diff --git a/apps/api/src/api/v1/signoz_webhook.py b/apps/api/src/api/v1/signoz_webhook.py
index 289dcaf6..f9d136fe 100644
--- a/apps/api/src/api/v1/signoz_webhook.py
+++ b/apps/api/src/api/v1/signoz_webhook.py
@@ -151,7 +151,8 @@ async def process_signoz_alert(
             "error_type": labels.get("error_type", severity),
         }
         frequency = await anomaly_counter.record_anomaly(anomaly_signature)
-        anomaly_frequency = frequency.model_dump() if frequency else None
+        # 2026-03-31 ogt: AnomalyFrequency 是 dataclass，使用 to_dict() 而非 model_dump()
+        anomaly_frequency = frequency.to_dict() if frequency else None
 
         logger.info(
             "signoz_anomaly_recorded",