diff --git a/flowchart/serializers.py b/flowchart/serializers.py
index ad5cc22..e240d75 100644
--- a/flowchart/serializers.py
+++ b/flowchart/serializers.py
@@ -4,7 +4,7 @@ from .models import FlowchartSubmission
 
 class CreateFlowchartSubmissionSerializer(serializers.Serializer):
     problem_id = serializers.IntegerField()
-    mermaid_code = serializers.CharField()
+    mermaid_code = serializers.CharField(max_length=50000)
     flowchart_data = serializers.JSONField(required=False, default=dict)
 
     def validate_mermaid_code(self, value):
@@ -12,6 +12,12 @@ class CreateFlowchartSubmissionSerializer(serializers.Serializer):
             raise serializers.ValidationError("Mermaid代码不能为空")
         return value
 
+    def validate_flowchart_data(self, value):
+        import json
+        if len(json.dumps(value)) > 500 * 1024:
+            raise serializers.ValidationError("流程图数据过大")
+        return value
+
 
 class FlowchartSubmissionSerializer(serializers.ModelSerializer):
     class Meta:
diff --git a/flowchart/tasks.py b/flowchart/tasks.py
index 4f0b7c4..8a3939e 100644
--- a/flowchart/tasks.py
+++ b/flowchart/tasks.py
@@ -24,29 +24,27 @@ def evaluate_flowchart_task(submission_id):
         
         # 构建用户提示词，包含标准答案对比
         user_prompt = f"""
-        请对以下Mermaid流程图进行评分：
-        
-        学生提交的流程图：
-        ```mermaid
-        {submission.mermaid_code}
-        ```
+请对以下Mermaid流程图进行评分：
 
-        标准答案参考：
-        ```mermaid
-        {submission.problem.mermaid_code}
-        ```
-        """
-        # 如果有流程图提示，添加到提示词中
-        if submission.problem.flowchart_hint:
+学生提交的流程图：
+```mermaid
+{submission.mermaid_code}
+```
+"""
+        if submission.problem.mermaid_code:
             user_prompt += f"""
-        
-        设计提示：{submission.problem.flowchart_hint}
-        """
-        
-        user_prompt += """
-        
-        请按照评分标准进行详细评估，并给出0-100的分数。
-        """
+标准答案参考：
+```mermaid
+{submission.problem.mermaid_code}
+```
+"""
+        else:
+            user_prompt += "\n注意：此题没有标准答案，请根据题目描述和流程图的逻辑合理性进行评分。\n"
+
+        if submission.problem.flowchart_hint:
+            user_prompt += f"\n设计提示：{submission.problem.flowchart_hint}\n"
+
+        user_prompt += "\n请按照评分标准进行详细评估，并给出0-100的分数。\n"
         
         # 调用AI进行评分
         client = get_ai_client()
@@ -155,26 +153,15 @@ def build_evaluation_prompt(problem):
 """
 
 def parse_ai_evaluation_response(ai_response):
-    """解析AI评分响应"""
-    try:
-        import re
-        json_match = re.search(r'\{.*\}', ai_response, re.DOTALL)
-        if json_match:
-            data = json.loads(json_match.group())
-        else:
-            data = {
-                "score": 60,
-                "grade": "C",
-                "feedback": "AI评分解析失败，请重新提交",
-                "suggestions": "",
-                "criteria_details": {}
-            }
-        return data
-    except Exception:
-        return {
-            "score": 60,
-            "grade": "C", 
-            "feedback": "AI评分解析失败，请重新提交",
-            "suggestions": "",
-            "criteria_details": {}
-        }
+    """解析AI评分响应，解析失败时抛出异常由调用方处理"""
+    import re
+    json_match = re.search(r'\{.*\}', ai_response, re.DOTALL)
+    if not json_match:
+        raise ValueError("AI响应中未找到JSON数据")
+
+    data = json.loads(json_match.group())
+
+    if "score" not in data or "grade" not in data:
+        raise ValueError("AI响应缺少必要字段: score 或 grade")
+
+    return data
diff --git a/flowchart/views/oj.py b/flowchart/views/oj.py
index 4e43e19..9100c3c 100644
--- a/flowchart/views/oj.py
+++ b/flowchart/views/oj.py
@@ -22,8 +22,6 @@ class FlowchartSubmissionAPI(APIView):
 
         # 验证题目存在
         try:
-            from problem.models import Problem
-
             problem = Problem.objects.get(id=data["problem_id"])
         except Problem.DoesNotExist:
             return self.error("Problem doesn't exist")
@@ -65,6 +63,7 @@ class FlowchartSubmissionAPI(APIView):
 
 
 class FlowchartSubmissionListAPI(APIView):
+    @login_required
     def get(self, request):
         """获取流程图提交列表"""
         username = request.GET.get("username")
@@ -158,12 +157,18 @@ class FlowchartSubmissionDetailAPI(APIView):
             problem=problem,
             status=FlowchartSubmissionStatus.COMPLETED,
         ).order_by("create_time")
+        count = submissions.count()
+        if count == 0:
+            return self.success({"submission": None, "count": 0})
+        # page=0 means latest; page=N means the Nth submission (1-indexed, chronological)
         if page == 0:
             submission = submissions.last()
         else:
+            if page < 0 or page > count:
+                return self.error("Page out of range")
             submission = submissions[page - 1]
         serializer = FlowchartSubmissionSerializer(submission)
-        return self.success({"submission": serializer.data})
+        return self.success({"submission": serializer.data, "count": count})
 
 
 class FlowchartSubmissionCurrentAPI(APIView):