Jeva: System One Models from First Principles
Qi Li · Xinchao Wang
National University of Singapore
Overview
Jeva-4B is a System One model for fast, direct decision-making over text. It selects among alternatives, assesses whether a statement is true, and assigns scores according to a rubric—without generating lengthy explanations.
Jeva is built around a simple idea: a decision model should respect the structure of the decision itself. The order of the options should not determine the answer, more options should not require a different model, and choosing nothing should be possible when nothing fits. Familiar options should also remain useful across new requests.
Four Defining Properties
- Null-aware. Supports an explicit “none of the above” outcome when no candidate applies.
- Permutation-invariant. Designed to preserve decisions when the same candidates are reordered.
- Option-scalable. Accommodates both small and large candidate sets within the same model.
- Cacheable. Reuses previously processed options across new requests, including new combinations of familiar options.
Reuse the options. Recompute the decision.
What You Can Do with Jeva
| Capability | Purpose | Example question |
|---|---|---|
| Choice | Select a candidate, with an optional “none of the above” outcome. | Which team should handle this request? |
| Noul | Estimate the probability that a statement is true. | Does this report describe an ongoing service outage? |
| Score | Produce a numerical assessment using a supplied rubric. | How severe is the reported customer impact? |
These capabilities support request routing, document selection, relevance assessment, and rubric-based evaluation. The same familiar alternatives can be used across different questions without reusing an earlier answer.
Explore Jeva
- Project Page: Discover the work, illustrations, and research findings.
- GitHub Repository: Find usage examples and guidance for running Jeva with your own tasks.
- Questions and Feedback: Share questions, report issues, or suggest improvements.
Intended Use
Jeva is intended for research and decision-support applications. Evaluate its suitability for your task and retain human oversight for consequential decisions.
License
Jeva-4B model weights, including the decision head (head.pt) and adapter weights (backbone_adapter/), are released under the Apache License 2.0.
The bundled Qwen3.5-4B backbone retains its upstream Apache-2.0 license. See NOTICE for attribution.
Acknowledgment
If Jeva contributes to your work, please cite our work:
@article{li2026jeva,
title={Jeva: System One Models from First Principles},
author={Li, Qi and Wang, Xinchao},
journal={arXiv preprint arXiv:2610.18773},
year={2026}
}