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Research paperReasoning & Planning · Large Language Models2 sources · Oct 6, 2026

SanSi: A Looped Typed Decision Model for System 1.5 Thinking

We propose SanSi, which turns a pre-trained looped language model into a typed decision model.

Key points

  • Typed decision models answer a declared question without generating text: a decision head returns a probability for each of the declared options in a single forward pass.
  • We study what lies between one pass and generated reasoning: looping, in which the same layers are recursively applied several times before one typed readout.
  • Each loop lets the model revise its hidden state before it commits to an answer, without generating a token; we call this System 1.5 thinking.
  • Used as the judge for policy optimization with reinforcement learning, without gold answers, SanSi raises the generator's F1 by 7.7 points.

Sources (2)

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