When Chain-of-Thought Fails, the Solution Hides in the Hidden States

• URL: http://arxiv.org/abs/2604.23351v1
• Date: Sat, 25 Apr 2026 15:33:21 GMT
• Title: When Chain-of-Thought Fails, the Solution Hides in the Hidden States
• Abstract summary: We use activation patching to transfer token-level hidden states from a CoT generation to a direct-answer run for the same question.<n>We find that individual CoT tokens can encode sufficient information to recover the correct answer, even when the original trace is incorrect.
• Score: 6.13796568285414
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Whether intermediate reasoning is computationally useful or merely explanatory depends on whether chain-of-thought (CoT) tokens contain task-relevant information. We present a mechanistic causal analysis of CoT on GSM8K using activation patching: transferring token-level hidden states from a CoT generation to a direct-answer run for the same question, then measuring the effect on final-answer accuracy. Across models, generating after patching yields substantially higher accuracy than both direct-answer prompting and the original CoT trace, revealing that individual CoT tokens can encode sufficient information to recover the correct answer, even when the original trace is incorrect. This task-relevant information is more prevalent in correct than incorrect CoT runs and is unevenly distributed across tokens, concentrating in mid-to-late layers and appearing earlier in the reasoning trace. Moreover, patching language tokens such as verbs and entities carry task-solving information that steers generation toward correct reasoning, whereas mathematical tokens encode answer-proximal content that rarely succeeds. Patched outputs are often shorter and yet exceed the accuracy of a full CoT trace, suggesting complete reasoning chains are not always necessary. Together, these findings demonstrate that CoT encodes recoverable, token-level problem-solving information, offering new insight into how reasoning is represented and where it breaks down.

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