Learning Auxiliary Tasks Improves Reference-Free Hallucination Detection in Open-Domain Long-Form Generation
Authors: Chengwei Qin, Wenxuan Zhou, Karthik Abinav Sankararaman, Nanshu Wang, Tengyu Xu, Alexander Radovic, Eryk Helenowski, Arya Talebzadeh, Aditya Tayade, Sinong Wang, Shafiq Joty, Han Fang, Hao Ma
Accepted at: acl25

Learning Auxiliary Tasks Improves Reference-Free Hallucination Detection in Open-Domain Long-Form Generation

Chengwei Qin†*, Wenxuan Zhou*, Karthik Abinav Sankararaman†,
Nanshu Wang*, Tengyu Xu*, Alexander Radovic*, Eryk Helenowski†,
Arya Talebzadeh*, Aditya Tayade*, Sinong Wang*, Shafiq Joty*, Han Fang*, Hao Ma*
†The Hong Kong University of Science and Technology (Guangzhou)
*Nanyang Technological University *GenAI, Meta

Abstract

Hallucination, the generation of factually incorrect information, remains a significant challenge for large language models (LLMs), especially in open-domain long-form generation. Existing approaches for detecting hallucination in long-form tasks either focus on limited domains or rely heavily on external fact-checking tools, which may not always be available.

In this work, we systematically investigate reference-free hallucination detection in open-domain long-form responses. Our findings reveal that internal states (\emph{e.g.}, model’s output probability and entropy) alone are insufficient for reliably (\emph{i.e.}, better than random guessing) distinguishing between factual and hallucinated content. To enhance detection, we explore various existing approaches, including prompting-based methods, probing, and fine-tuning, with fine-tuning proving the most effective. To further improve the accuracy, we introduce a new paradigm, named RATE-FT, that augments fine-tuning with an auxiliary task for the model to jointly learn with the main task of hallucination detection. With extensive experiments and analysis using a variety of model families & datasets, we demonstrate the effectiveness and generalizability of our method, \emph{e.g.}, +3% over general fine-tuning methods on LongFact.

1 Introduction

With the recent advancements in model scale and pretraining data, large language models (LLMs) have demonstrated remarkable capabilities in various natural language processing (NLP) tasks (Brown et al., 2020). Despite these successes, hallucination, where models tend to produce content that conflicts with real-world facts, remains a significant challenge (Zhang et al., 2023). Most existing research on hallucination detection has focused on short-form tasks, where the output consists of one or a few tokens. While these methods are effective for short-form content (Manakul et al., 2023; Mahaut et al., 2024; Yehuda et al., 2024; Zhang et al., 2024a), extending them to open-domain long-form generation presents additional complexities and new challenges. Unlike short-form tasks, long-form responses can span hundreds or even thousands of tokens, requiring models to generate detailed and nuanced answers to broad fact-seeking prompts (Wei et al., 2024). This necessitates that LLMs synthesize information across multiple knowledge domains, increasing the risk of generating content that sounds plausible yet is factually incorrect. For example, when answering ‘What is the significance of Amber Room?’, LLMs may generate responses that mix accurate historical information with fabricated details, complicating the task of distinguishing fact from hallucination.

Recent efforts have sought to address hallucination detection in long-form tasks. However, they either focus on limited domains, \emph{e.g.}, biography generation (Min et al., 2023; Fadeeva et al., 2024) or rely heavily on external fact-checking tools or

![Comparison between Fine-Tuning and RATE-FT for hallucination detection. RATE-FT improves Fine-Tuning by incorporating rationales and an auxiliary task (question answering) into the training process.](page_1012_370_388_246.png)

Figure 1: Comparison between Fine-Tuning and RATE-FT for hallucination detection. RATE-FT improves Fine-Tuning by incorporating rationales and an auxiliary task (question answering) into the training process.
knowledge bases, e.g., Google Search (Wei et al., 2024). While these tools offer valuable support, they are not always available or scalable. This raises an important question: can we develop hallucination detectors that rely solely on the model itself, without the need for external fact-checking resources? So far, little attention has been given to systematically exploring how the model’s own mechanisms can be used for detecting hallucinations in open-domain long-form generation.

To address this gap, we start by investigating hallucination detection in open-domain long-form responses using the model’s internal states, e.g., output probability and entropy. Specifically, we decompose long-form responses into atomized claims using the model and verify each claim’s correctness using Google Search to construct benchmark data following Wei et al. (2024). Our analysis reveals that these internal states alone are insufficient for reliably (i.e., better than random guessing) distinguishing between correct and incorrect claims, indicating that the mechanisms for detecting hallucinations in long-form outputs differ significantly from those in short-form tasks. To enhance detection, we explore several existing methods, including prompting, probing, and fine-tuning LLMs. Our experimental results show that fine-tuning LLMs is the most effective method to detect hallucinations.

Building on this, we introduce a novel method Rationale and Auxiliary Task Enhanced Fine-Tuning (RATE-FT) (Figure 1). Specifically, we convert the original claims into auxiliary question answering (QA) examples for augmentation, providing a complementary learning perspective for the model, which enables better generalization. Additionally, we incorporate collected rationales into the training process for better reasoning. Extensive experiments and analysis using different models demonstrate the effectiveness and generalizability of our approach. Furthermore, we investigate the integration of model uncertainty into hallucination detection in Appendix A.8. In summary, our main contributions are:

• We are the first to systematically investigate reference-free hallucination detection in open-domain long-form generation by analyzing a representative set of existing methods.

• We introduce a novel approach that incorporates rationales and an auxiliary question answering task into fine-tuning, achieving significant performance improvements.

2 Related Work

Large Language Models (LLMs) often generate content that appears plausible but is factually unsupported, a phenomenon commonly known as hallucination (Zhang et al., 2023). Based on whether the hallucinated content contradicts real-world facts or the input context, hallucination can be categorized into two main groups: factuality hallucination and faithfulness hallucination (Huang et al., 2023). Extensive research has been conducted on exploring the causes (Onoe et al., 2022; Kang and Choi, 2023; Wei et al., 2023; Liu et al., 2024), detection (Min et al., 2023; Zhao et al., 2023; Chen et al., 2024a; Fadeeva et al., 2024; Wei et al., 2024), and mitigation (Gao et al., 2023; Ji et al., 2023; Tian et al., 2024; Zhang et al., 2024b; Kang et al., 2024; Lin et al., 2024) of hallucination in LLMs. However, most existing hallucination detection methods have primarily focused on short-form tasks, where the output consists of one or a few tokens. In this work, we shift the focus to the more challenging problem of reference-free hallucination detection in open-domain long-form generation, where outputs are substantially longer and require a more nuanced evaluation of actuality.

3 Are LLMs’ Internal States Sufficient for Open-Domain Long-Form Generation?

The internal states of LLMs, such as output probability and entropy, have been shown to be effective in detecting hallucinations in short-form tasks, where outputs are typically limited to only a few tokens. By analyzing these signals, models can often differentiate between factual and hallucinated information. However, their applicability in open-domain long-form generation remains underexplored. A key question is whether LLMs can depend solely on their internal states to identify hallucinations in long-form generation, without using external fact-checking tools. To answer it, we conduct some pilot experiments on LongFact (Wei et al., 2024), a long-form generation dataset spanning 38 different domains. Specifically, for each prompt in the sampled subset (200 prompts), we obtain a long-form response from Llama-3-8B-Instruct with greedy decoding. Following Wei et al. (2024), we employ the model to decompose long-form responses into atomized claims and label them as ‘factual’ or ‘hallucinated’ together with the reasons (see Appendix A.1 for construction details).

For each claim, we mainly focus on two types
of internal states to estimate factual confidence following SelfCheckGPT (Manakul et al., 2023): the probability or the entropy (uncertainty) of output tokens. Specifically, we examine the arithmetic and geometric \(^1\) averages of all tokens, the average of tokens with the top-\(K\) lowest probability or highest entropy (\(K = 1, 3, 5\)), and the average of tokens with the top-\(P\%\) lowest probability or highest entropy (\(P = 5, 10, 15\)). The results in Figure 2 and Appendix A.2 suggest that neither internal state reliably, *i.e.*, better than random guessing, predicts the correctness of a given claim, which may be due to the presence of numerous insignificant tokens within the claim, such as stop words. To address this, we consider variants that focus only on output tokens related to entities. The results, shown in Appendix A.2, reveal similar patterns (see Appendix A.3 for a detailed comparison with the findings in Manakul et al. (2023)). We analyze the underlying reasons as follows. In open-domain long-form generation, claims are not limited to a few tokens, which introduces multiple sources of uncertainty. Specifically, *the probability or entropy reflects the model’s confidence in how a claim is expressed, i.e., its confidence in the claim as a sequence of output tokens, rather than in the correctness of the claim.* Different surface forms of the claim yield different confidence levels, leading to unreliable estimates.

Considering the unreliability of LLMs’ internal states in hallucination detection, there are several promising alternative approaches, including prompting, probing and fine-tuning LLMs, which we explore in the next section.

4 Prompting, Probing and Fine-Tuning

Based on a review of the research area, we identify three groups of existing hallucination detection methods, which we discuss below.

Prompting  Prompting-based approaches involve directly prompting LLMs to assess the correctness of a given claim without additional training. We investigate the following three different methods: (*i*) Prompting the model to output *‘True’* or *‘False’* for a given claim, referred to as Prompt\(_{\text{TF}}\). The probability assigned to the token ‘True’ represents \(P_{\text{factual}}\), while the probability assigned to ‘False’ represents \(P_{\text{hallucinated}}\). (*ii*) Prompting the model to output the *probability* that it considers the given claim to be correct, referred to as Prompt\(_{\text{Prob}}\). This number directly represents \(P_{\text{factual}}\). (*iii*) SelfCheck-GPT, which detects hallucinations by sampling additional responses from the model and assessing inconsistencies between each response and the target claim. The *proportion* of responses that support the claim is taken as \(P_{\text{factual}}\). Following Manakul et al. (2023), we sample 20 responses for detection.

Probing  Following Su et al. (2024a), we train a multilayer perceptron (MLP) on the contextualized embeddings of LLMs to perform binary classification for hallucination detection, while keeping the base LLM frozen. The trained MLP outputs \(P_{\text{factual}}\) as an indicator for classification.

Fine-Tuning  We fine-tune the base LLM with LoRA to enhance its ability to output ‘True’ or ‘False’ for a given claim (Kapoor et al., 2024). Similar to Prompt\(_{\text{TF}}\), the probabilities assigned to the tokens ‘True’ and ‘False’ correspond to \(P_{\text{factual}}\) and \(P_{\text{hallucinated}}\), respectively. Note that LoRA fine-tuning allows us to easily use the original model for general tasks while applying the trained LoRA specifically for hallucination detection.

Following the data construction process outlined in Appendix A.1, we conduct experiments on the full set of LongFact using Llama-3-8B-Instruct. This process yields 2,711 factual and hallucinated claims, which are subsequently split into training (70%), validation (20%), and test (10%) sets. For all three types of methods, we use \(P_{\text{factual}}\) as the classification indicator. Specifically, a claim is classified as ‘factual’ if \(P_{\text{factual}}\) exceeds a predefined threshold; otherwise, it is classified as ‘hallucinated’. The optimal threshold is determined through a search on the validation set. Consistent with Tang et al. (2024); Chen et al. (2024b), we employ balanced accuracy (BAcc) as the evaluation metric: \[
\text{BAcc} = \frac{1}{2}(\frac{\text{TP}}{\text{TP+FN}} + \frac{\text{TN}}{\text{TN+FP}})
\]
where TP, TN, FP, and FN stand for true/false positives/negatives.

The results of different methods on the test set, as

Figure 2: Detection results based on token probability.

\( ^1 \)Commonly known as perplexity
<table>
  <tr>
    <th rowspan="2">Dataset</th>
    <th colspan="5">Method</th>
  </tr>
  <tr>
    <th>Prompt<sub>TF</sub></th>
    <th>Prompt<sub>Prob</sub></th>
    <th>SelfCheckGPT</th>
    <th>Probing</th>
    <th>Fine-Tuning</th>
  </tr>
  <tr>
    <td>LongFact</td>
    <td>69.9</td>
    <td>53.4</td>
    <td>69.1</td>
    <td>74.4</td>
    <td><b>76.1</b></td>
  </tr>
  <tr>
    <td>Biography</td>
    <td>72.3</td>
    <td>56.3</td>
    <td>71.9</td>
    <td>77.0</td>
    <td><b>78.2</b></td>
  </tr>
</table>

Table 1: BAcc (%) of existing hallucination detection methods on LongFact and biography generation.

<table>
  <tr>
    <th rowspan="2">Dataset</th>
    <th colspan="5">Method</th>
  </tr>
  <tr>
    <th>Prompt<sub>TF</sub></th>
    <th>Prompt<sub>CoT-TF</sub></th>
    <th>Probing</th>
    <th>Fine-Tuning</th>
    <th>RATE-FT</th>
  </tr>
  <tr>
    <td>LongFact</td>
    <td>69.9</td>
    <td>74.9</td>
    <td>74.4</td>
    <td>76.1</td>
    <td><b>79.6</b></td>
  </tr>
  <tr>
    <td>Biography</td>
    <td>72.3</td>
    <td>74.8</td>
    <td>77.0</td>
    <td>78.2</td>
    <td><b>80.9</b></td>
  </tr>
</table>

Table 2: BAcc (%) of RATE-FT and baseline methods.

shown in Table 1, indicate that fine-tuning LLMs is the most effective among all existing methods (see Appendix A.4 for an analysis of fine-tuning effectiveness in Out-of-Distribution (OOD) scenarios). While both Prompt<sub>TF</sub> and SelfCheckGPT achieve decent performance, Probing yields notable improvements by incorporating additional training with labels obtained from external search. Fine-Tuning further enhances performance by updating the internal features of LLMs, enabling more effective learning. In contrast, Prompt<sub>Prob</sub> performs significantly worse, likely due to LLMs’ tendency to output high probabilities for hallucinated claims, leading to overconfidence. Additionally, we extend the experiments to biography generation (Min et al., 2023). The results presented in Table 1 demonstrate that the observations and conclusions can be generalized to different datasets.

Building on these findings, a natural question arises: can Fine-Tuning be further improved to develop more effective hallucination detectors? We answer this question by <i>incorporating rationales and an auxiliary task into the training process</i>.

5 Rationale and Auxiliary Task Enhanced Fine-Tuning (RATE-FT)

While hallucination detection is not regarded as a reasoning task in the conventional sense, incorporating Chain-of-Thought (CoT) (Wei et al., 2022) explaining the judgment can still be beneficial for distinguishing factual content from hallucinated information as it enables LLMs to better evaluate the correctness of claims by systematically analyzing underlying components. To examine the impact of rationales, we prompt the model to generate a reasoning path before making a judgment (<i>i.e.</i>, ‘True’ or ‘False’), referred to as Prompt<sub>CoT-TF</sub>. This approach improves performance from 69.9 (using Prompt<sub>TF</sub>) to 74.9, highlighting the effectiveness of incorporating CoT reasoning.

Augmenting Fine-Tuning with Rationales
Building on the above observation, we augment the fine-tuning dataset with rationales generated by the model during data construction, explaining whether the search results support the claims. Notably, we adopt the ‘label-rationale’ format to maintain the same inference cost as the baseline Fine-Tuning. This allows us to directly derive \( P_{factual} \) from the first output token without requiring the generation of the complete reasoning path.

Consolidating knowledge through repetition in diverse contexts is a fundamental principle of effective human learning (Ausubel, 2012). For example, medical students deepen their understanding of anatomy by studying diagrams, practicing in simulations, and engaging in hands-on dissections, each offering a unique perspective on the same foundational knowledge. Drawing inspiration from this paradigm, we introduce an auxiliary question answering (QA) task into the fine-tuning process to further strengthen the model’s understanding and enhance its generalization capabilities. This auxiliary QA task serves as a complementary component to the primary hallucination detection task, offering the model an alternative but closely related perspective on the problem (see Appendix A.5 for more analysis on the auxiliary task).

Augmenting Fine-Tuning with QA Task
Specifically, for each claim, we first prompt the model to generate a question about the key information within it. If the claim is factual, we ask the model to extract the correct answer directly from the claim and provide an explanation, forming a QA example. For hallucinated claims, we leverage the augmented rationale to guide the model in generating an appropriate correct answer along with an explanation. After constructing these QA examples, they are combined with the original data for fine-tuning.

By integrating these two strategies, we propose <b>Rationale and Auxiliary Task Enhanced Fine-Tuning (RATE-FT)</b> (Figure 1). RATE-FT requires the model to systematically analyze and explain its judgments and allows the model to benefit from complementary learning perspectives, reinforcing its understanding of claims through diverse yet interconnected tasks. Following the experimental setup described in Section 4, we show the comparison between RATE-FT and baseline approaches
<table>
  <tr>
    <th rowspan="2">Dataset</th>
    <th colspan="3">Method</th>
  </tr>
  <tr>
    <th>Fine-Tuning</th>
    <th>w.o. aux.</th>
    <th>RATE-FT</th>
  </tr>
  <tr>
    <td>LongFact</td>
    <td>76.1</td>
    <td>77.5</td>
    <td><b>79.6</b></td>
  </tr>
  <tr>
    <td>Biography</td>
    <td>78.2</td>
    <td>79.4</td>
    <td><b>80.9</b></td>
  </tr>
</table>

Table 3: Results of different ablations.

<table>
  <tr>
    <th rowspan="2">Model</th>
    <th colspan="2">Prompt<sub>pr</sub></th>
    <th>Prompt<sub>pr,aux</sub></th>
    <th>Probing</th>
    <th>Fine-Tuning</th>
    <th>RATE-FT</th>
  </tr>
  <tr>
    <th>Prompt<sub>pr</sub></th>
    <th>Prompt<sub>pr,aux</sub></th>
    <th>Probing</th>
    <th>Fine-Tuning</th>
    <th>RATE-FT</th>
  </tr>
  <tr>
    <td>Llama-3-1.70B-Instruct</td>
    <td>73.2</td>
    <td>76.8</td>
    <td>79.4</td>
    <td>80.6</td>
    <td><b>83.8</b></td>
  </tr>
  <tr>
    <td>Mistral-7B-Instruct</td>
    <td>61.8</td>
    <td>64.1</td>
    <td>68.4</td>
    <td>70.8</td>
    <td><b>73.4</b></td>
  </tr>
  <tr>
    <td>Qwen2.5-7B-Instruct</td>
    <td>72.5</td>
    <td>75.5</td>
    <td>77.0</td>
    <td>78.4</td>
    <td><b>81.1</b></td>
  </tr>
</table>

Table 4: Results using different models.

in Table 2, which demonstrates the superiority of RATE-FT across different datasets (see Appendix A.6 for an analysis of the effect of additional data augmentation compared to the auxiliary QA task).

5.1 Further Analysis

Ablation Study We analyze the contribution of different components of RATE-FT by investigating the variant of RATE-FT without the auxiliary task (w.o. aux). Table 3 presents the performance of different methods, highlighting that each component plays an important role in achieving the overall performance.

Generalization to Different Models Our experiments and analysis so far use Llama-3-8B-Instruct as the backbone model. To verify whether the performance gain of RATE-FT is consistent across different backbone models, we extend the experiments to Llama-3.1-70B-Instruct (Dubey et al., 2024), Mistral-7B-Instruct (Jiang et al., 2023), and Qwen2.5-7B-Instruct (Yang et al., 2024) on LongFact (see Appendix A.7 for details on data collection). From the results shown in Table 4, we can observe that RATE-FT consistently outperforms baseline approaches across all models, demonstrating its robustness and generalizability to diverse model architectures and scales.

In addition, we provide results of incorporating uncertainty for hallucination detection, all prompts used in our experiments, and implementation details in Appendix A.8 ~ A.12, respectively.

6 Conclusion

In this work, we systematically investigate reference-free hallucination detection in open-domain long-form generation. Our study begins with an analysis of the model’s internal states, demonstrating that these states alone cannot reliably detect hallucinations. We then evaluate several existing approaches, including prompting, probing, and fine-tuning, with fine-tuning emerging as the most effective method. Building on these findings, we introduce Rationale and Auxiliary Task Enhanced Fine-Tuning (RATE-FT), a novel approach that leverages rationales and an auxiliary task to achieve significant improvements in detection performance across two datasets and various LLMs.

Limitations

One limitation of our work is its focus solely on improving the performance of the hallucination detector. A potential improvement could be to explore leveraging the detector’s feedback as a reward signal to guide LLMs to generate more factual responses. Additionally, developing a more comprehensive benchmark for hallucination detection in open-domain long-form generation that covers a broader range of domains would further enhance its applicability.

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A Appendix

A.1 Benchmark Construction Details
For each prompt in the sampled subset (200 prompts), we obtain a long-form response from Llama-3-8B-Instruct with greedy decoding. Following Wei et al. (2024), we employ the model to decompose long-form responses into atomized claims and assess whether each claim is relevant to answering the corresponding prompt. For each relevant claim, we use the model to generate multi-step Google Search queries and reason about whether the search results support the claim. Claims supported by the search results are labeled as “factual”, while those contradicted by the results are categorized as “hallucinated”. After construction, we obtain 2394 factual claims and 223 hallucinated claims, respectively. We then randomly selected an equal number (223) of factual and hallucinated claims for experiments.

A.2 Hallucination Detection Results using Internal States
We show the hallucination detection results using different internal states in Figure 3 ~ 5

A.3 Detailed Comparison with Findings in SelfCheckGPT
(i) While SelfCheckGPT (Manakul et al., 2023) explores several internal states of LLMs, our work covers a broader range of variants. As illustrated in Section 3, we examine the arithmetic and geometric averages (perplexity) of all tokens, the average of tokens with the top-K lowest probability or highest entropy (\( K = 1, 3, 5 \)), and the average of tokens with the top-\( P\% \) lowest probability or highest entropy (\( P = 5, 10, 15 \)). In contrast, SelfCheckGPT only examines the arithmetic average of all tokens and the average of tokens with the top-1 lowest probability or highest entropy.
(ii) Our findings differ significantly from those reported in SelfCheckGPT. While SelfCheckGPT suggests that LLM probabilities correlate well with factuality, our experiments demonstrate that neither internal state reliably, i.e., better than random
guessing, predicts the correctness of a given claim. One possible explanation for this is the presence of many insignificant tokens, such as stop words, within the claim. To address this, we further investigate variants that focus only on output tokens related to entities (Appendix A.2), and the results exhibit similar patterns. Importantly, our findings are consistent with those in Kapoor et al. (2024).

A.4 Out-of-Distribution Results

We verify the effectiveness of fine-tuning in Out-of-Distribution (OOD) scenarios by training the model on LongFact and evaluating its performance on Biography. The results reported in Table 5 demonstrate that fine-tuning effectively generalizes to OOD scenarios.

A.5 More Analysis on Auxiliary Task

Comparison with F2  F2 (Hu et al., 2024) also integrates rationales and auxiliary tasks into the training process. However, its main goal is to enhance the faithfulness of model responses while we focus on improving the accuracy of hallucination detection.

Further Clarification on Motivation  The underlying motivation for introducing the auxiliary question answering (QA) task into fine-tuning is that hallucination detection and mitigation are complementary and closely related tasks. This auxiliary QA task—where a question about the key information in the claim is posed, and the model is trained to provide the correct answer—helps improve the factuality of the model’s responses through supervised fine-tuning. It acts as a complementary component to the primary hallucination detection task, offering the model an alternative yet closely related perspective, thereby enhancing its generalization capabilities.

A.6 Additional Data Augmentation versus Auxiliary QA Task

To isolate the effect of additional data augmentation versus the auxiliary QA task, we design two variants: (i) we paraphrase the original claim using GPT-4 for data augmentation and fine-tune the model on the combined data, referred to as Fine-Tuning_{para}, which has roughly the same amount of training data as RATE-FT; and (ii) we reduce the training data for RATE-FT by half (approximately the same amount as Fine-Tuning), referred to as RATE-FT_{half}. We conduct experiments on LongFact using Llama-3-8B-Instruct and present the results in Table 6 and 7, which demonstrate that the performance improvement primarily comes from our designed auxiliary task, rather than from additional data augmentation.

![Hallucination detection results based on token entropy (uncertainty).](page_1012_120_393_312.png)

Figure 3: Hallucination detection results based on token entropy (uncertainty).

<table>
  <tr>
    <th>Prompt_{FT}</th>
    <th>Prompt_{Pub}</th>
    <th>SelfCheckGPT</th>
    <th>Probing</th>
    <th>Fine-Tuning</th>
  </tr>
  <tr>
    <td>72.3</td>
    <td>56.3</td>
    <td>71.9</td>
    <td>71.1</td>
    <td><b>74.7</b></td>
  </tr>
</table>

Table 5: Results of different methods in OOD scenarios.

A.7 Data Collection Process for Other Models

When conducting experiments using other models, we follow the exact same settings as those used for Llama-3-8B-Instruct. Specifically, for each prompt, we obtain a long-form response from the model under investigation with greedy decoding. Following Wei et al. (2024), we employ the model to decompose long-form responses into atomized claims and assess whether each claim is relevant to answering the corresponding prompt. For each relevant claim, we use the model to generate multi-step Google Search queries and reason about whether the search results support the claim. Claims supported by the search results are labeled as “factual”, while those contradicted by the results are categorized as “hallucinated”.

Our constructed benchmarks align well with Su et al. (2024b), as both include responses and internal states from various LLMs. The key difference is that the LLMs we investigate are all modern models (Llama-3-8B-Instruct, Llama-3.1-70B-Instruct, Mistral-7B-Instruct, and Qwen2.5-7B-Instruct), whereas the models used in Su et al. (2024b) are relatively outdated (such as LLaMA-2 and GPT-J).
Figure 4: Hallucination detection results based on the probability of entity-related tokens.

Figure 5: Hallucination detection results based on the entropy of entity-related tokens.

<table>
  <tr>
    <th>Fine-Tuning<sub>para</sub></th>
    <th>RATE-FT</th>
  </tr>
  <tr>
    <td>76.8</td>
    <td><b>79.6</b></td>
  </tr>
</table>
Table 6: Comparison between Fine-Tuning<sub>para</sub> and RATE-FT.

<table>
  <tr>
    <th>Fine-Tuning</th>
    <th>RATE-FT<sub>half</sub></th>
  </tr>
  <tr>
    <td>76.1</td>
    <td><b>78.5</b></td>
  </tr>
</table>
Table 7: Comparison between Fine-Tuning and RATE-FT<sub>half</sub>.

A.8 Incorporating Uncertainty for Hallucination Detection

To enhance hallucination detection, we propose incorporating model uncertainty into the detection process, enabling a hybrid pipeline that combines the strengths of the model and external tools. Specifically, when the model is uncertain about whether a claim is factual or hallucinated, we leverage external tools to handle ambiguous cases, improving overall performance. The process involves setting two thresholds, \( \alpha_{low} \) and \( \alpha_{high} \), for classification. A claim is classified as ‘factual’ if \( P_{factual} > \alpha_{high} \) and ‘hallucinated’ if \( P_{factual} < \alpha_{low} \). Claims falling between these thresholds are classified as ‘unknown’ and delegated to external tools for further evaluation. Assuming the external tools’ output is the ground truth, predictions classified as ‘unknown’ are treated as correct. To evaluate the hybrid pipeline, we define the BAcc-unknown metric as follows:

\[
\text{BAcc-unknown} = \frac{1}{2} \left( \frac{\# \text{ Correct Factual Predictions}}{\# \text{ Total Factual Claims}} + \frac{\# \text{ Correct Hallucinated Predictions}}{\# \text{ Total Hallucinated Claims}} \right)
\]

The optimal thresholds, \( \alpha_{low} \) and \( \alpha_{high} \), are determined through a search on the validation set. This process ensures that BAcc on the validation set exceeds 70%, while also maximizing BAcc-unknown. The goal is to strike a balance between performance and efficiency by achieving high BAcc-unknown without generating an excessive number of ‘unknown’ predictions, which could substantially increase detection costs. We conduct experiments on LongFact using Llama-3-8B-Instruct and report the results in Table 8, which demonstrate that incorporating model uncertainty greatly enhances hallucination detection, as evidenced by the BAcc-unknown metric’s superior performance compared to standard BAcc in resolving ambiguous cases. Moreover, RATE-FT continues to outperform all other methods with respect to the BAcc-unknown metric, highlighting its robustness and effectiveness.

<table>
  <tr>
    <th>Prompt<sub>CUT-TF</sub></th>
    <th>Probing</th>
    <th>Fine-Tuning</th>
    <th>RATE-FT</th>
  </tr>
  <tr>
    <td>80.4</td>
    <td>81.1</td>
    <td>82.4</td>
    <td><b>85.0</b></td>
  </tr>
</table>
Table 8: BAcc-unknown (%) of different methods on Longfact with Llama-3-8B-Instruct.

![Prompt for extracting the original output given an atomized claim.](page_1012_1047_377_120.png)
Figure 6: Prompt for extracting the original output given an atomized claim.

A.9 Prompt for Output Extraction

After decomposition, the atomized claims may differ from the original expression in the response. To address this, we use the prompt shown in Figure 6 to retrieve the original output corresponding to a given atomized claim.
Figure 7: Prompts for different prompting methods.

Figure 8: Prompts for different components of RATE-FT.

A.10 Prompts for Baseline Approaches

Figure 7 illustrates the prompts used for different prompting methods. The prompt used for constructing training data in Probing and Fine-Tuning is the same as the prompt employed by the PromptTF method.

A.11 Prompts Used in RATE-FT

Figure 8 presents all the prompts used in RATE-FT.

A.12 Implementation Details

For PromptTF and PromptProb, we obtain the response from the model with greedy decoding. Following Manakul et al. (2023), we set the temperature to 1.0 and generate 20 additional responses for SelfCheckGPT.

We evaluate 4 different types of contextualized embeddings for Probing: (1) the final token from the last layer (type_1), (2) the average of all tokens in the last layer (type_2), (3) the average of the final token across all layers (type_3), and (4) the average of type_1 and type_2 (type_4). The optimal embedding type, along with other hyperparameters, e.g., learning rate, is selected through a search on the validation set. For Fine-Tuning and RATE-FT, we leverage the LLaMA-Factory library (Zheng et al., 2024) and perform a search on the validation set for important hyperparameters.

Reviewer paper summaries: This manuscript investigates reference-free hallucination detection in open-domain long-form generation. The authors explore various methods, including prompting, probing, and fine-tuning, and find that internal states are insufficient and fine-tuning proves the most effective. Then, they introduce RATE-FT, which incorporates rationales and an auxiliary question-answering task into the fine-tuning process. Experiments demonstrate that RATE-FT outperforms existing methods across different datasets and models.

The paper addresses the challenge of reference-free hallucination detection in open-domain long-form generation by LLMs. Existing methods either focus on limited domains or rely heavily external fact-checking tools, which lack scalability and availability. The authors first demonstrate that internal states of LLMs (e.g., token probability and entropy) are insufficient for reliable reference-free hallucination detection in open-domain long-form generation. They then evaluate three methods—prompting, probing (using the contextualized embeddings of LLMs to train a multilayer perceptron), and fine-tuning—with fine-tuning emerging as the most effective baseline. 
They find that incorporating Chain-of-Thought helps LLMs better distinguish between factual content and hallucinations. Based on this idea, they propose RATE-FT (Rationale and Auxiliary Task Enhanced Fine-Tuning), which integrates rationales and an auxiliary question-answering task during fine-tuning. Experiments on datasets like LongFact and Biography show that RATE-FT consistently outperforms baselines across multiple LLMs (e.g., Llama-3-8B-Instruct and Llama-3.1-70B-Instruct). The method also generalizes well to out-of-domain scenarios. Limitations include a focus solely on detection (not mitigation) and the need for broader benchmarks.

This paper investigates reference-free hallucination detection in open-domain long-form generation, which remains a significant challenge for large language models (LLMs). Specifically, the authors analyze LLMs' internal states (e.g., output probability and entropy) and find them insufficient to distinguish between factual and hallucinated content in long-form responses.

Therefore, the author proposes Rationale and Auxiliary Task Enhanced Fine-Tuning (RATE-FT). This novel method augments fine-tuning with rationales and an auxiliary question-answering task to improve hallucination detection accuracy. They demonstrate the effectiveness and generalizability of RATE-FT through extensive experiments and analysis using various model families and datasets, outperforming general fine-tuning methods.
Summary of strengths: 1. The manuscript includes a thorough analysis of the limitations of using internal states and provides interesting insights into their failing cases.
2. The manuscript conducts comprehensive additional experiments including “out-of-distribution” results, “data argumentation” results, and “incorporating uncertainty” results, improving the soundness of RATE-FT.

1. The paper systematically evaluates the performance of several existing methods (internal states of LLMs, prompting, probing, and fine-tuning) in reference-free hallucination detection in open-domain long-form generation, providing a comprehensive reference for future researchers.
2. Compared to previous studies on short-form generation or relying on external tools, reference-free hallucination detection in open-domain long-form generation has higher practical value.
3. RATE-FT mainly enhances fine-tuning by incorporating rationales and auxiliary QA, without requiring large-scale external retrieval systems or complex model architectures. The method is simple and feasible.

1. The paper is well-organized and largely easy to follow.

2. The authors present their ideas clearly and systematically, enhancing readability and comprehension.

3. Hallucination detection is a crucial topic for applying large language models.

4. The experiments are thorough. Compared to the previous submission, the authors add additional experiments such as the effectiveness of fine-tuning in OOD scenarios, which addresses my concerns.
Summary of weaknesses: 1. This manuscript focuses on long-form hallucination detection, yet the characteristics of “long-form” in RATE-FT are unclear. It seems that augmentations of both rational and QA are more related to normal “open-ended” generation rather than specialized in long-form paragraphs. Also, how are the performances of baselines and RATE-FT scale with the length?
2. Unclear about the scale of the datasets used. How long are the model-generated paragraphs?
3. The margins of the figures are too large. The distribution of the token probability of the fine-tuned model is warranted for a clear comparison with Fig 2-5.

1. Although RATE-FT focus on reference-free, the data construction process for fine-tuning still relies on external search (e.g., Google Search) for annotation. This may encounter scalability and availability challenges in real-world applications.
2. In appendix A.10 ablation study, I see the results without auxiliary QA, but what would happen if there were no Rationale?
3. The paper does not focus on specific types of hallucinations. It might be worth further discussing how the method performs in detecting different types of hallucinations (e.g., overconfidence, consistency).
4. The method focuses on reference-free detection, but a comparison with methods that use external fact-checking tools would provide a more comprehensive understanding of practical trade-offs.

I do not have any major reasons to reject this paper, since my previous concerns are addressed in this version of submission.
Comments, suggestions and typos: See weaknesses.

None

It is recommended that the authors move the Main Contributions section from Appendix A.1 to the introduction in the next version to more effectively highlight the key advantages of the proposed method.
Overall assessment scores: 4

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