We're open sourcing Northstar CUA Fast, a frontier 4B open-source Computer Use Action (CUA) model, built for accuracy and long-horizon action planning.
Trained with native GUI reinforcement learning, Northstar CUA Fast achieves emergent error recovery and exploration behaviour that generalizes to web and desktop environments, outperforming models up to 8x its size.
Model
| Parameters | 4B |
| Context | 64K |
| Training | GUI reinforcement learning |
| Input | Text + screenshot |
| Output | GUI actions — click, type, scroll, key, drag, ... |
| Pricing | < $1/M tokens |
What it can do
Fill out forms. Point it at any web form — signup flows, admin panels, expense reports — and let it fill fields, click checkboxes, and submit.
Extract data from any UI. Scrape dashboards, pull numbers from analytics tools, read tables — no brittle selectors needed. The model sees the screen.
QA & UI testing. Run end-to-end tests described in plain English. No Selenium scripts to maintain — the model adapts when the UI changes.
Desktop automation. Not just browsers — automate native desktop apps, file management, terminal commands, and IDE workflows.
OSWorld Benchmark
Evaluated on OSWorld — 369 real-world desktop tasks (pass@1).
| Domain | UI-TARS 2 | Qwen3 Flash | Northstar CUA Fast (4B) |
|---|---|---|---|
| Chrome | 62.96% | 56.43% | 55.30% |
| Thunderbird | 73.33% | 66.67% | 62.40% |
| LibreOffice Writer | 60.87% | 56.52% | 56.94% |
| OS | 41.67% | 54.17% | 46.26% |
| VLC | 49.94% | 34.41% | 43.87% |
| Overall | 53.1% | 41.6% | 37.01% |
At 4B parameters, Northstar CUA Fast is competitive with open-source models at twice its size on single-app tasks.
Open source
The agent harness — Lightcone — is fully open source under Apache 2.0. It wraps Northstar CUA Fast into a complete desktop automation loop: screenshot, think, act, repeat. The Python SDK (pip install tzafon) and the OpenAI-compatible endpoint are also open.
Model weights are available on Huggingface here.
To support open science, we're opening a 500-seat research preview of Northstar CUA Fast at Lightcone, our enterprise-ready platform for deploying computer-use agents.
Access the repo here: github.com/tzafon/lightcone