decision model
In plain wordsA kind of AI language model that reads text but answers only with numbers — a yes-or-no probability, a choice or a score — instead of writing words.
A language modellanguage model, n.: A system that has learned how likely different sequences of words are, so it can predict the next word in a text and, by doing that repeatedly, write text of its own. that takes unstructured text but returns typed numerical outputs instead of generated text: a probability for a yes/no question, a probability distribution over a fixed set of choices, or a value on a numeric scale. According to TypeSafe AI, several questions about the same input can be answered in parallel at roughly the cost of one, and since nothing is generated, pricing can rest on input tokenstoken, n.: The small chunk of text an AI language model reads and writes — often a word, part of a word or a punctuation mark. AI usage, limits and prices are counted in tokens. alone. Decision models target work such as spam detection, labeling, prioritization and ranking, where a chat modelchat model, n.: A language model trained to hold a back-and-forth conversation, keeping track of who said what and replying as a helpful assistant. It is the engine inside chat apps.'s prose would otherwise have to be parsed into a decision. They revive the output format of a classic classifiertext classification, n.: Sorting pieces of text into categories automatically — what topic an article covers, whether a message is spam, which team should handle a customer's complaint. but, unlike a classifier trained for one label set, accept new questions at inference time. Critics note that a bare score cannot explain which part of the input drove it, which makes careful evaluationevaluation, n.: Testing an AI system to find out what it can and cannot do, how well, and how safely — from exam-style quizzes to hands-on trials and expert reviews. and bias testing essential.
We swapped the chat model for a decision model and our triage pipeline stopped choking on malformed JSON.
Origin
Both names date from September 2026, when the start-up TypeSafe AI released Jev and marketed it as the first of a new kind of 'System One' model — after the fast, intuitive System 1 of dual-process psychology; explainers such as DigitalOcean's and DataCamp's use that name. Simon Willison proposed 'decision model' as the clearer label.
Usage note
As of October 2026 the vendor's 'System One model' is the more common name; 'decision model' is a proposed alternative. Not to be confused with decision models in operations research and business-rules engineering (for example the Decision Model and Notation standard).
Related
- text classificationtext classification, n.: Sorting pieces of text into categories automatically — what topic an article covers, whether a message is spam, which team should handle a customer's complaint.
- structured outputsstructured outputs, n.: Getting an AI model to answer in an exact, machine-readable format — like a form with fixed fields — so other software can use the answer without guesswork.
- System 1 and System 2System 1 and System 2, phr.: A popular way of describing two modes of thinking: fast, automatic and intuitive (System 1) versus slow, effortful and deliberate (System 2). AI researchers borrow it to contrast instant answers with step-by-step reasoning.
- calibrationcalibration, n.: How well a model's stated confidence matches reality: when a well-calibrated system says it is 80% sure, it turns out to be right about 80% of the time.
- logitlogit, n.: A raw score that a model gives each possible answer before the scores are turned into probabilities. In statistics, the 'log-odds': a probability rewritten on a scale running from minus to plus infinity.
- large language modellarge language model, n.: A computer program trained on enormous amounts of text to predict the words that come next. At vast scale, that simple skill lets it chat, answer questions, translate, summarize and write code.
- chat modelchat model, n.: A language model trained to hold a back-and-forth conversation, keeping track of who said what and replying as a helpful assistant. It is the engine inside chat apps.
Sources
- Willison, S. (2026). Jev introduces a new shape of LLM - System One, aka Decision Models. Simon Willison's Weblog, 21 September 2026. simonwillison.net/2026/Sep/21/jev
- DigitalOcean (2026). What is Jev (2026)? TypeSafe AI's System One model. digitalocean.com/resources/articles/what-is-jev