What is a System One Model?

A System One model makes fast, structured decisions that software can use directly. Jev is TypeSafe's flagship model and the first System One model.

This page explains the class in order to use Jev. Broader Decision AI benchmarks, datasets, and calibration studies belong on a future dedicated project, not on this hub.

How it differs from an LLM

	LLM	System One (Jev)
Trained to	Produce text people read	Return calibrated decisions
Output	Tokens of prose or JSON-as-text	Typed answers + probabilities
Control flow	Often hidden in the prompt	Stays in your code
Confidence	Not the same contract	Choice/Score include confidence

Official training name you will see in docs: RLCD (Reinforcement Learning for Calibrated Decisions). Calibration is across many predictions, not a promise about one ticket.

The decision primitives

You do not prompt Jev to “be a JSON API”. You pick a primitive:

• Choice — which option?
• Score — where on this rubric?
• Noul — is this true?

Those three are how a System One model exposes an answer space.

A refund workflow

Official System One page walks a refund case:

1. Build state: message, transactions, policy.
2. Ask independent questions together.
3. Combine answers with deterministic checks, then route.

Jev supplies the judgments. Code supplies eligibility math, idempotency, and the actual refund call.

Naming

TypeSafe chose “System One” for speed and focus, not because Jev is unconscious or approximate in the Kahneman sense. Do not stretch the metaphor into a product claim.

Jev's own name, per third-party reporting, nods at Jevons' paradox: cheaper decisions get used in more places. That is commentary, not an official SLA.

When to reach for a System One model

High-volume, low-latency gates in front of humans or LLMs: routing, filtering, scoring, tool selection.

When not to

If the user is waiting on a written answer, you still need a generator. If you need a ranked list of open-ended ideas, System One is the wrong class.

Common mistakes

• Treating “System One” as a second API beside Jev. The HTTP API is still POST /v1/systemone with model: "jev-latest".
• Turning this site into a generic Decision AI platform. We stop at what you need to ship Jev.

Next: How Jev works or the API.