Bug: VLM-generated Markdown with bracketed LaTeX causes crash in document assembly

Bug

Hello, I'm sharing Gemini diagnostic, but not entirely sure that is relevant:

The VlmPipeline fails during the document assembly stage when processing Markdown generated from a PDF that contains a multi-line LaTeX array environment. The VLM model (mistral-community/pixtral-12b-240910) sometimes wraps complex LaTeX blocks in square brackets ([...]) instead of the standard ... delimiters.The internal docling Markdown parser then incorrectly interprets this block as a single element with nested children (, , etc.). This complex, nested structure is passed to document.append_child_item(), which expects a "flat" element, triggering a ValueError: Can not append a child with children.The error originates in docling/pipeline/vlm_pipeline.py during the _turn_md_into_doc step, which attempts to build the document from the faulty Markdown.

Steps to reproduce

import logging
from pathlib import Path

from docling.datamodel.base_models import InputFormat
from docling.datamodel.pipeline_options import (
    VlmPipelineOptions,
)
from docling.datamodel.pipeline_options_vlm_model import ApiVlmOptions, ResponseFormat
from docling.document_converter import DocumentConverter, PdfFormatOption
from docling.pipeline.vlm_pipeline import VlmPipeline


def lms_vlm_options(model: str, prompt: str, format: ResponseFormat):
    options = ApiVlmOptions(
        url="http://localhost:8000/v1/chat/completions",
        api_key="", 
        params=dict(
            model=model,
        ),
        prompt=prompt,
        timeout=90,
        max_size=16024,
        temperature=0.0,
        response_format=format,
    )
    return options

def main():
    logging.basicConfig(level=logging.DEBUG)

    # Download the file from: http://www.doctortang.com/Algebra%202/Essential%20Formulas%20for%20Algebra%202%20Final%20Exam.pdf
    # And save it as "algebra_formulas.pdf"
    input_doc_path = Path("algebra_formulas.pdf")

    pipeline_options = VlmPipelineOptions(
        enable_remote_services=True
    )

    pipeline_options.vlm_options = lms_vlm_options(
        model="mistral-community/pixtral-12b-240910",
        prompt="OCR the full page to markdown.",
        format=ResponseFormat.MARKDOWN,
    )

    doc_converter = DocumentConverter(
        format_options={
            InputFormat.PDF: PdfFormatOption(
                pipeline_options=pipeline_options,
                pipeline_cls=VlmPipeline,
            )
        }
    )

    try:
        result = doc_converter.convert(input_doc_path)
        print(result.document.export_to_markdown())
    except ValueError as e:
        print(f"Caught expected error: {e}")


if __name__ == "__main__":
    main()

Logs

The script will fail with the following traceback when processing page 4 of the PDF:DEBUG:docling.backend.md_backend: Some other element:

<Paragraph children=[<Literal children='['>,
<LineBreak children='\n'>,
<RawText children='\\begin{array}{cc}'>,
<LineBreak children='\n'>,
<RawText children='\\text{Domain: } x \\in R & \\text{Domain: } x \\in R '>,
<Literal children='\\'>,
<RawText children=('\n'
'\\text{Range: } f(x) = \\lfloor \\lfloor x \\rfloor \\rfloor \\text{ or } '
'\\text{int}(x) & \\text{Range: } f(x) \\in I ')>,
<Literal children='\\'>,
<RawText children='\n\\end{array}'>,
<LineBreak children='\n'>,
<Literal children=']'>]>
...
Traceback (most recent call last):
  File "/demo/docling/demo.py", line 69, in <module>
    main()
    ~~~~^^
  File "/demo/docling/demo.py", line 66, in main
    result = doc_converter.convert(input_doc_path)
  ...
  File "/software/miniconda3/envs/docling_demo/lib/python3.13/site-packages/docling/pipeline/base_pipeline.py", line 47, in execute
    conv_res = self._assemble_document(conv_res)
  File "/software/miniconda3/envs/docling_demo/lib/python3.13/site-packages/docling/pipeline/vlm_pipeline.py", line 147, in _assemble_document
    conv_res.document = self._turn_md_into_doc(conv_res)
  File "/software/miniconda3/envs/docling_demo/lib/python3.13/site-packages/docling/pipeline/vlm_pipeline.py", line 300, in _turn_md_into_doc
    conv_res.document.append_child_item(child=item)
  File "/software/miniconda3/envs/docling_demo/lib/python3.13/site-packages/docling_core/types/doc/document.py", line 1956, in append_child_item
    raise ValueError("Can not append a child with children")
ValueError: Can not append a child with children

Docling version

Docling version: 2.41.0 Docling Core version: 2.42.0 Docling IBM Models version: 3.8.1 Docling Parse version: 4.1.0 Python: cpython-313 (3.13.5) Platform: Linux-5.15.0-143-generic-x86_64-with-glibc2.35

Python version

Python 3.13.5

Thank you for your help! :)