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Closed reasoning models expose no chain of thought at all, so RECAST reconstructs one from the answer alone

AspireAI community · arXiv (cs.AI)
Large closed-source reasoning models expose only their final answers; the intermediate reasoning is hidden behind API surfaces, tool-call policies and safety filters. Existing CoT extraction methods assume a side channel such as a visible reasoning field or a scratchpad carried through tool calls. The authors reproduce three representative state-of-the-art extraction methods and find that on Claude-Sonnet-5.5 through the standard chat-completions interface, all three return zero CoT tokens, and that the failure is structural rather than a bug. RECAST takes a different route: it conditions a decoder on the question and the target model's final answer, reconstructs a first-person structured rationale, cleans it and builds SFT-ready data. It is described as the first system to reconstruct CoT from frontier closed models such as Claude-Sonnet-5.5, GPT-6.1 and Gemini 3 without hidden reasoning channels. The authors are careful about scope: the preprint describes the system, its threat model and a controlled artifact validation, and explicitly does not claim large-scale benchmark improvements.
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