LettuceDetect goes agentic — span-level hallucination detection across RAG, code, and tool output.

Paper: Beyond Document Grounding: Span-Level Hallucination Detection over Code, Tool Output, and Documents (GroundLM 2026 workshop at EMNLP 2026) · arXiv:2607.00895

lettucedect-v2-qwen-2b: Generative Hallucination Span Detection

TL;DR — results

A 2B model that localizes and types hallucinated spans across code, tool output, and prose in one pass:

• Unified test set (10,698): span-F1 0.689, example-F1 0.921, IoU 0.758.
• Code-agent answers: span-F1 0.602 / example-F1 0.835 — where it beats every alternative we tested: our own 8B sibling (LFM-8B, 0.507), a self-hosted 120B judge (0.21) and a 550B judge (0.22), and off-the-shelf detectors (HHEM / Lynx-8B / Granite-Guardian / MiniCheck, all ≈ chance). Large general judges over-flag generated code; this model doesn't.
• Established prose benchmarks: RAGTruth example-F1 0.818 (> LettuceDetect-large 0.792); PsiloQA strong across 14 languages.

One small, fast model — no separate judge, no giant LLM — for span-level hallucination detection from RAG to agentic coding.

Overview

lettucedect-v2-qwen-2b is a generative hallucination detector for Retrieval-Augmented Generation (RAG) and coding-agent settings. Given a user request, the supporting context, and an answer, it emits the exact spans of the answer that are not supported by the context, each tagged with a hallucination category and subcategory. Unlike the encoder models in the LettuceDetect family (token classifiers), this is an instruction-tuned generative model that returns structured JSON, so it localizes and types hallucinations in a single pass — across prose and code.

It is the first LettuceDetect model trained on a unified benchmark spanning code (SWE-bench-derived coding-agent traces) and prose (RAGTruth, PsiloQA, and synthetic ACL / README / tool-output / Wikipedia sources), in 14 languages.

Model Details

• Base model: Qwen3.5-2B (hybrid Gated DeltaNet + attention)
• Training: LoRA supervised fine-tuning (full bf16), merged to a standalone model
• Task: generative span detection — outputs {"hallucinated_spans": [{"text", "category", "subcategory"}]}
• Taxonomy: 3 categories (contradiction, fabricated_reference, unsupported_addition) × 13 subcategories
• Context: long-context (handles the full request + retrieved context + answer)
• Languages: English plus 13 more (via the multilingual PsiloQA portion)

How It Works

The model is prompted with a fixed, data-agnostic system prompt that defines a hallucination ("a substring of the answer not supported by the context") and enumerates the taxonomy, followed by the user request, the context, and the answer to verify. It replies with a JSON object listing each hallucinated span verbatim with its category and subcategory; a fully supported answer returns an empty list. Span offsets are recovered by matching each returned substring back into the answer.

Usage

The model is a generative span detector: serve it with vLLM (OpenAI-compatible) and send the LettuceDetect detection prompt, which defines a hallucination and enumerates the taxonomy. It replies with JSON; match each returned text back into the answer to get character offsets.

vllm serve KRLabsOrg/lettucedect-v2-qwen-2b --served-model-name lettucedect-v2-qwen-2b

The model expects the exact detection prompt it was trained on (below). Send the user request + context as the user turn, ending with the answer to verify.

import json
from openai import OpenAI

SYSTEM = """You are an expert annotator who identifies hallucinated spans in a generated answer with respect to a given context (the only trusted evidence). A hallucinated span is a substring of the answer that is not supported by the context. Spans consistent with the context are not hallucinations.

Quote each hallucinated span verbatim from the answer and classify it into exactly one category and one subcategory.

Categories (the kinds of unsupported span):
- contradiction: conflicts with the context (a wrong value, number, date, name, or relationship)
- fabricated_reference: an entity, name, identifier, or section that is absent from the context
- unsupported_addition: a claim, detail, or behavior the context never states

Subcategories:
- entity: a wrong or invented name, entity, or object
- temporal: an incorrect date, time, duration, or ordering
- numerical: an incorrect number, quantity, or amount
- value: a wrong value, setting, or attribute value
- relational: an incorrect relationship or association between things
- identifier: an invented identifier or name not found in the context
- section: a reference to a section, part, or location that does not exist
- attribute: an invented or incorrect attribute or property
- claim: an added factual claim the context does not support
- behavior: an added or changed action or behavior the context never states
- elaboration: extra detail or elaboration beyond what the context supports
- subjective: an unsupported subjective or evaluative statement
- unspecified: unsupported, with no more specific subtype

Reply with ONLY a JSON object (no markdown, no code fences): {"hallucinated_spans": [{"text": "...", "category": "...", "subcategory": "..."}]}. If nothing is unsupported, reply {"hallucinated_spans": []}."""

client = OpenAI(base_url="http://localhost:8000/v1", api_key="EMPTY")
context = "France is a country in Europe. Its capital is Paris."
question = "What is the capital of France? What is its population?"
answer = "The capital of France is Paris. Its population is 2 million."
user = f"User request: {question}\n\n{context}\n\nAnswer to verify:\n{answer}"

resp = client.chat.completions.create(
    model="lettucedect-v2-qwen-2b", temperature=0.0,
    messages=[{"role": "system", "content": SYSTEM}, {"role": "user", "content": user}],
)
spans = json.loads(resp.choices[0].message.content)["hallucinated_spans"]
for s in spans:  # recover character offsets in the answer
    s["start"] = answer.find(s["text"])
    s["end"] = s["start"] + len(s["text"])
# [{"text": "Its population is 2 million.", "category": "unsupported_addition",
#   "subcategory": "numerical", "start": 32, "end": 60}]

Or via the LettuceDetect package: serve the model with vLLM, point OPENAI_API_BASE at it, and call

from lettucedetect.models.inference import HallucinationDetector
det = HallucinationDetector(method="llm", model="KRLabsOrg/lettucedect-v2-qwen-2b")
spans = det.predict(context=[context], question=question, answer=answer, output_format="spans")

It auto-routes this model to its native training prompt and hallucinated_spans output (typed category/subcategory). Pass native=True if served under a different name, and include_reasoning=True to request per-span explanations.

With explanations (pure prompts)

To get a short per-span explanation without the LettuceDetect package, use the explanation variant of the system prompt. It differs from the base prompt in exactly two places — the instruction sentence gains one clause, and the output contract gains an "explanation" field:

SYSTEM_EXPL = """You are an expert annotator who identifies hallucinated spans in a generated answer with respect to a given context (the only trusted evidence). A hallucinated span is a substring of the answer that is not supported by the context. Spans consistent with the context are not hallucinations.

Quote each hallucinated span verbatim from the answer and classify it into exactly one category and one subcategory, and give a short explanation of why it is unsupported.

Categories (the kinds of unsupported span):
- contradiction: conflicts with the context (a wrong value, number, date, name, or relationship)
- fabricated_reference: an entity, name, identifier, or section that is absent from the context
- unsupported_addition: a claim, detail, or behavior the context never states

Subcategories:
- entity: a wrong or invented name, entity, or object
- temporal: an incorrect date, time, duration, or ordering
- numerical: an incorrect number, quantity, or amount
- value: a wrong value, setting, or attribute value
- relational: an incorrect relationship or association between things
- identifier: an invented identifier or name not found in the context
- section: a reference to a section, part, or location that does not exist
- attribute: an invented or incorrect attribute or property
- claim: an added factual claim the context does not support
- behavior: an added or changed action or behavior the context never states
- elaboration: extra detail or elaboration beyond what the context supports
- subjective: an unsupported subjective or evaluative statement
- unspecified: unsupported, with no more specific subtype

Reply with ONLY a JSON object (no markdown, no code fences): {"hallucinated_spans": [{"text": "...", "category": "...", "subcategory": "...", "explanation": "..."}]}. If nothing is unsupported, reply {"hallucinated_spans": []}."""

The user turn is unchanged. Each returned span then carries an explanation, e.g. {"text": "Its population is 2 million.", "category": "unsupported_addition", "subcategory": "numerical", "explanation": "The context does not state the population."}.

Production tip: enforce the output schema

At temperature=0 the model reliably emits valid JSON, but when embedding it in a serving stack (e.g. a router response filter), pass a strict schema so parsing can never fail — vLLM supports OpenAI structured outputs:

resp = client.chat.completions.create(
    model="lettucedect-v2-qwen-2b", temperature=0.0,
    messages=[{"role": "system", "content": SYSTEM}, {"role": "user", "content": user}],
    response_format={"type": "json_schema", "json_schema": {
        "name": "hallucination_detection", "strict": True,
        "schema": {
            "type": "object",
            "properties": {"hallucinated_spans": {"type": "array", "items": {
                "type": "object",
                "properties": {
                    "text": {"type": "string"},
                    "category": {"type": "string", "enum": ["contradiction", "fabricated_reference", "unsupported_addition"]},
                    "subcategory": {"type": "string", "enum": ["entity", "temporal", "numerical", "value", "relational", "identifier", "section", "attribute", "claim", "behavior", "elaboration", "subjective", "unspecified"]},
                },
                "required": ["text", "category", "subcategory"],
                "additionalProperties": False,
            }}},
            "required": ["hallucinated_spans"],
            "additionalProperties": False,
        },
    }},
)

(For the explanation variant, add "explanation": {"type": "string"} to properties and to required.)

Plain transformers (no vLLM, no lettucedetect)

import json, torch
from transformers import AutoModelForCausalLM, AutoTokenizer

model_id = "KRLabsOrg/lettucedect-v2-qwen-2b"
tok = AutoTokenizer.from_pretrained(model_id)
model = AutoModelForCausalLM.from_pretrained(model_id, torch_dtype="auto", device_map="auto")

SYSTEM = "..."  # the detection prompt shown above (verbatim)
context = "France is a country in Europe. Its capital is Paris."
question = "What is the capital of France? What is its population?"
answer = "The capital of France is Paris. Its population is 2 million."
user = f"User request: {question}\n\n{context}\n\nAnswer to verify:\n{answer}"

inputs = tok.apply_chat_template(
    [{"role": "system", "content": SYSTEM}, {"role": "user", "content": user}],
    add_generation_prompt=True, enable_thinking=False, return_tensors="pt", return_dict=True,
).to(model.device)
out = model.generate(**inputs, max_new_tokens=512, do_sample=False)
reply = tok.decode(out[0, inputs["input_ids"].shape[1]:], skip_special_tokens=True)

# raw_decode reads the first JSON object and ignores any trailing text
spans = json.JSONDecoder().raw_decode(reply[reply.index("{"):])[0]["hallucinated_spans"]
for s in spans:  # recover character offsets in the answer
    s["start"] = answer.find(s["text"]); s["end"] = s["start"] + len(s["text"])
print(spans)
# -> [{'text': '2 million', 'category': 'unsupported_addition', 'subcategory': 'claim', 'start': 50, 'end': 59}]

Performance

Char-level span-F1 / example-F1 / IoU on the unified test set (10,698 samples). The model beats the mmBERT-base encoder on every source, span- and example-level.

Its strength is unified cross-domain coverage — one model handling code, prose, and 14 languages — rather than a single-benchmark record.

Prose benchmarks: RAGTruth & PsiloQA

On the established prose benchmarks the same model is competitive with or better than specialized methods, so the code/tool sources extend rather than trade off against prose.

RAGTruth (official test set, n=2700) — example-level F1, on the standard leaderboard:

Second only to a fine-tuned 8B, and above the v1 detector, the fine-tuned 13B, Luna, and GPT-4. (Our span-level on RAGTruth: P 0.601 / R 0.548 / F1 0.574 / IoU 0.765.)

PsiloQA (multilingual) — span IoU. Our generative 2B matches PsiloQA's best fine-tuned encoder and is far above the strongest LLM judge:

Across all 14 languages our IoU is 0.689. Numbers for the other methods are from the PsiloQA paper (arXiv 2510.04849); our IoU uses the unified char-overlap scorer, so cross-paper rows are indicative rather than an identical protocol. Full per-language results:

PsiloQA — per language (n=2897, 14 languages):

Example-F1 stays high across all 14 languages (answer-level detection is reliable); span-F1 is strongest for higher-resource languages, as expected.

Baselines on code-agent

Code-agent (2,015 samples) is where off-the-shelf detectors and even frontier LLM judges collapse, and where this model's value is clearest:

The off-the-shelf detectors and the 550B judge over-flag on code-agent (balanced accuracy near chance) because a generated code patch is not literally present in the context.

Citing

@inproceedings{kovacs-etal-2026-beyond,
    title = "Beyond Document Grounding: Span-Level Hallucination Detection over Code, Tool Output, and Documents",
    author = "Kov{\'a}cs, {\'A}d{\'a}m  and
      He, Bowei  and
      Liu, Xue  and
      Boros, Istv{\'a}n  and
      T{\'o}th, Szilveszter  and
      Recski, Gabor",
    editor = "Wang, Yimu  and
      Choi, Yee Man  and
      Wu, Di  and
      Ouyang, Siqi  and
      Azadani, Mozhgan Nasr  and
      Fung, Yi R. (May)",
    booktitle = "Proceedings of the 1st Workshop on Grounding Language Models: Learning Faithfully and Efficiently ({G}round{LM} 2026)",
    month = oct,
    year = "2026",
    address = "Budapest, Hungary",
    publisher = "Association for Computational Linguistics",
    url = "https://aclanthology.org/2026.groundlm-1.44/",
    pages = "566--577",
    ISBN = "979-8-89176-478-1"
}

Also on arXiv: 2607.00895.