GLOSSARY
What is a System Two model?
System Two modelA System Two model reasons by generating text token by token — in practice, a large language model. The term contrasts it with System One models, which return a typed decision in a single pass.
Generation is what makes a language model flexible: it can explain, plan, write and call tools. It is also what makes it slow and awkward to consume from software. Every answer is text that has to be parsed, the parse can fail, and the time to answer grows with the length of what is written.
Most applications need both kinds. A System Two model handles the open-ended work; a System One model handles the many small decisions in the request path — route this, score that, allow or block — where a typed answer in milliseconds matters more than an explanation.
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