Simon Willison writes about Jev, a new category of models from TypeSafe AI called "System One models" or decision models. Unlike standard large language models that output text, Jev accepts unstructured input and returns structured probabilistic decisions such as floating-point numbers for yes/no questions (Noul), choices between options, or numeric scores. These models are designed to be extremely fast and inexpensive, charging only for input tokens while providing free output.
- Jev is optimized for classification tasks like spam detection, ranking, and labeling.
- The model's "Noul" question type refers to the Bernoulli distribution.
- Using such black-box decision models raises concerns about hidden biases that are difficult to audit without explanations.
- There is an emerging trend of open-weight recreations of Jev-class models, including projects like Kev and benchmarks like JevBench.
- Jev is optimized for classification tasks like spam detection, ranking, and labeling.
- The model's "Noul" question type refers to the Bernoulli distribution.
- Using such black-box decision models raises concerns about hidden biases that are difficult to audit without explanations.
- There is an emerging trend of open-weight recreations of Jev-class models, including projects like Kev and benchmarks like JevBench.
Diogo Almeida writes that TypeSafe AI is releasing Jev, its first System One Model—a new class of frontier model built for fast, structured decisions that software can consume directly. Unlike autoregressive language models that generate strings token by token, Jev outputs type-safe structured values with calibrated probabilities in a single parallel query, achieving frontier-level intelligence on decision tasks at roughly 40–200× lower latency and cost. The company's new training method, Reinforcement Learning for Calibrated Decisions (RLCD), optimizes for epistemically honest probability estimates rather than human preference or verifiable rewards, and the architecture is mathematically incapable of producing type errors or hallucinations.
- Named after William Stanley Jevons, whose paradox predicted that efficiency gains would increase (not decrease) total demand; TypeSafe expects each order-of-magnitude cost drop to unlock orders of magnitude more use cases.
- Workflow evals benchmark Jev against the average of GPT-6 Astra and Fable 5.1 as reference probabilities, claiming 193.6× speed and 444.6× cost advantages on production-shaped tasks.
- The team demonstrated real-time intelligence with a Doom bot making 10 structured queries per second (~$7/hour) and a Wikiracing bot that outperforms LLMs at high-cardinality link selection.
- Jev supports output cardinality up to 255; for higher-cardinality choices it falls back to a two-stage scoring system that scores independently then makes an explicit selection.
- Named after William Stanley Jevons, whose paradox predicted that efficiency gains would increase (not decrease) total demand; TypeSafe expects each order-of-magnitude cost drop to unlock orders of magnitude more use cases.
- Workflow evals benchmark Jev against the average of GPT-6 Astra and Fable 5.1 as reference probabilities, claiming 193.6× speed and 444.6× cost advantages on production-shaped tasks.
- The team demonstrated real-time intelligence with a Doom bot making 10 structured queries per second (~$7/hour) and a Wikiracing bot that outperforms LLMs at high-cardinality link selection.
- Jev supports output cardinality up to 255; for higher-cardinality choices it falls back to a two-stage scoring system that scores independently then makes an explicit selection.
