Chain of thought

Chain of thought is the technique of having a model write out its intermediate reasoning steps before giving a final answer, instead of jumping straight to the result (Chapter 1). The written steps act as scratch paper: each one becomes part of what the model reads next, giving it room to work through a problem it could not solve in a single leap.

Chain of thought is the seed from which most of modern reasoning grew. The longer, self-checking reasoning of advanced models like DeepSeek-R1 (Chapter 4) is essentially a chain of thought that the model learned to produce and extend on its own.