What is a decision model?

Decision models return structured answers, such as categories, scores or yes/no probabilities. Jev returns these without generating text; some alternatives generate an option letter.

Definition source: Simon Willison

How decision models work

You send the context your app has, such as a message or an agent's history. A game state or web page can also be input. For a Choice question, you supply the allowed options and receive probabilities for them. You can use those probabilities to decide which answers to act on and which to send to a person or a bigger model.

Evidence: Simon Willison · 21 Sep 2026 · Source ↗

Decision models and generative LLMs

A generative LLM can return structured output that your code reads. It generates its answer one token at a time. Jev returns structured answers without generating text. Together reports that Tev1 keeps a text-generation head and is trained to return one option letter.

Evidence: Simon Willison · 21 Sep 2026 · Source ↗ · Source: huggingface.co

Choice, Noul and Score

Jev, and the models built to its API, take three question types. Choice returns probabilities for the options you supply. A Noul question returns a yes/no probability; the name comes from the Bernoulli distribution. A Score question returns a value along your defined scale, which can fall between the numeric levels you supply.

Evidence: Simon Willison · 21 Sep 2026 · Source ↗

Decision models and System One models

TypeSafe calls Jev a "System One model", after Daniel Kahneman's split between fast, intuitive thinking and slow, deliberate reasoning. Liquid AI uses "decision models" for d1.

Source: typesafe.ai · Source: docs.liquid.ai

Are decision models deterministic?

Returning probabilities without generating text does not guarantee that repeated requests return identical answers. A dated Jev test found changing individual answers despite stable overall accuracy. See the determinism guide for the repeated-call and batching results.

Evidence: ElyasEbrahim · pgdocket Phase 0 · 17 to 19 Oct 2026 · Source ↗

Architecture and RLCD

TypeSafe documents the output contract: a Choice answer includes the selected option and a probability for every supplied option, with probabilities summing to one. Its introduction says Jev returns structured answers without text generation. That describes what callers receive; it does not disclose the internal readout implementation.

Source: docs.typesafe.ai · Source: docs.typesafe.ai

Open readouts: Kev

Kev's README describes a pointer head that compares each option's closing-token hidden state with the question's decision-token hidden state. Softmax converts the scores to probabilities. Kev documents cross-entropy training and stored calibration temperatures. This is Kev's disclosed implementation, not a specification of Jev's internals.

Kev model facts

Source: github.com

Open readouts: SemIf and Tev1

SemIf reads declared option logits in a forward pass without sampling an answer token. Its authors explicitly say it reproduces the interface pattern, not Jev's undisclosed model or training. Together's Tev1 retains a standard next-token head and generates an option letter. A decision-model interface therefore does not identify one shared architecture.

SemIf model facts · Tev1 model facts

Source: github.com · Source: huggingface.co

What TypeSafe means by RLCD

TypeSafe's AI primer expands RLCD as reinforcement learning for calibrated decisions. It describes training Jev to return decisions and probabilities, with higher probabilities intended to correspond to more frequent correct outcomes. Calibration concerns groups of predictions and does not guarantee an individual answer. VENDOR-REPORTED: this is TypeSafe's account of its training goal, not an independently reproduced training method.

Source: docs.typesafe.ai

Not public

The TypeSafe introduction, Choice documentation, AI primer and launch post linked here do not disclose the following details (checked 5 November 2026). Open implementations cannot fill these gaps.

  • Jev's base checkpoint and parameter count.
  • The training corpus and its selection or labelling procedure.
  • The RLCD reward formula, loss and full training recipe.
  • The internal probability head and parallel serving implementation.

Jev model facts · Decision models versus classifiers

Source: docs.typesafe.ai · Source: docs.typesafe.ai · Source: docs.typesafe.ai · Source: typesafe.ai

Can I run a decision model locally?

Open models have several local routes: MLX on Apple Silicon, ONNX in a browser and GGUF through supported runtimes. The local guide explains which listed projects use each route and where their hardware measurements stop. TypeSafe’s hosted Jev weights remain private.

Choose a local runtime

Source: github.com · Source: github.com · Source: github.com

Confidence after reading the logits

In her 6 Nov 2026 talk thread, Leonie Monigatti describes a bare option-logit readout as naive: changing option order can change its answer, and training can leave it overconfident. She describes fine-tuning and lifting the causal mask, then temperature scaling to calibrate the probabilities. Her slide deck also names RLCD. Its calibration diagrams identify no model, dataset or measured run, so their percentages are illustrations. Task-level evidence is needed before setting an automation threshold.

Calibration evidence and FAQ · Evaluate a decision model on your task

Source: github.com · Source: x.com

Training and fine-tuning a decision model

Specialist fine-tunes can change which errors a decision model makes. The recorded releases below support checking your labels, deployment format and held-out task before adopting a checkpoint. Public weights alone do not supply a complete training recipe.

Evidence: Amaljith · medgemma-decide-4b-v2 card and results · 7 Nov 2026 · Source ↗ · Evidence: Thomas Tesche · Laya Minecraft card and training report · 7 Nov 2026 · Source ↗ · Source: github.com · Source: github.com

Choose the labels and error rule first

medgemma-decide-4b-v2 oversamples minority answers to improve one skewed question. VENDOR-REPORTED: its balanced accuracy on that question rises from 0.625 to 0.825, while the macro across sources slips from 0.689 to 0.686. Evaluate every question you need, with the same decision rule, rather than treating one corrected failure as an overall gain. The model is research only.

Medical-text fine-tune and limits · Compare with a classifier baseline

Evidence: Amaljith · medgemma-decide-4b-v2 card and results · 7 Nov 2026 · Source ↗

Keep calibration and evaluation separate

Laya Minecraft freezes its encoder and trains from verified game ticks, then fits temperatures on 284 calibration items. VENDOR-REPORTED: its 415-item evaluation contains four overlapping training or calibration items. Its accuracy and ECE changes therefore cannot establish an untouched-test gain. Preserve independent final examples when selecting weights and fitting thresholds.

Minecraft checkpoint and overlap · Held-out evaluation checklist

Evidence: Thomas Tesche · Laya Minecraft card and training report · 7 Nov 2026 · Source ↗ · Source: huggingface.co

Retain the actual training data and inference path

medical-jev-4b publishes a reconstructed training pool because its original file was lost. Its later 1,557-item evaluation may include sources seen by the released adapter. The default runtime accepts 52 options; the repository’s experimental tournament changes the benchmark result. Pin the data, checkpoint and deployed scoring path together. A candidate-count change can change what you are measuring.

Medical choice model facts · Source, code and GPU record

Source: github.com · Source: github.com

Check what the release actually includes

MoDe publishes inference code and three weight packages, but not its training code or data. Its weights use the FAIR licence for noncommercial research; the inference code is Apache-2.0. Laya Minecraft’s model card uses Apache-2.0, while medgemma-decide uses HAI-DEF terms. Check the specific model and data licences before reusing a recipe. These records do not supply a verified end-to-end fine-tuning tutorial or a universal compute budget.

MoDe access and licence · Documented local runtimes

Source: github.com · Source: huggingface.co · Evidence: Thomas Tesche · Laya Minecraft card and training report · 7 Nov 2026 · Source ↗ · Evidence: Amaljith · medgemma-decide-4b-v2 card and results · 7 Nov 2026 · Source ↗ · Source: github.com

Multiple intents, duplicate picks and stopping

A message can ask for a refund and for a person. If both conditions can be true, ask a separate Noul for each in one request. Each answer estimates its own yes probability. A single Choice returns one selected option, so it does not enumerate every matching intent. TypeSafe’s fan-out pattern lets code ignore answers that do not apply.

Jev facts and sources · Request limits · Evaluate your own task

Source: docs.typesafe.ai · Source: docs.typesafe.ai

When the next question depends on the last answer

TypeSafe’s cart cookbook starts with four menu items and one hard-coded cafe order. Code asks which remaining item comes first, saves that selection and removes it before the next request. Each question includes nothing_else. The loop stops on that option, an empty menu or a six-request cap. Removing a selected option prevents picking the same menu item twice. The recorded run finds a latte and a muffin, then stops. It is a worked example, not a general extraction benchmark.

Source: docs.typesafe.ai

What the loop leaves out

Each menu item can appear only once. The cookbook explicitly says two flat whites still produce one latte because the schema has no quantity field. It suggests adding quantity choices. Known fields such as size and milk are asked together after item discovery, with unspecified available when the order leaves a field out. Whether to ask independent yes/no questions or use a loop depends on whether the candidate questions are already known. Neither pattern supplies a universal accuracy or latency guarantee.

Source: docs.typesafe.ai · Source: docs.typesafe.ai

Vision decisions, OpenCV and numeric rules

A decision model can sit after image processing. In the laya-opencv workbench, OpenCV measures areas, counts and lengths, hard rules decide numeric limits, and a text Laya checkpoint cross-checks the written result. Disagreements between rule and model, marginal values and missing inputs flag human review; a low-confidence model answer is flagged only where the model, not a hard rule, decides. That pipeline gives the components different jobs.

Source: github.com

Check what the model actually sees

The workbench also offers experimental Jev image paths. Its local text Laya path receives measurements in words, so it cannot independently inspect pixels that OpenCV missed. Choose between a measurement check and a direct image judgment based on the information the question needs.

Laya model facts · Inspection workbench · Jev image inputs

Source: github.com

Training and image preparation change the result

The Glimmer inspection project reports 90.2% defect accuracy on 866 held-out images from known products after training its head on half of each product's test images. Its MVTec data is restricted to research. Separately, clef-playa's author reports lower vehicle counts on dense aerial tiles than two LLM annotators gave, and splitting tiles into quadrants narrowed that gap; true counting accuracy is not verified. Neither record compares OpenCV and a decision model on the same images, so these examples do not establish which is more accurate.

Glimmer inspection · Aerial counting limits · Evaluate on your own task

Source: github.com · Source: github.com

Jev sentiment analysis: positive, neutral or negative

For one overall sentiment label, send the text as state and ask a Choice question with positive, neutral and negative criteria. The saved multilingual benchmark uses the same question for all eight languages, without examples or tuning. It records the selected label and all three probabilities. A sentiment category is different from confidence in that category.

Jev model and access · Sentiment code and saved predictions · Evaluation checklist · Overlapping labels and separate questions

Source: github.com · Source: github.com · Source: docs.typesafe.ai

A question you can reuse

This is the benchmark's question object, to place under questions in a System One request. Supply the text in state and select the model separately. No response or accuracy on a new article is implied by this example.

{
  "sentiment": {
    "type": "choice",
    "instructions": "What is the overall sentiment expressed by the author of text ?",
    "criteria": {
      "positive": "The text expresses a positive opinion, emotion or attitude.",
      "neutral": "The text is factual or expresses neither a clearly positive nor a clearly negative sentiment.",
      "negative": "The text expresses a negative opinion, emotion or attitude."
    }
  }
}

What the saved test answers

The author reports Jev macro-F1 61.7 averaged equally across eight languages and 12,970 items; English is 68.0. These three-label benchmark scores do not give an accuracy rate for a new application's articles. Five LLM columns reuse paper results with different input formats. The report scores Jev's returned choice, including 53 items tied at the highest printed probability to two decimals.

Evidence: Uran Lajci · zero-shot conference report · run 6 Nov 2026, discovered 9 Nov · Source ↗

Frequently asked questions

When is a decision model good enough to replace an LLM call?

Whether a model can replace an LLM call depends on the task and error budget. In Vals AI's claim-verification test, Jev tied GPT-6-class models at 0.975. With a 1% error budget, Jev automated 95% at 1.6% error, over budget; Mercury Decide automated 94% at 0.8% error. In a study of conflicting retrieved sources, sending each decision model's least confident quarter to Gemini 3.8 Flash scored no differently from Gemini 3.8 Flash, statistically, for 8 of 11 models, at 25 to 37% of its cost. A 16-day Claude Code field report had mixed results. Of 240 scope judgments, 172 were uncertain or lacked evidence. The team removed Jev; token savings were never measured.

Evidence: Source: Vals AI · Jev evaluation 6 Oct 2026, Mercury Decide evaluation 8 Oct 2026 ↗ · Source: Khatri, Jain, Kapoor, Sah · Do We Need an LLM to Know Why Sources Conflict? · arXiv 2610.36915 · 27 Oct 2026 ↗ · Source: sash-ua · Accurate and Useless · 30 Oct 2026 ↗

Why does the answer change when I reword the question?

The linked studies tested option lists and option order. They did not systematically test rewording. Across 1,700 questions, shuffling the options moved Jev's accuracy by under two points but grew the set of questions it ever got wrong by 27 to 29%, more than plain repetition. A small hand-written field guide offers thin evidence: Jev often confidently chose the nearest option when the correct answer was missing. In a replay after the locked tests, adding a "none of these" exit raised correct handling from 11 of 14 requests to 13.

Evidence: Source: Song, Zheng, Ma, Zhou · Understanding Jev-Style System One Models: Does Structure Reduce Selection Bias? · arXiv 2610.35826 · 26 Oct 2026 ↗ · Source: WeBuildDatabases · Jev, job by job · 29 Oct 2026 ↗

Can I trust the probabilities, including on coin-flip questions?

None of these studies tested calibration on coin flips. On other tasks, the results varied. In SILT's preregistered test of 799 unpublished yes/no questions about coding-agent work and destructive shell commands, Decider V1.1-27B was the only one of seven models to pass SILT's chosen calibration rule. Jev's result was inconclusive. So were the results for OpenAI's Decisions API and Laya. A separate banking-intent test found Jev's choice answers well calibrated but its yes/no answers underconfident on true cases. On MedQA, answers assigned at least 0.90 probability were still wrong 1.71% to 12.77% of the time, depending on the model.

Evidence: Source: SILT · Do Decision Models Know When They're Wrong? · 11 Oct 2026 ↗ · Source: ankur287 · jev-open-calibration, code and data for a Towards Data Science article · Oct 2026 ↗ · Source: Freyer et al. · Decision models vs generative LLMs on medical questions · arXiv 2610.38448 · 28 Oct 2026 ↗

How cheap is a moderation check, and where should the threshold sit?

The reports give workload-specific thresholds. Squigdle's developer reported 6 cents for about 1,000 doodle checks, about $0.00006 each, using 512×512 images plus titles and the game's own thresholds. The reported misses were occasional false alarms. This single developer's live-game report gives little evidence about other workloads. In a separate moderation simulation, a fine-tuned Laya router used a threshold set with labelled calibration data. It needed one extra moderator instead of three at 100 extra posts an hour.

Evidence: Source: James O'Halloran · Moderating 500 daily anonymous doodles with Jev · 30 Oct 2026 ↗ · Source: Enayat Hassani · Exposure-aware routing · 29 Oct 2026 ↗

Can a decision model sit in front of risky actions as a guardrail?

Two tests point both ways. In a USC input-gate test, Jev reduced answered harmful-labelled prompts from 527 to 17 out of 720, but refused 27 of 250 safe prompts, against 3 for the LLM alone. The study counted answered prompts as a proxy for harm; it did not check the completions. In Datadog's prompt-injection detector test, Jev caught 82.90% of labelled attacks; prepending a fake policy inside the untrusted text reduced that to 3.88%. That study distinguishes a classifier bypass from a full agent compromise.

Evidence: Source: Cognetti, Krishnamachari (USC) · Jev as an LLM Safety Guardrail · 2026, code and tables ↗ · Source: Datadog Security Labs · 15 Oct 2026 ↗

Do decision models work outside English?

Performance depends on the language and task. Šuppa and Kopčan's XDecideBench tests nine suites across 122 languages. The English-best model, picked in hindsight, loses significantly in 56 of 232 suite-language cells. A threshold fitted on English to admit 5% errors instead admits 7.8% to 12.4% on German and French XNLI. Check the model's accuracy and confidence threshold on labelled examples from the language you plan to use.

Evidence: Source: Marek Šuppa, Jaroslav Kopčan · XDecideBench · arXiv 2611.01370 · 01 Nov 2026 ↗

Decision models versus classifiers · Are decision models deterministic? · Compare decision models · Jev alternatives · Decision model pricing · JevBench scores · Model registry