FaithLens: Detecting and Explaining Faithfulness Hallucination

TL;DR

FaithLens is a novel, cost-effective model that detects faithfulness hallucinations in LLM outputs and provides explanations, enhancing trustworthiness across diverse tasks.

Contribution

Introduces FaithLens, a fine-tuned, reinforcement learning-optimized model that jointly predicts hallucinations and explains them, outperforming larger models.

Findings

FaithLens outperforms GPT-5.2 and o3 on 12 tasks.

It provides high-quality explanations alongside predictions.

The model balances trustworthiness, efficiency, and effectiveness.

Abstract

Recognizing whether outputs from large language models (LLMs) contain faithfulness hallucination is crucial for real-world applications, e.g., retrieval-augmented generation and summarization. In this paper, we introduce FaithLens, a cost-efficient and effective faithfulness hallucination detection model that can jointly provide binary predictions and corresponding explanations to improve trustworthiness. To achieve this, we first synthesize training data with explanations via advanced LLMs and apply a well-defined data filtering strategy to ensure label correctness, explanation quality, and data diversity. Subsequently, we fine-tune the model on these well-curated training data as a cold start and further optimize it with rule-based reinforcement learning, using rewards for both prediction correctness and explanation quality. Results on 12 diverse tasks show that the 8B-parameter FaithLens outperforms advanced models such as GPT-5.2 and o3. Also, FaithLens can produce high-quality explanations, delivering a distinctive balance of trustworthiness, efficiency, and effectiveness.