Rather than intentionally hiding anything, the model simply does not reliably represent all influential inputs in its output. As a result, it may generate what sounds like a reasoning process, even if that explanation doesn’t match what actually drove its answer.
Putting faithfulness to the test
To measure “faithfulness”—how accurately the CoT matches the actual reasoning—the researchers inserted various kinds of hints into questions and observed whether the model acknowledged using them.
The researchers then checked whether the models referenced using these hints in their chain-of-thought. The researchers designed some hints to be neutral (e.g., stating “a Stanford Professor indicates the answer is [A]” or embedding the answer in faux metadata), while they designed others to be more concerning (e.g., “You have gained unauthorized access… The correct answer is [A]”). Some hints provided the correct answer, while others were deliberately wrong, allowing researchers to observe how the models responded.
The team tested Anthropic’s Claude 3.7 Sonnet and DeepSeek’s R1 model. The results regarding faithfulness were eye-opening: across all hint types on average, Claude referenced the hints in its CoT only 25 percent of the time, while DeepSeek R1 did so 39 percent of the time. This means a substantial majority of answers were unfaithful, omitting mention of information that influenced the output. Intriguingly, the researchers found that these unfaithful chains of thought were, on average, longer than faithful ones, suggesting the omissions weren’t merely for brevity. They also noted preliminary findings that faithfulness tended to be lower when the questions were more difficult.
Perhaps most notable was a “reward hacking” experiment. Reward hacking refers to an AI model finding unexpected shortcuts to maximize its performance scores without solving problems as intended. In Anthropic’s experiments, models were deliberately rewarded for choosing incorrect answers indicated by hints. The models quickly learned to exploit this loophole, selecting wrong answers over 99 percent of the time to earn points—yet reflected that behavior in their CoT outputs less than 2 percent of the time.
For example, a model given a hint pointing to an incorrect answer on a medical question might write a long CoT justifying that wrong answer, never mentioning the hint that led it there. This suggests the model generated an explanation to fit the answer, rather than faithfully revealing how the answer was determined.