Jev Can't Be Calibrated

September 23, 2026 · 3 mins · 617 words Share on: X · HN

Unless you’ve been living under a rock, you’ve probably heard about Jev. Simon Willison’s post is a good overview, and this one shows how to implement it in a few lines of Python. In short, Jev is TypeSafe’s first “ System One Model ”: instead of generating text, it takes unstructured input and returns typed decisions from a set of outputs you define in advance, each with a probability attached. One of its selling points is that “all answers are accompanied with calibrated probabilities and confidence scores”. In this post I argue that Jev is useful, but that the calibration claim can’t hold in general, and that you should treat its outputs as scores rather than probabilities.