feat(webui): persist fork boundary metadata

nanobot/channels/websocket.py

@@ -28,7 +28,11 @@ from nanobot.security.workspace_access import (
     WorkspaceScopeError,
 )
 from nanobot.session.goal_state import goal_state_ws_blob
-from nanobot.session.webui_turns import websocket_turn_wall_started_at
+from nanobot.session.webui_turns import (
+    WEBUI_TITLE_METADATA_KEY,
+    clean_generated_title,
+    websocket_turn_wall_started_at,
+)
 from nanobot.utils.media_decode import (
     FileSizeExceeded,
     save_base64_data_url,
@@ -46,6 +50,7 @@ from nanobot.webui.http_utils import (
 )
 from nanobot.webui.mcp_presets_api import normalize_mcp_preset_mentions
 from nanobot.webui.transcript import (
+    append_fork_marker,
     delete_webui_transcript,
     fork_transcript_before_user_index,
     write_session_messages_as_transcript,
@@ -709,6 +714,13 @@ class WebSocketChannel(BaseChannel):
                 )
                 if not transcript_ok:
                     write_session_messages_as_transcript(target_key, forked.messages)
+                append_fork_marker(target_key)
+                fork_title = clean_generated_title(
+                    envelope.get("title") if isinstance(envelope.get("title"), str) else None,
+                )
+                if fork_title:
+                    forked.metadata[WEBUI_TITLE_METADATA_KEY] = fork_title
+                    self.gateway.session_manager.save(forked, fsync=True)
             except Exception as exc:
                 delete_webui_transcript(target_key)
                 self.gateway.session_manager.delete_session(target_key)

nanobot/session/manager.py

@@ -648,8 +648,8 @@ class SessionManager:
         ``before_user_index`` is zero-based over user messages in the full session:
         ``0`` means "before the first user message", ``1`` means "before the
         second user message", and so on. A value equal to the total user-message
-        count copies the full session prefix. The target user message itself is
-        not copied; the WebUI pre-fills it in the composer for editing and resend.
+        count copies the full session prefix. WebUI assistant-reply forks pass
+        the next user index so the selected completed assistant turn is included.
         """
         if before_user_index < 0:
             return None

nanobot/webui/transcript.py

@@ -17,6 +17,7 @@ from nanobot.config.paths import get_webui_dir
 from nanobot.session.manager import SessionManager
 
 WEBUI_TRANSCRIPT_SCHEMA_VERSION = 3
+WEBUI_FORK_MARKER_EVENT = "fork_marker"
 _MAX_TRANSCRIPT_FILE_BYTES = 8 * 1024 * 1024
 _WEBUI_TURN_ID_RE = re.compile(r"^[A-Za-z0-9._:-]{1,128}$")
 WEBUI_TURN_METADATA_KEY = "webui_turn_id"
@@ -306,6 +307,8 @@ def fork_transcript_before_user_index(
     user_index = 0
     found_target = False
     for row in lines:
+        if row.get("event") == WEBUI_FORK_MARKER_EVENT:
+            continue
         if _is_user_transcript_row(row):
             if user_index == before_user_index:
                 found_target = True
@@ -340,6 +343,17 @@ def fork_transcript_before_user_index(
     return True
 
 
+def append_fork_marker(session_key: str) -> None:
+    """Mark the UI-only boundary where a WebUI fork starts accepting new turns."""
+    append_transcript_object(
+        session_key,
+        {
+            "event": WEBUI_FORK_MARKER_EVENT,
+            "chat_id": _chat_id_from_session_key(session_key),
+        },
+    )
+
+
 def write_session_messages_as_transcript(
     target_key: str,
     messages: list[dict[str, Any]],
@@ -1397,6 +1411,28 @@ def replay_transcript_to_ui_messages(
     return messages
 
 
+def fork_boundary_message_count(
+    lines: list[dict[str, Any]],
+    *,
+    augment_user_media: Callable[[list[str]], list[dict[str, Any]]] | None = None,
+    augment_assistant_media: Callable[[list[str]], list[dict[str, Any]]] | None = None,
+    augment_assistant_text: Callable[[str], str] | None = None,
+) -> int | None:
+    """Return the replayed UI message count before the first fork marker, if any."""
+    for idx, rec in enumerate(lines):
+        if rec.get("event") != WEBUI_FORK_MARKER_EVENT:
+            continue
+        return len(
+            replay_transcript_to_ui_messages(
+                lines[:idx],
+                augment_user_media=augment_user_media,
+                augment_assistant_media=augment_assistant_media,
+                augment_assistant_text=augment_assistant_text,
+            ),
+        )
+    return None
+
+
 def build_webui_thread_response(
     session_key: str,
     *,
@@ -1410,14 +1446,23 @@ def build_webui_thread_response(
     if not lines:
         return None
     lines = inject_missing_user_events_from_session(session_key, lines, session_messages)
+    fork_boundary = fork_boundary_message_count(
+        lines,
+        augment_user_media=augment_user_media,
+        augment_assistant_media=augment_assistant_media,
+        augment_assistant_text=augment_assistant_text,
+    )
     msgs = replay_transcript_to_ui_messages(
         lines,
         augment_user_media=augment_user_media,
         augment_assistant_media=augment_assistant_media,
         augment_assistant_text=augment_assistant_text,
     )
-    return {
+    payload = {
         "schemaVersion": WEBUI_TRANSCRIPT_SCHEMA_VERSION,
         "sessionKey": session_key,
         "messages": msgs,
     }
+    if fork_boundary is not None:
+        payload["fork_boundary_message_count"] = fork_boundary
+    return payload