diff --git a/backend/app/api/v1/router.py b/backend/app/api/v1/router.py
index f72ad44..1d4c8d2 100644
--- a/backend/app/api/v1/router.py
+++ b/backend/app/api/v1/router.py
@@ -818,7 +818,7 @@ def agent_heartbeat(payload: AgentHeartbeat, authorization: str | None = Header(
     agent.status = "online"
     agent.version = payload.version
     agent.last_seen_at = datetime.utcnow()
-    agent.last_payload = payload.model_dump()
+    agent.last_payload = payload.model_dump(mode="json")
     for old_flow in db.scalars(select(TrafficFlow).where(TrafficFlow.node_id == node.id)).all():
         db.delete(old_flow)
     for raw_flow in payload.flows[:1000]: