Reasoning-Trace Collapse: Evaluating the Loss of Explicit Reasoning During Fine-Tuning

ArXi:2605.21127v1 Announce Type: new Explicit reasoning models are trained to produce intermediate reasoning traces before final answers, but downstream fine-tuning is often performed on ordinary instruction-response data that contains no such traces. We show that this mismatch can induce reasoning-trace collapse: a fine-tuned model continues to produce plausible final answers while losing the structurally valid explicit reasoning traces that made it a reasoning model in the first place. We.