A query-conditioned token classifier that highlights supporting evidence spans in scientific paper chunks. Fine-tuned from Alibaba-NLP/gte-reranker-modernbert-base on silver spans from KRLabsOrg/acl-verbatim-spans.

Input: (question, context) — output: character spans in context that support the answer, with confidence scores.

The model uses the full 8192-token ModernBERT context, so long paper chunks are handled without aggressive truncation. On the current all-row ACL gold benchmark, this 150M-parameter model achieves the best committed word-level F1 among the evaluated extractors.

Quick Start

python

from transformers import AutoModel

model = AutoModel.from_pretrained(
    "KRLabsOrg/acl-verbatim-modernbert",
    trust_remote_code=True,
)

question = "What is ModernBERT?"
context = (
    "ModernBERT is a long-context encoder for NLP. "
    "It supports sequences up to 8192 tokens. "
    "Unlike earlier BERT variants, it uses rotary position embeddings."
)

result = model.process(
    question=question,
    context=context,
    threshold=0.2,
    return_sentence_metrics=True,
)

for span in result["spans"]:
    print(f"[{span['score']:.2f}] {span['text']}")

Example output:

[0.93] ModernBERT is a long-context encoder for NLP.
[0.87] It supports sequences up to 8192 tokens.

Parameters

arg	default	notes
`question`	—	Query string
`context`	—	Passage to search for supporting spans
`threshold`	`0.2`	Probability cutoff for marking a token as evidence. Use `0.2` for balanced F1, `0.5` for high precision
`max_length`	`8192`	Max tokens per window (ModernBERT supports 8192)
`doc_stride`	`256`	Overlap between windows for long contexts
`min_span_chars`	`10`	Drop predicted spans shorter than this many characters
`merge_gap_chars`	`20`	Merge adjacent predicted spans separated by ≤ this many characters
`return_sentence_metrics`	`False`	Also return per-sentence mean evidence score

min_span_chars and merge_gap_chars together clean up token-level fragmentation: without them, binary token labels often produce a "shotgun" of 3–10-character pseudo-spans that hurt span-level metrics. The defaults are what we use in our evaluation.

Return shape

python

{
    "spans": [
        {"start": int, "end": int, "text": str, "score": float},
        ...
    ],
    "sentences": [  # only when return_sentence_metrics=True
        {"start": int, "end": int, "text": str, "score": float},
        ...
    ],
}

Spans are character offsets into the input context. They are merged across sliding windows, so callers do not need to deduplicate.

Raw Inference

If you prefer to skip the .process() helper and do the post-processing yourself:

python

from transformers import AutoTokenizer, AutoModelForTokenClassification

tokenizer = AutoTokenizer.from_pretrained("KRLabsOrg/acl-verbatim-modernbert")
model = AutoModelForTokenClassification.from_pretrained(
    "KRLabsOrg/acl-verbatim-modernbert"
)

enc = tokenizer(
    question, context,
    return_offsets_mapping=True,
    max_length=8192,
    truncation="only_second",
    return_tensors="pt",
)
logits = model(
    input_ids=enc["input_ids"], attention_mask=enc["attention_mask"]
).logits
labels = logits.argmax(dim=-1)

Label 0 is "outside", label 1 is "evidence" (binary scheme).

Training

item	value
base model	`Alibaba-NLP/gte-reranker-modernbert-base`
dataset	`KRLabsOrg/acl-verbatim-spans` (`encoder` config)
label scheme	binary (`0` outside, `1` evidence)
max_length	8192
doc_stride	256
batch size	8
learning rate	2e-5
epochs	5
best checkpoint	silver-dev F1 = 0.642 at epoch 3.21

We started from Alibaba-NLP/gte-reranker-modernbert-base rather than vanilla answerdotai/ModernBERT-base because the reranker backbone has already been post-trained on query/passage relevance — the semantic prior it provides gives a large head start on query-conditioned span extraction.

Reproduce with:

bash

python acl_verbatim/span_training/train_token_cls.py \
  --hf-dataset KRLabsOrg/acl-verbatim-spans \
  --hf-config encoder \
  --train-split train \
  --eval-split validation \
  --model Alibaba-NLP/gte-reranker-modernbert-base \
  --output-dir runs/models/acl-verbatim-modernbert \
  --batch-size 8 \
  --lr 2e-5 \
  --epochs 5 \
  --label-scheme binary

Evaluation

Scored on the canonical/test split of KRLabsOrg/acl-verbatim-spans (20 queries × 5 retrieved chunks: 100 rows total, 47 relevant rows with 78 gold spans, and 53 irrelevant negative rows) with the shared span metrics in acl_verbatim.eval.span_metrics. Irrelevant rows have empty gold spans, so false-positive extracted text lowers precision.

Headline numbers (balanced config: threshold=0.2 + merge)

metric	value
word-F1 (micro)	0.536
word precision	0.654
word recall	0.454
span F1 @ IoU 0.3	0.473
span F1 @ IoU 0.5	0.389
containment F1 @ 0.5	0.527
containment F1 @ 0.8	0.343
containment F1 @ 1.0	0.273
gold-coverage recall @ 0.5	0.423
gold-coverage recall @ 0.8	0.372
recall @ any-overlap	0.500
over-prediction ratio	0.846
mean latency (local eval run)	0.468 s

How this compares

On the same benchmark and harness:

system	word-F1	IoU F1 @ 0.5	any-overlap R
acl-verbatim-modernbert (this model)	0.536	0.389	0.500
glm-5	0.487	0.287	0.795
mistral-small-2603	0.469	0.173	0.782
qwen-3.6-paragraph	0.467	0.424	0.692
mistral-small-2603-paragraph	0.466	0.346	0.795
qwen-3.6-default	0.427	0.234	0.641
nemotron-120b-a12b	0.409	0.302	0.667
nemotron-120b-paragraph	0.407	0.435	0.667
provence-reranker-pruner	0.344	0.153	0.718
zilliz semantic-highlight	0.301	0.113	0.513

These are all-row scores: irrelevant retrieved chunks are included as negative examples and false-positive evidence on those rows lowers precision.

Threshold / post-processing ablation

The released default is threshold=0.2, min-span length 10, and merge gap 20. Additional threshold ablations can be regenerated with acl_verbatim/span_training/evaluate_token_cls.py. The threshold=0.2 + merge configuration is the default in model.process().

See the acl-verbatim repo for the full benchmark harness, LLM extractor scripts, and qualitative analysis.

Intended Use

Query-conditioned evidence highlighting over scientific text
Re-ranking or filtering of retrieval outputs for extractive QA
Dataset annotation assistance
Local alternative to LLM extractors for evidence selection

Limitations

Trained on ACL Anthology markdown; transfer to other scientific domains (biomedical, legal, patents) is not evaluated.
Silver supervision inherits noise from the LLM teacher and the retriever. Recall in particular reflects teacher behaviour: the model rarely extracts a passage the teacher would have skipped.
The gold benchmark is small (20 queries, 100 query--chunk rows, 47 relevant chunks, 78 gold spans) and single-annotator; confidence intervals on the headline numbers are wide.
Tables and figures are represented through their caption text; the model has no structural awareness of tabular data.
Any-overlap recall (0.500) lags the LLM extractors rerun here, meaning the model sometimes predicts nothing on chunks that contain relevant evidence. For high-recall applications, lower threshold further or combine with an LLM fallback.

Citation

bibtex

@misc{Recski:2026,
      title={ACL-Verbatim: hallucination-free question answering for research}, 
      author={Gábor Recski and Szilveszter Tóth and Nadia Verdha and István Boros and Ádám Kovács},
      year={2026},
      eprint={2605.21102},
      archivePrefix={arXiv},
      primaryClass={cs.CL},
      url={https://arxiv.org/abs/2605.21102}, 
}