Sanity Checks for Reasoning Based Hallucination Detection

Hallucination detection methods for large language models increasingly operate on chain-of-thought reasoning traces, yet it remains unclear whether they evaluate the reasoning itself or merely exploit surface correlates of the final answer. We introduce a controlled-invariance methodology that exposes this distinction through two oracle tests: Force, which replaces each response's final answer with the ground truth while preserving the reasoning trace, and Remove, which strips answer-announcement steps while leaving the trajectory intact. This reveals if their predictive power derives from answer-level artifacts rather than from the structure or validity of intermediate reasoning. We further show that once these artifacts are controlled for, effective discrimative power does still exist within the trace itself. To verify, we introduce TRACT, a lightweight scorer built on explainable lexical trajectory features (hedging trends, step-length dynamics, and cross-response vocabulary convergence), verifying that traces still contain important discrimative signatures, while remaining competitive with or outperforming existing baselines on unperturbed traces. These findings suggest that the current central challenge in reasoning-aware hallucination detection is not the absence of signal in the trace, but the failure to isolate it from endpoint cues.