Jev introduces a new shape of LLM - System One, aka Decision Models

Simon Willison's Weblog · 2d ago
Model Releases LLMs

How-To How to actually use this

What changed: TypeSafe AI released Jev, a "System One" or "decision model" LLM that outputs floating point numbers for categories or yes/no instead of text.

How to use it:

  1. Identify a task requiring a numerical score or binary classification rather than generated text.
  2. Send your text prompt to the Jev API as you would to a standard LLM.
  3. Parse the returned floating point values to map to your specific categories or decision thresholds.
  4. Integrate the numerical outputs directly into your automation or scoring logic.

Good for: developers building classification or scoring systems without text-parsing overhead.

Last week TypeSafe AI unveiled Jev , their first example of a new category of model that they are calling "System One models" (I'm with Maggie Appleton, I think "decision models" is a better name for these). Jev is an interesting variant on the usual LLM format: it still accepts text inputs, but instead of text output it returns floating point numbers corresponding to categories, yes/no…

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