12/29 papers had zero citations: OpenAlex indexes arXiv preprints and theses
without a parsed reference list (verified referenced_works_count=0 server-side
for all 11 with an openalex_id). Not an ingest bug — a source-coverage limit.
- ingest.py: when OpenAlex returns no references, fall back to a Semantic
Scholar reference supplement (_s2_reference_supplement); citing_id rewritten
to canonical papers.id, DOI cited-ids lowercased. Removed a now-redundant
discarded S2 probe in the OpenAlex-404 arXiv branch.
- semanticscholar.py: fix TypeError on S2's HTTP-200 {"data": null} no-refs
responses (.get("data", []) returns None when the key is present-but-null).
- tests: regression test for the OpenAlex-empty -> S2 path.
- Live DB backfilled idempotently: +330 citations (590->920), zero-out-edge
papers 12->2, citing coverage 17->27/29. Only the 2 TU-Berlin depositonce
theses remain (no references in any source).
- docs: DATA-QUALITY-2026-06-15.md — DQ-1 resolution, DQ-4 result, and a
free-source acquisition roadmap (R-A..R-E).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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