groundcheck-modernbert

GroundCheck v2 (ModernBERT-base)

A ~150M-parameter encoder that checks whether a RAG answer is supported by the source it was given. Given an answer and a source (and optionally the question), it returns grounded or hallucinated with P(grounded). It runs on CPU.

• Labels: 0 = grounded, 1 = hallucinated, and P(grounded) = softmax(logits)[0].
• Code, evaluation harness and provenance: https://github.com/Pranshurs/groundcheck
• Every number below is traceable to a committed report in that repository.

Which weights

Hashes for every file are in the repository's MODEL_PROVENANCE.json. python -m eval.provenance --verify re-checks them.

Results

All measured on CPU against the pinned weights, using test sets rebuilt from pinned public datasets. F1 is for the hallucinated class, and the 95% CIs come from a 2,000-resample bootstrap.

• 512 tokens reproduces the training run's published numbers (F1 0.6824, acc 0.7468; VitaminC acc 0.8495) to within one prediction in 2,500.
• On the same rows, 2048 tokens scores a paired ΔF1 of +0.014 (95% CI −0.001 to +0.028). That's a small gain, at about 2.5× the CPU time on long documents.
• The flipped-fact holdout is regenerated. The original run's sample depended on Python's per-process hash seed and can't be rebuilt. The original sample reported 80.4% caught and 76.2% kept.
• RAGTruth uses the first 2,500 of the 2,700 test responses, the same subset the training run evaluated on.

External published reference (not a controlled comparison)

The RAGTruth paper (Niu et al., 2024; tabulated in LettuceDetect, 2025) reports F1 0.634 for a zero-shot GPT-4-turbo prompt judge. That figure comes from a different protocol: a prompted judge scored on all 2,700 test responses. It was not run head-to-head with GroundCheck, so it's there for orientation and doesn't support a "beats GPT-4" claim.

Latency (one machine, not a guarantee)

Apple M1, CPU, 4 torch threads, single requests after warm-up, at max_length 2048:

Other hardware will differ. Measure with python -m bench.latency.

Usage

from transformers import AutoTokenizer, AutoModelForSequenceClassification
import torch

name, rev = "Pranshurs/groundcheck-modernbert", "998cec35563d6b90947409d1c7510adac7f7c80c"
tok = AutoTokenizer.from_pretrained(name, revision=rev)
model = AutoModelForSequenceClassification.from_pretrained(name, revision=rev).eval()

source = "France's capital and largest city is Paris."
answer = "Paris is the capital of France."
enc = tok(source, answer, truncation="only_first", max_length=512, return_tensors="pt")
with torch.no_grad():
    grounded = torch.softmax(model(**enc).logits, dim=-1)[0, 0].item()
print("grounded" if grounded >= 0.5 else "hallucinated", round(grounded, 3))

Or use the library (pip install "groundcheck-rag[model]"), which pins the revision, truncates only the source, and raises an error rather than substituting a heuristic when the model can't load:

from groundcheck import GroundCheck
print(GroundCheck().check(source="...", answer="..."))

Training

Fine-tuned from answerdotai/ModernBERT-base (pinned at 8949b909) as a sequence-pair classifier: the premise is the optional question plus the source, and the hypothesis is the answer. The training run was on a single Kaggle P100 with torch 2.4.1 and transformers 4.49.0: 3 epochs, batch 16, learning rate 2e-5, linear schedule, warmup 0.06, weight decay 0.01, seed 42, fp16, sequence length 512.

The 28,500 training rows break down as: - 10,000 from RAGTruth (wandb/RAGTruth-processed @ eb4f4b9d), with the question dropped on ~50% of rows. - 16,000 from VitaminC (tals/vitaminc @ be6febb7): SUPPORTS → grounded; REFUTES and NOT ENOUGH INFO → hallucinated. - 2,500 rule-based one-fact flips (a number, date, direction word or entity) of grounded RAGTruth answers.

There is no LLM-generated augmentation and no private data. The recipe and data builders are in the repository's training/ directory.

Intended use

Use it as a post-generation check in RAG pipelines: flag answers the retrieved source doesn't support, so they can be routed to review, regeneration or a stronger checker. It checks support against the provided source only and isn't a world-knowledge fact-checker.

Limitations

• English only. Verdicts are per answer, not per span.
• Long sources are truncated from the end. 76% of RAGTruth test pairs exceed 512 tokens and 19 of 2,500 exceed 2,048, so chunk long documents.
• RAGTruth precision is about 0.63, so roughly a third of hallucinated verdicts on long RAG answers are false alarms. Scores aren't calibrated probabilities; tune the threshold on your own data.
• About a quarter of unedited grounded answers in the minimal-edit holdout are flagged.
• It hasn't been evaluated on adversarial or out-of-domain inputs.

License and data terms

The upstream data terms (detailed in the repository's DATA_LICENSES.md): - RAGTruth is MIT. Its source passages come from: - MS MARCO: Microsoft's terms allow non-commercial research use only. - The Yelp Open Dataset: academic and non-commercial use only. - CNN/DailyMail: the articles are copyrighted by their publishers. - VitaminC is CC BY-SA 3.0.

Whether a dataset's non-commercial terms extend to a model trained on it is legally unsettled. Review the data terms before any commercial use. This card is not legal advice.