基本完成，舒适性修补

backend/services/chat_service.py

@@ -561,6 +561,98 @@ class ChatService:
         except Exception as e:
             logger.error(f"总结聊天消息失败 {role_name}/{chat_name}: {str(e)}")
             raise
+    
+    def create_branch(
+        self,
+        role_name: str,
+        chat_name: str,
+        target_floor: int,
+        new_chat_name: Optional[str] = None
+    ) -> Dict:
+        """
+        创建聊天分支
+        
+        复制目标楼层及之前的所有内容到一个新的聊天记录
+        
+        Args:
+            role_name: 角色名称
+            chat_name: 原聊天名称
+            target_floor: 目标楼层（包含该楼层及之前的内容）
+            new_chat_name: 新聊天名称（可选，默认自动生成）
+            
+        Returns:
+            Dict: {
+                "success": bool,
+                "new_chat_name": str,
+                "message_count": int
+            }
+            
+        Raises:
+            FileNotFoundError: 聊天不存在
+            ValueError: 楼层不存在
+        """
+        chat_file = self.chat_dir / role_name / f"{chat_name}.jsonl"
+        
+        if not chat_file.exists():
+            raise FileNotFoundError(f"Chat not found: {role_name}/{chat_name}")
+        
+        try:
+            # 读取所有行
+            with open(chat_file, 'r', encoding='utf-8') as f:
+                lines = f.readlines()
+            
+            if not lines:
+                raise ValueError(f"Empty chat file: {role_name}/{chat_name}")
+            
+            # 验证目标楼层
+            # floor + 1 是因为第0行是header
+            message_line_index = target_floor + 1
+            if message_line_index >= len(lines):
+                raise ValueError(f"Floor {target_floor} not found in chat (total messages: {len(lines) - 1})")
+            
+            # 生成新聊天名称
+            if not new_chat_name:
+                timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
+                new_chat_name = f"branch_{chat_name}_{timestamp}"
+            
+            # 创建新聊天文件
+            new_chat_file = self.chat_dir / role_name / f"{new_chat_name}.jsonl"
+            
+            if new_chat_file.exists():
+                raise FileExistsError(f"Branch chat already exists: {new_chat_name}")
+            
+            # 复制 header 和目标楼层及之前的消息
+            branch_lines = [lines[0]]  # header
+            for i in range(1, message_line_index + 1):
+                msg_data = json.loads(lines[i])
+                # 重新分配 floor（从0开始）
+                msg_data["floor"] = i - 1
+                branch_lines.append(json.dumps(msg_data, ensure_ascii=False) + '\n')
+            
+            # 写入新文件
+            with open(new_chat_file, 'w', encoding='utf-8') as f:
+                f.writelines(branch_lines)
+            
+            message_count = len(branch_lines) - 1  # 减去header
+            
+            logger.info(
+                f"[ChatService] 创建分支成功: {role_name}/{chat_name} -> {new_chat_name}, "
+                f"楼层: 0-{target_floor}, 消息数: {message_count}"
+            )
+            
+            return {
+                "success": True,
+                "new_chat_name": new_chat_name,
+                "message_count": message_count,
+                "branched_from": chat_name,
+                "target_floor": target_floor
+            }
+            
+        except (FileExistsError, ValueError):
+            raise
+        except Exception as e:
+            logger.error(f"创建分支失败 {role_name}/{chat_name}: {str(e)}")
+            raise
 
 
 # 全局实例