Fathom v19 / styxx v3.9.1: Cross-Dataset Validated Hallucination Prevention via the Trust Layer

Headline: v3. 9. 1 is the cross-dataset validated correction of v3. 9. 0. We caught our own overfitting in public and shipped the fix in the same day. What v3. 9. 0 claimed: AUC 0. 9012 on HaluEval-QA. What cross-dataset validation revealed: v3. 9. 0 collapsed to AUC 0. 56-0. 63 on HaluEval-Dialog, HaluEval-Summarization, and TruthfulQA. The 0. 90 was a single-benchmark overfit. What v3. 9. 1 ships: four new response-novelty signals (contentₙovelty, entityₙovelty, numberₙovelty, bigramₙovelty, trigramₙovelty) that ask what the response ADDED that the reference doesn't support. Refit a pooled logistic regression on all four datasets combined (n=800 train, n=400 held-out test, seed 31, L2=0. 05, 8 features). Honest cross-dataset held-out AUC: HaluEval-QA: 1. 0000 (was 0. 9049) TruthfulQA: 0. 9767 (was 0. 6261) HaluEval-Summarization: 0. 5954 (was 0. 5897) HaluEval-Dialog: 0. 6014 (was 0. 5984) mean: 0. 7934 (was 0. 6548) Honest limits: Dialog and summarization remain at AUC ~0. 60. The fundamental issue is that faithful dialog/summary responses naturally add content not verbatim in the reference, so pure-novelty signals can't discriminate. True cross-dataset generalization needs NLI-style entailment. That is v4. 0. What survives: on the two largest hallucination-detection QA benchmarks (HaluEval-QA, TruthfulQA), styxx. guardrail reaches AUC 1. 00 and 0. 98 respectively — above every published baseline we have compared against (SelfCheckGPT 0. 71-0. 79, KnowHalu 0. 74, HaluCheck 0. 82). This is a real and defensible claim, narrower than "solves hallucination" but substantiated. API: unchanged from v3. 9. 0. from styxx import trust followed by @trust on any LLM-calling function. Zero config. Shape-preserving. Sync and async. Four halt policies. Tests: 11 new tests for response-novelty signals. Full suite: 573 pass, 1 skip, 0 fail. Installation: pip install styxx==3. 9. 1 Bundled files: styxx-v3. 9. 1-zenodo-osf-bundle. zip — wheel + sdist + README + CHANGELOG + LICENSE + trustdemo. py + cross-dataset result JSONs + paper PDF crossdatasetbenchmark. json — raw benchmark output (v3. 9. 0 weights on 4 datasets) crossdatasetcalibration. json — v3. 9. 1 pooled LR weights and per-dataset held-out AUC fathom-paper-3-guardrail. pdf — paper (will be revised for v4. 0 with cross-dataset update) The meta-move: we are the lab that catches its own overfitting in public and ships the fix the same day. Credibility over hype. License: CC-BY-4. 0 (this deposit, data, paper) / MIT (styxx code). Repository: github. com/fathom-lab/styxx (tag v3. 9. 1). Package: pypi. org/project/styxx/3. 9. 1. Predecessor: v18 (10. 5281/zenodo. 19702107) — retained in the record for historical accuracy; the v18 description is correct for v3. 9. 0's HaluEval-QA-only claim but does not reflect the cross-dataset validation we ran afterward.