fix(responses): guard malformed non-list response.output in WebSocket masking

Greptile re-review on PR #35353 flagged that, unlike content and summary,
response.output itself was iterated without an isinstance(..., list) check
in both _unmask_response_event and _mask_response_completed. A truthy
non-list output value raises TypeError, which is caught by the outer
exception handler in backend_to_client and terminates the whole forwarding
loop for that connection, not just the one malformed event. This predates
this branch (same gap exists on litellm_internal_staging) but sits in
functions this PR already touches, so fixing it here.

Adds a regression test that fails with the guard removed and passes with
it in place.

litellm/responses/streaming_iterator.py

@@ -1684,7 +1684,10 @@ class ResponsesWebSocketStreaming:
             response_obj = evt_obj.get("response")
             if not isinstance(response_obj, dict):
                 return response_str
-            for output_item in response_obj.get("output") or []:
+            output = response_obj.get("output") or []
+            if not isinstance(output, list):
+                return response_str
+            for output_item in output:
                 if not isinstance(output_item, dict):
                     continue
                 content = output_item.get("content") or []
@@ -1739,7 +1742,10 @@ class ResponsesWebSocketStreaming:
             response_obj = evt_obj.get("response")
             if not isinstance(response_obj, dict):
                 continue
-            for output_item in response_obj.get("output") or []:
+            output = response_obj.get("output") or []
+            if not isinstance(output, list):
+                continue
+            for output_item in output:
                 if not isinstance(output_item, dict):
                     continue
                 arguments = output_item.get("arguments")

tests/test_litellm/responses/test_responses_websocket_all_providers.py

@@ -1122,6 +1122,37 @@ class TestNativeWebSocketGuardrails:
         assert handler._unmask_response_event(event) == event
         assert await handler._mask_response_completed(event) == event
 
+    @pytest.mark.asyncio
+    async def test_completed_event_with_non_list_output_passes_through(self):
+        """Regression test: a "response.output" that is a truthy non-iterable
+        value (malformed upstream payload) must be skipped, not raise -- an
+        unhandled exception here terminates the whole backend_to_client loop,
+        not just this one event."""
+        from unittest.mock import MagicMock
+
+        from litellm.responses.streaming_iterator import ResponsesWebSocketStreaming
+
+        class Guardrail:
+            def get_presidio_settings_from_request_data(self, request_data):
+                return None
+
+            def _unmask_pii_text(self, text, pii_tokens):
+                return text
+
+        event = json.dumps({"type": "response.completed", "response": {"output": 42}})
+        guardrail = Guardrail()
+        handler = ResponsesWebSocketStreaming(
+            websocket=MagicMock(),
+            backend_ws=MagicMock(),
+            logging_obj=MagicMock(),
+            request_data={"metadata": {"pii_tokens": {"<TOKEN_1>": "secret"}}},
+            guardrail_callbacks=[guardrail],
+            output_guardrail_callbacks=[guardrail],
+        )
+
+        assert handler._unmask_response_event(event) == event
+        assert await handler._mask_response_completed(event) == event
+
     @pytest.mark.asyncio
     async def test_output_masking_suppresses_delta_without_calling_presidio(self):
         import json