fix(guardrails): keep a Gemini upstream error frame as its own terminal frame after masking

litellm/proxy/guardrails/gemini_sse.py

@@ -3,10 +3,11 @@
 The Google ``:streamGenerateContent`` route relays the upstream ``data:`` frames as raw bytes, so a
 guardrail's ``async_post_call_streaming_iterator_hook`` cannot read them as chunk objects. These
 helpers fold such a stream into the one response body a non-streaming call would have returned,
-rewrite its text and function-call arguments in place, and re-emit it as one frame. Every other
-field (thought signatures, function-call ids, model version, response id, safety ratings, usage)
-is carried through untouched, because a client echoes the model turn back on its next request and
-Gemini 3 rejects a function call whose thought signature is missing.
+rewrite its text and function-call arguments in place, and re-emit it as one frame, followed by any
+upstream error frame kept as its own terminal frame. Every other field (thought signatures,
+function-call ids, model version, response id, safety ratings, usage) is carried through untouched,
+because a client echoes the model turn back on its next request and Gemini 3 rejects a function
+call whose thought signature is missing.
 """
 
 from __future__ import annotations
@@ -63,7 +64,8 @@ async def mask_gemini_sse_stream(all_chunks: Sequence[object], mask_text: TextMa
 
     Text arrives split across frames, so the fragments of one text part are joined before they are
     scanned; PII that only exists once the fragments meet is otherwise forwarded in halves. Top-level
-    keys and candidate keys take the last frame's value, and candidates are merged by index.
+    keys and candidate keys take the last frame's value, and candidates are merged by index. An
+    upstream error frame stays its own terminal frame after the masked response, as it arrived.
     """
     sse_stream: Final = joined_sse_stream(all_chunks)
     if sse_stream is None:
@@ -76,8 +78,20 @@ async def mask_gemini_sse_stream(all_chunks: Sequence[object], mask_text: TextMa
         return unreadable
     if masked_candidates == candidates:
         return GeminiStreamUnchanged()
-    response: Final = MappingProxyType({**_later_wins(events), "candidates": masked_candidates})
-    return GeminiStreamMasked((f"data: {_json_text(response)}\n\n".encode(),))
+    response: Final = MappingProxyType({**_without_error(_later_wins(events)), "candidates": masked_candidates})
+    return GeminiStreamMasked((_frame(response), *map(_frame, _error_frames(events))))
+
+
+def _frame(event: _JsonObject) -> bytes:
+    return f"data: {_json_text(event)}\n\n".encode()
+
+
+def _without_error(event: _JsonObject) -> _JsonObject:
+    return MappingProxyType({key: value for key, value in event.items() if key != "error"})
+
+
+def _error_frames(events: Sequence[_JsonObject]) -> Iterator[_JsonObject]:
+    return (MappingProxyType({"error": event["error"]}) for event in events if "error" in event)
 
 
 def _json_text(value: object) -> str:

tests/test_litellm/proxy/guardrails/guardrail_hooks/test_presidio.py

@@ -2808,10 +2808,10 @@ async def test_apply_to_output_streaming_gemini_masked_function_call_arguments_t
 
 
 @pytest.mark.asyncio
-async def test_apply_to_output_streaming_gemini_error_frame_mid_stream_is_carried_on_the_masked_frame():
+async def test_apply_to_output_streaming_gemini_error_frame_mid_stream_keeps_its_own_terminal_frame():
     """
-    Gemini can end a stream with an error frame after content frames, and the
-    client reads the error off the last frame, so the masked re-emit keeps it.
+    Gemini can end a stream with an error frame after content frames, and a client
+    reads the failure off that terminal frame, so the masked re-emit keeps it separate.
     """
     frames = [
         _gemini_frame({"text": "The architect was John Smith."}),
@@ -2828,9 +2828,10 @@ async def test_apply_to_output_streaming_gemini_error_frame_mid_stream_is_carrie
         await _collect_masked_output(guardrail, mock_stream(), collected)
         await guardrail._close_http_session()
 
-    (frame,) = _gemini_frames(collected)
-    assert frame["candidates"][0]["content"]["parts"] == [{"text": "The architect was <PERSON>."}]
-    assert frame["error"] == {"code": 503, "message": "overloaded", "status": "UNAVAILABLE"}
+    masked, error = _gemini_frames(collected)
+    assert masked["candidates"][0]["content"]["parts"] == [{"text": "The architect was <PERSON>."}]
+    assert "error" not in masked
+    assert error == {"error": {"code": 503, "message": "overloaded", "status": "UNAVAILABLE"}}
 
 
 @pytest.mark.asyncio