if not errors:
            # Shared serializer for stream items (JSONL and SSE).
            # Validates against stream_item_field when set, then
            # serializes to JSON bytes.
            def _serialize_data(data: Any) -> bytes:
                if stream_item_field:
                    value, errors_ = stream_item_field.validate(
                        data, {}, loc=("response",)
                    )