diff --git a/backend/xingbang/workflow.py b/backend/xingbang/workflow.py
index dbad315..4ca78e2 100644
--- a/backend/xingbang/workflow.py
+++ b/backend/xingbang/workflow.py
@@ -129,8 +129,14 @@ class XingbangWorkflow:
             "trajectory_id": traj_id,
             "customer": customer,
             "reward": reward,
-            "results": {k: {kk: vv for kk, vv in v.items() if kk in ("output", "file", "success")}
-                        for k, v in self._results.items()},
+            "results": {
+                k: {
+                    kk: (str(vv) if kk == "file" and vv is not None else vv)
+                    for kk, vv in v.items()
+                    if kk in ("output", "file", "success")
+                }
+                for k, v in self._results.items()
+            },
             "final_summary": summary_parts,
             "evolution_insights_file": str(insights_path),
         }