebelumnya",
     "fork": {
-      "failed": "Tidak dapat mem-fork chat ini. Coba lagi."
+      "failed": "Tidak dapat mem-fork chat ini. Coba lagi.",
+      "fromHistory": "Fork dari riwayat"
     },
     "promptNavigator": {
       "open": "Buka navigator prompt",
@@ -838,11 +840,11 @@
     "agentActivityLiveSummary": "Berjalan… · {{reasoning}} langkah · {{tools}} panggilan alat",
     "agentActivityLiveToolsOnly": "Berjalan… · {{tools}} panggilan alat",
     "imageAttachment": "Lampiran gambar",
-    "copyMessage": "Salin pesan",
-    "copiedMessage": "Pesan disalin",
-    "forkFromHere": "Fork dari sini",
-    "copyReply": "Salin balasan",
-    "copiedReply": "Balasan disalin",
+    "copyMessage": "Salin",
+    "copiedMessage": "Disalin",
+    "forkFromHere": "Fork",
+    "copyReply": "Salin",
+    "copiedReply": "Disalin",
     "turnLatencyTitle": "Waktu respons (ujung ke ujung)",
     "activityThinkingFor": "Berpikir selama {{duration}}",
     "activityThought": "Selesai berpikir",
diff --git a/webui/src/i18n/locales/ja/common.json b/webui/src/i18n/locales/ja/common.json
index 3686dcc92..4751f0e2d 100644
--- a/webui/src/i18n/locales/ja/common.json
+++ b/webui/src/i18n/locales/ja/common.json
@@ -509,6 +509,7 @@
   },
   "chat": {
     "fallbackTitle": "チャット {{id}}",
+    "forkTitle": "分岐：{{title}}",
     "loading": "読み込み中…",
     "noSessions": "まだセッションがありません。",
     "showMore": "さらに {{count}} 件表示",
@@ -811,7 +812,8 @@
     "scrollToBottom": "一番下へスクロール",
     "loadEarlier": "以前のメッセージを読み込む",
     "fork": {
-      "failed": "このチャットを分岐できませんでした。もう一度お試しください。"
+      "failed": "このチャットを分岐できませんでした。もう一度お試しください。",
+      "fromHistory": "履歴から分岐"
     },
     "promptNavigator": {
       "open": "プロンプトナビゲーターを開く",
@@ -838,11 +840,11 @@
     "agentActivityLiveSummary": "実行中… · {{reasoning}} ステップ · ツール呼び出し {{tools}} 回",
     "agentActivityLiveToolsOnly": "実行中… · ツール呼び出し {{tools}} 回",
     "imageAttachment": "画像の添付",
-    "copyMessage": "メッセージをコピー",
-    "copiedMessage": "メッセージをコピーしました",
-    "forkFromHere": "ここから分岐",
-    "copyReply": "返信をコピー",
-    "copiedReply": "返信をコピーしました",
+    "copyMessage": "コピー",
+    "copiedMessage": "コピー済み",
+    "forkFromHere": "分岐",
+    "copyReply": "コピー",
+    "copiedReply": "コピー済み",
     "turnLatencyTitle": "応答時間（全行程）",
     "activityThinkingFor": "{{duration}}考えています",
     "activityThought": "思考しました",
diff --git a/webui/src/i18n/locales/ko/common.json b/webui/src/i18n/locales/ko/common.json
index 0a77265fa..46ad9d913 100644
--- a/webui/src/i18n/locales/ko/common.json
+++ b/webui/src/i18n/locales/ko/common.json
@@ -509,6 +509,7 @@
   },
   "chat": {
     "fallbackTitle": "채팅 {{id}}",
+    "forkTitle": "분기: {{title}}",
     "loading": "불러오는 중…",
     "noSessions": "아직 세션이 없습니다.",
     "showMore": "{{count}}개 더 보기",
@@ -811,7 +812,8 @@
     "scrollToBottom": "맨 아래로 스크롤",
     "loadEarlier": "이전 메시지 불러오기",
     "fork": {
-      "failed": "이 채팅을 분기할 수 없습니다. 다시 시도해 주세요."
+      "failed": "이 채팅을 분기할 수 없습니다. 다시 시도해 주세요.",
+      "fromHistory": "기록에서 분기됨"
     },
     "promptNavigator": {
       "open": "프롬프트 탐색기 열기",
@@ -838,11 +840,11 @@
     "agentActivityLiveSummary": "진행 중… · {{reasoning}}단계 · 도구 호출 {{tools}}회",
     "agentActivityLiveToolsOnly": "진행 중… · 도구 호출 {{tools}}회",
     "imageAttachment": "이미지 첨부",
-    "copyMessage": "메시지 복사",
-    "copiedMessage": "메시지가 복사됨",
-    "forkFromHere": "여기서 분기",
-    "copyReply": "답변 복사",
-    "copiedReply": "답변이 복사됨",
+    "copyMessage": "복사",
+    "copiedMessage": "복사됨",
+    "forkFromHere": "분기",
+    "copyReply": "복사",
+    "copiedReply": "복사됨",
     "turnLatencyTitle": "응답 시간(엔드투엔드)",
     "activityThinkingFor": "{{duration}} 동안 생각 중",
     "activityThought": "생각함",
diff --git a/webui/src/i18n/locales/vi/common.json b/webui/src/i18n/locales/vi/common.json
index 07db71e82..628925b22 100644
--- a/webui/src/i18n/locales/vi/common.json
+++ b/webui/src/i18n/locales/vi/common.json
@@ -509,6 +509,7 @@
   },
   "chat": {
     "fallbackTitle": "Trò chuyện {{id}}",
+    "forkTitle": "Nhánh: {{title}}",
     "loading": "Đang tải…",
     "noSessions": "Chưa có phiên nào.",
     "showMore": "Hiển thị thêm {{count}}",
@@ -811,7 +812,8 @@
     "scrollToBottom": "Cuộn xuống cuối",
     "loadEarlier": "Tải tin nhắn trước đó",
     "fork": {
-      "failed": "Không thể rẽ nhánh cuộc trò chuyện này. Hãy thử lại."
+      "failed": "Không thể rẽ nhánh cuộc trò chuyện này. Hãy thử lại.",
+      "fromHistory": "Tách nhánh từ lịch sử"
     },
     "promptNavigator": {
       "open": "Mở trình điều hướng prompt",
@@ -838,11 +840,11 @@
     "agentActivityLiveSummary": "Đang chạy… · {{reasoning}} bước · {{tools}} lần gọi công cụ",
     "agentActivityLiveToolsOnly": "Đang chạy… · {{tools}} lần gọi công cụ",
     "imageAttachment": "Tệp hình ảnh đính kèm",
-    "copyMessage": "Sao chép tin nhắn",
-    "copiedMessage": "Đã sao chép tin nhắn",
-    "forkFromHere": "Rẽ nhánh từ đây",
-    "copyReply": "Sao chép trả lời",
-    "copiedReply": "Đã sao chép trả lời",
+    "copyMessage": "Sao chép",
+    "copiedMessage": "Đã sao chép",
+    "forkFromHere": "Tách nhánh",
+    "copyReply": "Sao chép",
+    "copiedReply": "Đã sao chép",
     "turnLatencyTitle": "Thời gian phản hồi (end-to-end)",
     "activityThinkingFor": "Đang suy nghĩ trong {{duration}}",
     "activityThought": "Đã suy nghĩ",
diff --git a/webui/src/i18n/locales/zh-CN/common.json b/webui/src/i18n/locales/zh-CN/common.json
index 7b96ba9fb..72acd3a74 100644
--- a/webui/src/i18n/locales/zh-CN/common.json
+++ b/webui/src/i18n/locales/zh-CN/common.json
@@ -509,6 +509,7 @@
   },
   "chat": {
     "fallbackTitle": "对话 {{id}}",
+    "forkTitle": "分叉：{{title}}",
     "loading": "加载中…",
     "noSessions": "还没有会话。",
     "showMore": "再显示 {{count}} 个",
@@ -811,7 +812,8 @@
     "scrollToBottom": "滚动到底部",
     "loadEarlier": "加载更早消息",
     "fork": {
-      "failed": "无法分叉这个对话，请重试。"
+      "failed": "无法分叉这个对话，请重试。",
+      "fromHistory": "从历史消息分叉"
     },
     "promptNavigator": {
       "open": "打开输入导航",
@@ -852,11 +854,11 @@
     "imageAttachment": "图片附件",
     "automationSourceFallback": "自动化",
     "automationTriggered": "自动触发",
-    "copyMessage": "复制消息",
-    "copiedMessage": "已复制消息",
-    "forkFromHere": "从这里分叉",
-    "copyReply": "复制回复",
-    "copiedReply": "已复制回复",
+    "copyMessage": "复制",
+    "copiedMessage": "已复制",
+    "forkFromHere": "分叉",
+    "copyReply": "复制",
+    "copiedReply": "已复制",
     "turnLatencyTitle": "本轮耗时（端到端）"
   },
   "lightbox": {
diff --git a/webui/src/i18n/locales/zh-TW/common.json b/webui/src/i18n/locales/zh-TW/common.json
index 4049c5913..f8a68134b 100644
--- a/webui/src/i18n/locales/zh-TW/common.json
+++ b/webui/src/i18n/locales/zh-TW/common.json
@@ -509,6 +509,7 @@
   },
   "chat": {
     "fallbackTitle": "對話 {{id}}",
+    "forkTitle": "分叉：{{title}}",
     "loading": "載入中…",
     "noSessions": "目前還沒有會話。",
     "showMore": "再顯示 {{count}} 個",
@@ -811,7 +812,8 @@
     "scrollToBottom": "捲動到底部",
     "loadEarlier": "載入更早訊息",
     "fork": {
-      "failed": "無法分叉這個對話，請重試。"
+      "failed": "無法分叉這個對話，請重試。",
+      "fromHistory": "從歷史訊息分叉"
     },
     "promptNavigator": {
       "open": "開啟輸入導覽",
@@ -838,11 +840,11 @@
     "agentActivityLiveSummary": "進行中… · {{reasoning}} 步 · {{tools}} 次工具呼叫",
     "agentActivityLiveToolsOnly": "進行中… · {{tools}} 次工具呼叫",
     "imageAttachment": "圖片附件",
-    "copyMessage": "複製訊息",
-    "copiedMessage": "已複製訊息",
-    "forkFromHere": "從這裡分叉",
-    "copyReply": "複製回覆",
-    "copiedReply": "已複製回覆",
+    "copyMessage": "複製",
+    "copiedMessage": "已複製",
+    "forkFromHere": "分叉",
+    "copyReply": "複製",
+    "copiedReply": "已複製",
     "turnLatencyTitle": "本輪耗時（端到端）",
     "activityThinkingFor": "思考中，已 {{duration}}",
     "activityThought": "已思考",
diff --git a/webui/src/lib/nanobot-client.ts b/webui/src/lib/nanobot-client.ts
index ee4e70a1e..9037a921e 100644
--- a/webui/src/lib/nanobot-client.ts
+++ b/webui/src/lib/nanobot-client.ts
@@ -352,6 +352,7 @@ export class NanobotClient {
   forkChat(
     sourceChatId: string,
     beforeUserIndex: number,
+    title?: string,
     timeoutMs: number = 5_000,
   ): Promise<string> {
     if (this.pendingNewChat) {
@@ -367,6 +368,7 @@ export class NanobotClient {
         type: "fork_chat",
         source_chat_id: sourceChatId,
         before_user_index: beforeUserIndex,
+        ...(title?.trim() ? { title: title.trim() } : {}),
       });
     });
   }
diff --git a/webui/src/lib/types.ts b/webui/src/lib/types.ts
index 7ab06c90a..438373a1f 100644
--- a/webui/src/lib/types.ts
+++ b/webui/src/lib/types.ts
@@ -862,6 +862,7 @@ export interface WebuiThreadPersistedPayload {
   sessionKey?: string;
   savedAt?: string;
   messages: UIMessage[];
+  fork_boundary_message_count?: number;
   workspace_scope?: WorkspaceScopePayload;
 }
 
@@ -877,7 +878,7 @@ export interface FilePreviewPayload {
 
 export type Outbound =
   | { type: "new_chat"; workspace_scope?: WorkspaceScopePayload }
-  | { type: "fork_chat"; source_chat_id: string; before_user_index: number }
+  | { type: "fork_chat"; source_chat_id: string; before_user_index: number; title?: string }
   | { type: "attach"; chat_id: string }
   | { type: "set_workspace_scope"; chat_id: string; workspace_scope: WorkspaceScopePayload }
   | { type: "transcribe_audio"; request_id: string; data_url: string; duration_ms?: number }
diff --git a/webui/src/tests/message-bubble.test.tsx b/webui/src/tests/message-bubble.test.tsx
index 38ab872e4..e8b907f52 100644
--- a/webui/src/tests/message-bubble.test.tsx
+++ b/webui/src/tests/message-bubble.test.tsx
@@ -76,8 +76,8 @@ describe("MessageBubble", () => {
 
     expect(row).toHaveClass("ml-auto", "flex");
     expect(pill).toHaveClass("ml-auto", "w-fit", "rounded-[18px]");
-    expect(screen.getByRole("button", { name: "Copy message" })).toBeInTheDocument();
-    expect(screen.queryByRole("button", { name: "Copy reply" })).not.toBeInTheDocument();
+    expect(screen.getByRole("button", { name: "Copy" })).toBeInTheDocument();
+    expect(screen.queryByRole("button", { name: "Fork" })).not.toBeInTheDocument();
   });
 
   it("does not render fork control for user messages", () => {
@@ -91,8 +91,8 @@ describe("MessageBubble", () => {
 
     render(<MessageBubble message={message} onForkFromHere={onForkFromHere} />);
 
-    expect(screen.getByRole("button", { name: "Copy message" })).toBeInTheDocument();
-    expect(screen.queryByRole("button", { name: "Fork from here" })).not.toBeInTheDocument();
+    expect(screen.getByRole("button", { name: "Copy" })).toBeInTheDocument();
+    expect(screen.queryByRole("button", { name: "Fork" })).not.toBeInTheDocument();
   });
 
   it("renders fork control in completed assistant action rows", () => {
@@ -107,7 +107,7 @@ describe("MessageBubble", () => {
 
     render(<MessageBubble message={message} onForkFromHere={onForkFromHere} />);
 
-    fireEvent.click(screen.getByRole("button", { name: "Fork from here" }));
+    fireEvent.click(screen.getByRole("button", { name: "Fork" }));
     expect(onForkFromHere).toHaveBeenCalledTimes(1);
   });
 
@@ -207,11 +207,11 @@ describe("MessageBubble", () => {
 
     render(<MessageBubble message={message} />);
 
-    fireEvent.click(screen.getByRole("button", { name: "Copy reply" }));
+    fireEvent.click(screen.getByRole("button", { name: "Copy" }));
 
     expect(writeText).toHaveBeenCalledWith("I can help with the next step.");
     await waitFor(() =>
-      expect(screen.getByRole("button", { name: "Copied reply" })).toBeInTheDocument(),
+      expect(screen.getByRole("button", { name: "Copied" })).toBeInTheDocument(),
     );
   });
 
@@ -235,11 +235,11 @@ describe("MessageBubble", () => {
     try {
       render(<MessageBubble message={message} />);
 
-      fireEvent.click(screen.getByRole("button", { name: "Copy reply" }));
+      fireEvent.click(screen.getByRole("button", { name: "Copy" }));
 
       await waitFor(() => expect(execCommand).toHaveBeenCalledWith("copy"));
       await waitFor(() =>
-        expect(screen.getByRole("button", { name: "Copied reply" })).toBeInTheDocument(),
+        expect(screen.getByRole("button", { name: "Copied" })).toBeInTheDocument(),
       );
     } finally {
       Reflect.deleteProperty(navigator, "clipboard");
@@ -268,12 +268,12 @@ describe("MessageBubble", () => {
     try {
       render(<MessageBubble message={message} />);
 
-      fireEvent.click(screen.getByRole("button", { name: "Copy reply" }));
+      fireEvent.click(screen.getByRole("button", { name: "Copy" }));
 
       expect(writeText).toHaveBeenCalledWith("Rejected clipboard copy.");
       await waitFor(() => expect(execCommand).toHaveBeenCalledWith("copy"));
       await waitFor(() =>
-        expect(screen.getByRole("button", { name: "Copied reply" })).toBeInTheDocument(),
+        expect(screen.getByRole("button", { name: "Copied" })).toBeInTheDocument(),
       );
     } finally {
       Reflect.deleteProperty(navigator, "clipboard");
@@ -292,7 +292,7 @@ describe("MessageBubble", () => {
 
     render(<MessageBubble message={message} />);
 
-    expect(screen.queryByRole("button", { name: "Copy reply" })).not.toBeInTheDocument();
+    expect(screen.queryByRole("button", { name: "Copy" })).not.toBeInTheDocument();
   });
 
   it("does not show copy when showAssistantCopyAction is false", () => {
@@ -305,7 +305,7 @@ describe("MessageBubble", () => {
 
     render(<MessageBubble message={message} showAssistantCopyAction={false} />);
 
-    expect(screen.queryByRole("button", { name: "Copy reply" })).not.toBeInTheDocument();
+    expect(screen.queryByRole("button", { name: "Copy" })).not.toBeInTheDocument();
   });
 
   it("renders trace messages as collapsible tool groups", () => {
diff --git a/webui/src/tests/thread-messages.test.tsx b/webui/src/tests/thread-messages.test.tsx
index 8fea32b47..5abcf6929 100644
--- a/webui/src/tests/thread-messages.test.tsx
+++ b/webui/src/tests/thread-messages.test.tsx
@@ -55,6 +55,23 @@ describe("ThreadMessages", () => {
     expect(rows[1]).toHaveClass("mt-4");
   });
 
+  it("renders a fork boundary divider after the copied history", () => {
+    const messages: UIMessage[] = [
+      { id: "u1", role: "user", content: "original", createdAt: 1 },
+      { id: "a1", role: "assistant", content: "answer", createdAt: 2 },
+      { id: "u2", role: "user", content: "branch prompt", createdAt: 3 },
+    ];
+
+    render(
+      <ThreadMessages
+        messages={messages}
+        forkBoundaryMessageCount={2}
+      />,
+    );
+
+    expect(screen.getByText("Forked from history")).toBeInTheDocument();
+  });
+
   it("keeps file edits as their own activity row inside a turn", () => {
     const messages: UIMessage[] = [
       {
@@ -639,7 +656,7 @@ describe("ThreadMessages", () => {
 
     render(<ThreadMessages messages={messages} isStreaming={false} />);
 
-    expect(screen.getAllByRole("button", { name: "Copy reply" })).toHaveLength(1);
+    expect(screen.getAllByRole("button", { name: "Copy" })).toHaveLength(1);
     expect(screen.getByText("final reply")).toBeInTheDocument();
   });
 
@@ -649,7 +666,7 @@ describe("ThreadMessages", () => {
       { id: "a2", role: "assistant", content: "part two", createdAt: 2 },
     ];
     render(<ThreadMessages messages={messages} isStreaming={false} />);
-    expect(screen.getAllByRole("button", { name: "Copy reply" })).toHaveLength(1);
+    expect(screen.getAllByRole("button", { name: "Copy" })).toHaveLength(1);
   });
 
   it("uses turn ids as activity grouping boundaries when available", () => {
diff --git a/webui/src/tests/thread-shell.test.tsx b/webui/src/tests/thread-shell.test.tsx
index ded9e65fa..e5b38e1ef 100644
--- a/webui/src/tests/thread-shell.test.tsx
+++ b/webui/src/tests/thread-shell.test.tsx
@@ -758,7 +758,7 @@ describe("ThreadShell", () => {
 
     const targetText = await screen.findByText("answer 100");
     fireEvent.click(within(targetText.closest(".w-full") as HTMLElement).getByRole("button", {
-      name: "Fork from here",
+      name: "Fork",
     }));
 
     await waitFor(() =>
@@ -804,7 +804,7 @@ describe("ThreadShell", () => {
       target: { value: "keep my current draft" },
     });
     fireEvent.click(within(targetText.closest(".w-full") as HTMLElement).getByRole("button", {
-      name: "Fork from here",
+      name: "Fork",
     }));
 
     await waitFor(() => expect(onForkChat).toHaveBeenCalledWith("chat-a", 1));
@@ -864,7 +864,7 @@ describe("ThreadShell", () => {
 
     const targetText = await screen.findByText("answer2");
     fireEvent.click(within(targetText.closest(".w-full") as HTMLElement).getByRole("button", {
-      name: "Fork from here",
+      name: "Fork",
     }));
 
     await waitFor(() => expect(onForkChat).toHaveBeenCalledWith("chat-a", 2));
@@ -962,7 +962,7 @@ describe("ThreadShell", () => {
     );
 
     await screen.findByText("answer1");
-    fireEvent.click(screen.getAllByRole("button", { name: "Fork from here" }).at(-1)!);
+    fireEvent.click(screen.getAllByRole("button", { name: "Fork" }).at(-1)!);
 
     await waitFor(() => expect(onForkChat).toHaveBeenCalledWith("chat-a", 1));
     await act(async () => {
diff --git a/webui/src/tests/useSessions.test.tsx b/webui/src/tests/useSessions.test.tsx
index 1d79b4673..e59a8eb2d 100644
--- a/webui/src/tests/useSessions.test.tsx
+++ b/webui/src/tests/useSessions.test.tsx
@@ -230,6 +230,24 @@ describe("useSessions", () => {
     expect(result.current.sessions[0]?.workspaceScope).toEqual(workspaceScope);
   });
 
+  it("keeps a fork title visible while the server session list catches up", async () => {
+    vi.mocked(api.listSessions).mockResolvedValue([]);
+    const client = fakeClient();
+    client.forkChat.mockResolvedValue("chat-fork");
+
+    const { result } = renderHook(() => useSessions(), {
+      wrapper: wrap(client),
+    });
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+    await act(async () => {
+      await result.current.forkChat("source", 2, "Fork: Original title");
+    });
+
+    expect(client.forkChat).toHaveBeenCalledWith("source", 2, "Fork: Original title");
+    expect(result.current.sessions[0]?.title).toBe("Fork: Original title");
+  });
+
   it("passes through WebUI transcript user media as images and media", async () => {
     vi.mocked(api.fetchWebuiThread).mockResolvedValue({
       schemaVersion: 3,