diff --git a/src/server/responses/combo-stream-preflight.ts b/src/server/responses/combo-stream-preflight.ts
index 5cdc1b4ddb6..e6a95e518ba 100644
--- a/src/server/responses/combo-stream-preflight.ts
+++ b/src/server/responses/combo-stream-preflight.ts
@@ -121,7 +121,8 @@ function retryableZeroOutputTerminal(payload: unknown): boolean {
   if (event.type === "response.failed") return true;
   if (event.type !== "response.incomplete") return false;
   const reason = event.response?.incomplete_details?.reason;
-  return typeof reason === "string" && RETRYABLE_ZERO_OUTPUT_INCOMPLETE_REASONS.has(reason);
+  return reason === undefined
+    || (typeof reason === "string" && RETRYABLE_ZERO_OUTPUT_INCOMPLETE_REASONS.has(reason));
 }
 
 /**
diff --git a/tests/routing/combo-stream-preflight.test.ts b/tests/routing/combo-stream-preflight.test.ts
index 29e6d34c3e7..a4487a4b7c3 100644
--- a/tests/routing/combo-stream-preflight.test.ts
+++ b/tests/routing/combo-stream-preflight.test.ts
@@ -116,6 +116,20 @@ describe("combo stream preflight", () => {
     }
   });
 
+  test("converts a zero-output incomplete without a stated reason into a retryable failure", async () => {
+    for (const response of [
+      { id: "r1", status: "incomplete" },
+      { id: "r1", status: "incomplete", incomplete_details: null },
+    ]) {
+      const result = await preflightComboStreamResponse(sse(
+        { type: "response.created", response: { id: "r1", status: "in_progress" } },
+        { type: "response.incomplete", response },
+      ), { model: "m1", provider: "a" });
+      expect(result.kind).toBe("failed");
+      expect(result.response.status).toBe(502);
+    }
+  });
+
   test("does not replay semantic incompletes that another provider cannot safely repair", async () => {
     const source = sse(
       { type: "response.created", response: { id: "r1", status: "in_progress" } },