Simple Factuality Probes Detect Hallucinations in Long-Form Natural Language Generation

Large language models (LLMs) often mislead users with confident hallucinations. Current approaches to detect hallucination require many samples from the LLM generator, which is computationally infeasible as frontier model sizes and generation lengths continue to grow. We present a remarkably simple baseline for detecting hallucinations in long-form LLM generations, with performance comparable to expensive multi-sample approaches while drawing only a single sample from the LLM generator. Our key finding is that LLM hidden states are highly predictive of factuality in long-form natural language generation and that this information can be efficiently extracted at inference time using a lightweight probe. We benchmark a variety of long-form hallucination detection methods across open-weight models up to 405B parameters and demonstrate that our approach achieves competitive performance with up to 100x fewer FLOPs. Furthermore, our probes generalize to out-of-distribution model outputs, evaluated using hidden states of smaller open-source models. Our results demonstrate the promise of hidden state probes in detecting long-form LLM hallucinations.