diff --git a/src/agents/tools/sessions-send-tool.ts b/src/agents/tools/sessions-send-tool.ts
index d34a013fc1a..1b7ebbbfa9b 100644
--- a/src/agents/tools/sessions-send-tool.ts
+++ b/src/agents/tools/sessions-send-tool.ts
@@ -231,7 +231,9 @@ async function startAgentRun(params: {
       } catch (err) {
         const queueSummary =
           formatEmbeddedPiQueueFailureSummary(queueOutcome) ?? "active run queue rejected";
-        throw new Error(`${queueSummary}; fallback_failed error=${formatErrorMessage(err)}`);
+        throw new Error(`${queueSummary}; fallback_failed error=${formatErrorMessage(err)}`, {
+          cause: err,
+        });
       }
     }
     const response = await params.callGateway<{ runId: string }>({