ToolACE Hallucination Detector

A lightweight ensemble classifier for detecting hallucinations in LLM responses that follow tool calls. It identifies three hallucination types — missing tool reference, unsupported overgeneration, and tool-output contradiction — and returns both a sample-level binary label and character-level span predictions.

Background

When an LLM answers a user query using tool outputs, it can hallucinate in distinct ways:

Type	Description
`missing_tool`	The assistant suggests using a tool that was not available
`overgeneration`	The answer contains plausible but unsupported extra facts
`tool_output_contradiction`	The answer contradicts specific facts returned by the tool

This model was trained on jameVee/ToolACE-Hallucination, a benchmark of ~4 100 examples derived from Team-ACE/ToolACE. Each of the three hallucination datasets was generated by corrupting ~50 % of clean ToolACE examples with a specific hallucination type (see dataset card for details).

Architecture

The detector is a three-branch soft-voting ensemble. Each branch independently produces a hallucination probability, and the three scores are averaged to give the final prediction.

                         ┌──────────────────────────────────┐
 query + context ──────► │  Branch A: Lexical Verifier       │ ──► P_A
 + tool output           │  (token overlap span detector     │
                         │   → StandardScaler + LogReg)      │
                         └──────────────────────────────────┘
                         ┌──────────────────────────────────┐
                    ───► │  Branch B: LettuceDetect          │ ──► P_B
                         │  (ModernBERT span model           │
                         │   → StandardScaler + LogReg)      │
                         └──────────────────────────────────┘
                         ┌──────────────────────────────────┐
                    ───► │  Branch C: LookBack-style         │ ──► P_C
                         │  (Qwen2.5-0.5B attention ratios  │
                         │   → StandardScaler + LogReg)      │
                         └──────────────────────────────────┘
                                          │
                              avg(P_A, P_B, P_C) ≥ 0.5
                                          │
                                     hallucinated?

Branch A — Lexical Span Verifier

Checks whether tokens in the assistant's answer are grounded in the (normalized) tool output using lexical overlap. Unsupported token sequences become candidate hallucination spans. Span-level features (count, coverage, max score) are fed into a logistic regression.

Branch B — LettuceDetect (supervised span model)

Uses KRLabsOrg/lettucedect-base-modernbert-en-v1, a ModernBERT-based transformer fine-tuned for grounded hallucination detection. It produces character-level spans scored by confidence. A logistic regression trained on span features converts those into a sample-level probability. Best single branch (AUROC 0.721, Span-F1 0.208).

Branch C — LookBack-style Detector

Passes the concatenation of context and answer through Qwen/Qwen2.5-0.5B and computes, for each answer token, the ratio of attention it pays to the context versus to previously generated tokens. Low-grounding tokens (ratio < 0.22) are merged into hallucination spans. Span and ratio features are fed to logistic regression.

Files in this repository

File	Description
`lexical_clf.joblib`	Branch A sklearn Pipeline (StandardScaler + LogisticRegression)
`lettuce_clf.joblib`	Branch B sklearn Pipeline (StandardScaler + LogisticRegression)
`lookback_clf.joblib`	Branch C sklearn Pipeline (StandardScaler + LogisticRegression)
`model_meta.json`	Feature column names, backbone IDs, train/test split sizes, test metrics
`evaluation_baselines_span_utils.py`	Span extraction helpers required at inference time

Performance (test set, 20 % held-out, grouped split by example_id)

Method	Accuracy	F1	AUROC	Span-F1
Lexical span verifier	0.566	0.523	0.603	0.053
LettuceDetect (Branch B)	0.678	0.632	0.721	0.208
LookBack (Branch C)	0.489	0.561	0.511	0.000
Soft-vote ensemble	0.676	0.659	0.721	0.179

Per-type F1 (ensemble):

Corruption type	F1
`missing_tool`	0.617
`overgeneration`	0.693
`tool_output_contradiction`	0.779
`clean`	—

How to use

1. Install dependencies

pip install joblib scikit-learn transformers lettucedetect torch

2. Load the classifiers

import joblib, json
from pathlib import Path

repo = Path("hallucination_detector")   # or your local clone path

lex_clf     = joblib.load(repo / "lexical_clf.joblib")
lettuce_clf = joblib.load(repo / "lettuce_clf.joblib")
lbl_clf     = joblib.load(repo / "lookback_clf.joblib")

with open(repo / "model_meta.json") as f:
    meta = json.load(f)

3. Load the backbone models

import torch
from lettucedetect.models.inference import HallucinationDetector
from transformers import AutoTokenizer, AutoModelForCausalLM

lettuce_detector = HallucinationDetector(
    method="transformer",
    model_path="KRLabsOrg/lettucedect-base-modernbert-en-v1",
)

DEVICE = "cuda" if torch.cuda.is_available() else "cpu"
lbl_tokenizer = AutoTokenizer.from_pretrained("Qwen/Qwen2.5-0.5B", use_fast=True)
lbl_model = AutoModelForCausalLM.from_pretrained(
    "Qwen/Qwen2.5-0.5B", torch_dtype=torch.bfloat16, attn_implementation="eager"
).to(DEVICE).eval()

4. Run inference

import sys
sys.path.insert(0, str(repo))    # make span utils importable
from evaluation_baselines_span_utils import (
    add_normalized_context_columns,
    aggregate_lookback_features,
    aggregate_span_features,
    lexical_hallucination_spans,
    spans_from_lookback_ratios,
    merge_spans,
)
import pandas as pd, numpy as np

def predict(query: str, context: str, output: str) -> dict:
    row = pd.Series({"query": query, "context": context, "output": output})
    # normalize context (converts raw tool JSON to readable text)
    row_df = add_normalized_context_columns(pd.DataFrame([row]))
    row = row_df.iloc[0]

    # Branch A — lexical
    lex_spans = lexical_hallucination_spans(row)
    lex_feats = aggregate_span_features(lex_spans, len(output))
    p_lex = lex_clf.predict_proba(pd.DataFrame([lex_feats]))[0, 1]

    # Branch B — LettuceDetect
    raw_spans = lettuce_detector.predict(
        context=[row["normalized_context"]],
        question=query, answer=output, output_format="spans",
    )
    lettuce_spans = merge_spans([
        {"start": int(s["start"]), "end": int(s["end"]),
         "text": output[int(s["start"]):int(s["end"])],
         "type": "hallucination", "score": float(s.get("score", 0.0))}
        for s in raw_spans if int(s.get("end", 0)) > int(s.get("start", 0))
    ])
    lettuce_feats = aggregate_span_features(lettuce_spans, len(output))
    p_lettuce = lettuce_clf.predict_proba(pd.DataFrame([lettuce_feats]))[0, 1]

    # Branch C — LookBack
    from evaluation_baselines_span_utils import aggregate_lookback_features
    # (reuse compute_lookback_ratios from the training notebook)
    # p_lbl = lbl_clf.predict_proba(pd.DataFrame([lbl_feats]))[0, 1]
    # For a self-contained example we skip Branch C and average A+B only:
    p_ensemble = np.mean([p_lex, p_lettuce])

    return {
        "hallucinated": bool(p_ensemble >= 0.5),
        "score": float(p_ensemble),
        "lex_score": float(p_lex),
        "lettuce_score": float(p_lettuce),
        "lettuce_spans": lettuce_spans,
    }

result = predict(
    query="What is the current price of AAPL?",
    context='Stock API: {"ticker": "AAPL", "price": 189.50, "change": "+1.2%"}',
    output="The current price of AAPL is $189.50, up 1.2%. It also hit an all-time high last Tuesday.",
)
print(result)

Training details

Source dataset: jameVee/ToolACE-Hallucination (1 034 base examples × 4 variants = 4 136 rows total)
Split: 80 / 20 grouped by example_id (no leakage between clean and corrupted variants of the same query)
Classifiers: scikit-learn LogisticRegression(max_iter=1000) wrapped in a StandardScaler pipeline
Random seed: 1241

About the training dataset

jameVee/ToolACE-Hallucination contains three JSONL files, each derived from Team-ACE/ToolACE:

missing_tool_dataset.jsonl — generated with unsloth/Meta-Llama-3.1-8B-Instruct-bnb-4bit (50 % corruption rate): a sentence is appended that refers to a non-existent tool.
overgeneration_dataset.jsonl — same generator (50 % corruption rate): a plausible but unsupported sentence is appended.
tool_output_contradiction_dataset.jsonl — generated with openai/gpt-4o-mini via OpenRouter (all entries attempted, strength 0.9): the answer is rewritten to contradict grounded facts from the tool output.

Each entry carries character-level hallucination_labels marking the corrupted span(s).

Limitations

The classifiers are trained on a relatively small dataset (~3 300 training rows). Performance may degrade on domains or tool schemas not represented in ToolACE.
Branch C (LookBack) shows weak span-level performance at the default threshold (0.22); tuning this on a validation split is recommended.
The ensemble does not produce type-specific labels — it only predicts binary hallucinated / clean at the sample level.

Citation

If you use this model or the associated datasets, please cite:

@misc{toolace_hallucination_detector,
  author    = {jameVee},
  title     = {ToolACE Hallucination Detector},
  year      = {2026},
  publisher = {Hugging Face},
  url       = {https://huggingface.co/jameVee/ToolACE-Hallucination-Detector}
}

@dataset{toolace_hallucination,
  author    = {jameVee},
  title     = {ToolACE-Hallucination},
  year      = {2026},
  publisher = {Hugging Face},
  url       = {https://huggingface.co/datasets/jameVee/ToolACE-Hallucination}
}

@dataset{toolace,
  author    = {Team-ACE},
  title     = {ToolACE},
  year      = {2024},
  publisher = {Hugging Face},
  url       = {https://huggingface.co/datasets/Team-ACE/ToolACE}
}

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Model tree for jameVee/ToolACE-Hallucination-Detector

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Finetuned

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