fix(guardrails): keep the stream error message a string and drop an unreachable branch

litellm/proxy/guardrails/anthropic_sse.py

@@ -102,10 +102,13 @@ def anthropic_sse_error_frames(message: str | Mapping[str, object]) -> tuple[byt
     """Anthropic error event, for a failure discovered after the response headers were flushed.
 
     Once a keepalive ping has been sent a raise cannot reach the client, so the failure has to
-    travel as a frame. A structured message is emitted as JSON rather than stringified, so the
-    guardrail's own detail stays machine readable.
+    travel as a frame. A structured detail is JSON-serialized into the string so it stays machine
+    readable without violating the string-typed contract.
     """
-    body: Final = json.dumps(message, default=str)
+    # AnthropicErrorDetail.message is a str, so a structured detail is serialized into the string
+    # rather than nested as an object
+    text: Final = message if isinstance(message, str) else json.dumps(message, default=str)
+    body: Final = json.dumps(text)
     return (
         f'event: error\ndata: {{"type": "error", "error": {{"type": "guardrail_error", '
         f'"message": {body}}}}}\n\n'.encode(),

litellm/proxy/guardrails/guardrail_hooks/bedrock_guardrails.py

@@ -2664,9 +2664,6 @@ class BedrockGuardrail(CustomGuardrail, BaseAWSLLM):
                 )
                 if _original_usage is not None:
                     assembled_model_response.usage = _original_usage
-                if raw_sse:
-                    assembled_model_response.id = _pre_block_response.id
-                    assembled_model_response.model = _pre_block_response.model
                 output_guardrail_response = None
 
             #########################################################################

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

@@ -2954,6 +2954,23 @@ async def test_streaming_hook_block_stream_keeps_upstream_identity():
     assert b"my-proxy-alias" not in body
 
 
+@pytest.mark.asyncio
+async def test_streaming_block_error_frame_message_is_a_string():
+    """AnthropicErrorDetail.message is typed str, so a structured guardrail detail is serialized."""
+    guardrail = _sse_guardrail()
+
+    with patch.object(guardrail, "make_bedrock_api_request", new_callable=AsyncMock) as mock_api:
+        mock_api.side_effect = HTTPException(
+            status_code=400, detail={"error": "Violated guardrail policy", "guardrailIdentifier": "gid"}
+        )
+        delivered = await _drain_streaming_hook(guardrail)
+
+    frame = next(line for line in b"".join(delivered).decode().splitlines() if line.startswith("data: "))
+    message = json.loads(frame[6:])["error"]["message"]
+    assert isinstance(message, str)
+    assert json.loads(message)["error"] == "Violated guardrail policy"
+
+
 @pytest.mark.asyncio
 async def test_streaming_hook_fails_closed_when_raw_sse_cannot_be_assembled():
     """An unscannable stream must not be delivered: forwarding it silently disables the guardrail."""