diff --git a/api/core/ops/entities/trace_entity.py b/api/core/ops/entities/trace_entity.py
index f0e34c0cd7..151fa2aaf4 100644
--- a/api/core/ops/entities/trace_entity.py
+++ b/api/core/ops/entities/trace_entity.py
@@ -3,7 +3,7 @@ from datetime import datetime
 from enum import StrEnum
 from typing import Any, Optional, Union
 
-from pydantic import BaseModel, ConfigDict, field_validator
+from pydantic import BaseModel, ConfigDict, field_serializer, field_validator
 
 
 class BaseTraceInfo(BaseModel):
@@ -24,10 +24,13 @@ class BaseTraceInfo(BaseModel):
             return v
         return ""
 
-    class Config:
-        json_encoders = {
-            datetime: lambda v: v.isoformat(),
-        }
+    model_config = ConfigDict(protected_namespaces=())
+
+    @field_serializer("start_time", "end_time")
+    def serialize_datetime(self, dt: datetime | None) -> str | None:
+        if dt is None:
+            return None
+        return dt.isoformat()
 
 
 class WorkflowTraceInfo(BaseTraceInfo):