80% of LLM 'Thinking' Is a Lie — What CoT Faithfulness Research Actually Shows

When You're Reading CoT, the Model Is Thinking Something Else Thinking models are everywhere now. DeepSeek-R1, Claude 3.7 Sonnet, Qwen3.5 — models that show you their reasoning process keep multiplying. When I run Qwen3.5-9B on an RTX 4060, the thinking block spills out lines of internal reasoning. "Wait, let me reconsider..." "Actually, this approach is better..." — it self-debates its way to an answer. It feels reassuring. You think: okay, it's actually thinking this through. That reassurance has no foundation. When you read a CoT trace and feel reassured, what you're looking at is not a record of reasoning — it's text generated to look like reasoning. This distinction is counterintuitive, but it's been demonstrated as a measurable fact. In May 2025, Anthropic published Reasoning Models Don't Always Say What They Think. Reasoning models don't always say what they actually think. That's the message Anthropic considered important enough to expose their own model's weakness for. The Exp