diff --git a/runtime/datamate-python/app/module/annotation/service/editor.py b/runtime/datamate-python/app/module/annotation/service/editor.py
index 1cd5587..878c389 100644
--- a/runtime/datamate-python/app/module/annotation/service/editor.py
+++ b/runtime/datamate-python/app/module/annotation/service/editor.py
@@ -992,11 +992,19 @@ class AnnotationEditorService:
 
         ls_task_id = self._make_ls_task_id(project_id, file_id)
 
+        segment_total_hint = None
+        if request.segment_index is not None:
+            segment_total_hint = self._resolve_segment_total(annotation_payload)
+            if segment_total_hint is None:
+                segment_total_hint = await self._compute_segment_total(project, file_record, file_id)
+
         existing_result = await self.db.execute(
-            select(AnnotationResult).where(
+            select(AnnotationResult)
+            .where(
                 AnnotationResult.project_id == project_id,
                 AnnotationResult.file_id == file_id,
             )
+            .with_for_update()
         )
         existing = existing_result.scalar_one_or_none()
 
@@ -1026,7 +1034,7 @@ class AnnotationEditorService:
         if request.segment_index is not None:
             segment_total = self._resolve_segment_total(final_payload)
             if segment_total is None:
-                segment_total = await self._compute_segment_total(project, file_record, file_id)
+                segment_total = segment_total_hint
             if segment_total and segment_total > 0:
                 final_payload[SEGMENT_TOTAL_KEY] = segment_total
                 segment_done = len(self._extract_segment_annotations(final_payload))