LettuceDetect 🥬🔍

Because even AI needs a reality check! 🥬

LettuceDetect is a lightweight and efficient tool for detecting hallucinations in Retrieval-Augmented Generation (RAG) systems. It identifies unsupported parts of an answer by comparing it to the provided context. The tool is trained and evaluated on the RAGTruth dataset and leverages ModernBERT for long-context processing, making it ideal for tasks requiring extensive context windows.

Our models are inspired from the Luna paper which is an encoder-based model and uses a similar token-level approach.

Highlights

• LettuceDetect addresses two critical limitations of existing hallucination detection models:
  • Context window constraints of traditional encoder-based methods
  • Computational inefficiency of LLM-based approaches
• Our models currently outperforms all other encoder-based and prompt-based models on the RAGTruth dataset and are significantly faster and smaller
• Achieves higher score than some fine-tuned LLMs e.g. LLAMA-2-13B presented in RAGTruth, coming up just short of the LLM fine-tuned in the RAG-HAT paper
• We release the code, the model and the tool under the MIT license

Get going

Features

• ✨ Token-level precision: detect exact hallucinated spans
• 🚀 Optimized for inference: smaller model size and faster inference
• 🧠 4K context window via ModernBERT
• ⚖️ MIT-licensed models & code
• 🤖 HF Integration: one-line model loading
• 📦 Easy to use python API: can be downloaded from pip and few lines of code to integrate into your RAG system

Installation

Install from the repository:

From pip:

pip install lettucedetect

Quick Start

Check out our models published to Huggingface:

• lettucedetect-base: https://huggingface.co/KRLabsOrg/lettucedect-base-modernbert-en-v1
• lettucedetect-large: https://huggingface.co/KRLabsOrg/lettucedect-large-modernbert-en-v1

You can get started right away with just a few lines of code.

from lettucedetect.models.inference import HallucinationDetector

# For a transformer-based approach:
detector = HallucinationDetector(
    method="transformer", model_path="KRLabsOrg/lettucedect-base-modernbert-en-v1"
)

contexts = ["France is a country in Europe. The capital of France is Paris. The population of France is 67 million.",]
question = "What is the capital of France? What is the population of France?"
answer = "The capital of France is Paris. The population of France is 69 million."

# Get span-level predictions indicating which parts of the answer are considered hallucinated.
predictions = detector.predict(context=contexts, question=question, answer=answer, output_format="spans")
print("Predictions:", predictions)

# Predictions: [{'start': 31, 'end': 71, 'confidence': 0.9944414496421814, 'text': ' The population of France is 69 million.'}]

Performance

Example level results

We evaluate our model on the test set of the RAGTruth dataset. Our large model, lettucedetect-large-v1, achieves an overall F1 score of 79.22%, outperforming prompt-based methods like GPT-4 (63.4%) and encoder-based models like Luna (65.4%). It also surpasses fine-tuned LLAMA-2-13B (78.7%) (presented in RAGTruth) and is competitive with the SOTA fine-tuned LLAMA-3-8B (83.9%) (presented in the RAG-HAT paper). Overall, lettucedetect-large-v1 and lettucedect-base-v1 are very performant models, while being very effective in inference settings.

The results on the example-level can be seen in the table below.

Span-level results

At the span level, our model achieves the best scores across all data types, significantly outperforming previous models. The results can be seen in the table below. Note that here we don't compare to models, like RAG-HAT, since they have no span-level evaluation presented.

How does it work?

The model is a token-level model that predicts whether a token is hallucinated or not. The model is trained to predict the tokens that are hallucinated in the answer given the context and the question.

flowchart LR
    subgraph Inputs
        Context["**Context**: France is a country in Europe. Population is 67 million."]
        Question["**Question**: What is the capital? What is the population?"]
        Answer["**Answer**: The capital of France is Paris. The population is 69 million."]
    end

    Model["**LettuceDetect**: Token Classification"]
    Tokens["**Token Probabilities**: <br> ... <br> The [0.01] <br> population [0.02] <br> is [0.01] <br> 69 [0.95] <br> million [0.95]"]

    Context --> Model
    Question --> Model
    Answer --> Model
    Model --> Tokens

Training a Model

You need to download the RAGTruth dataset first from here, then put it under the data/ragtruth directory. Then run

python lettucedetect/preprocess/preprocess_ragtruth.py --input_dir data/ragtruth --output_dir data/ragtruth

This will create a data/ragtruth/ragtruth_data.json file which contains the processed data.

Then you can train the model with the following command.

python scripts/train.py \
    --ragtruth-path data/ragtruth/ragtruth_data.json \
    --model-name answerdotai/ModernBERT-base \
    --output-dir output/hallucination_detector \
    --batch-size 4 \
    --epochs 6 \
    --learning-rate 1e-5 

We trained our models for 6 epochs with a batch size of 8 on a single A100 GPU.

Evaluation

You can evaluate the models on each level (example, token and span) and each data-type.

python scripts/evaluate.py \
    --model_path outputs/hallucination_detector \
    --data_path data/ragtruth/ragtruth_data.json \
    --evaluation_type example_level

Model Output Format

The model can output predictions in two formats:

Span Format

[{
    'text': str,        # The hallucinated text
    'start': int,       # Start position in answer
    'end': int,         # End position in answer
    'confidence': float # Model's confidence (0-1)
}]

Token Format

[{
    'token': str,       # The token
    'pred': int,        # 0: supported, 1: hallucinated
    'prob': float       # Model's confidence (0-1)
}]

Streamlit Demo

Check out the Streamlit demo to see the model in action.

Install streamlit:

Run the demo:

streamlit run demo/streamlit_demo.py

Use the Web API

LettuceDetect comes with it's own web API and python client library. To use it, make sure to install the package with the optional API dependencies:

or

pip install lettucedetect[api]

Start the API server with the scripts/start_api.py script:

python scripts/start_api.py dev  # use "prod" for production environments

Usage:

usage: start_api.py [-h] [--model MODEL] [--method {transformer}] {prod,dev}

Start lettucedetect Web API.

positional arguments:
  {prod,dev}            Choose "dev" for development or "prod" for production
                        environments. The serve script uses "fastapi dev" for "dev" or
                        "fastapi run" for "prod" to start the web server. Additionally
                        when choosing the "dev" mode, python modules can be directly
                        imported from the repositroy without installing the package.

options:
  -h, --help            show this help message and exit
  --model MODEL         Path or huggingface URL to the model. The default value is
                        "KRLabsOrg/lettucedect-base-modernbert-en-v1".
  --method {transformer}
                        Hallucination detection method. The default value is
                        "transformer".

Example using the python client library:

from lettucedetect_api.client import LettuceClient

contexts = [
    "France is a country in Europe. "
    "The capital of France is Paris. "
    "The population of France is 67 million.",
]
question = "What is the capital of France? What is the population of France?"
answer = "The capital of France is Paris. The population of France is 69 million."

client = LettuceClient("http://127.0.0.1:8000")
response = client.detect_spans(contexts, question, answer)
print(response.predictions)

# [SpanDetectionItem(start=31, end=71, text=' The population of France is 69 million.', hallucination_score=0.989198625087738)]

See demo/detection_api.ipynb for more examples. For async support use the LettuceClientAsync class instead.

License

MIT License - see LICENSE file for details.

Citation

Please cite the following paper if you use LettuceDetect in your work:

@misc{Kovacs:2025,
      title={LettuceDetect: A Hallucination Detection Framework for RAG Applications}, 
      author={Ádám Kovács and Gábor Recski},
      year={2025},
      eprint={2502.17125},
      archivePrefix={arXiv},
      primaryClass={cs.CL},
      url={https://arxiv.org/abs/2502.17125}, 
}