feat(webui): persist fork boundary metadata

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