Fully Open vs Open Weights: What It Buys You

The phrase “open source AI” hides a distinction that matters more every year. Most models people call open are open weights: you can download and run the trained model, but the training data and the full recipe stay private. A smaller group is fully open: weights, training data, and documentation all published. Switzerland’s Apertus model, released by EPFL, ETH Zurich, and CSCS, is a prominent example of the fully open end, publishing its architecture, weights, and training data together. You can read about it on the EPFL AI Center.

For everyday writing, both run the same way on your machine. But the distinction decides what you can verify, and that is worth understanding before you pick a model for sensitive work.

Open Weights: You Can Run It, Not Inspect It

An open-weights model gives you the finished thing. You can download it, run it offline, fine-tune it, and use it commercially if the license allows, all without the training data. This is the practical sweet spot for most people, because what you want is a capable model you can run privately, and the weights are enough for that.

What you cannot do is verify what went into it. You take the lab’s word on the training data, the filtering, and the safeguards. For running a model to rewrite your own text, that is usually fine, because the privacy guarantee comes from the model running locally, not from knowing its training set.

Fully Open: You Can Audit The Recipe

A fully open model publishes the training data and the method, so a researcher can in principle reproduce it and audit what it learned from. This matters for accountability, for reproducible research, and for anyone who needs to demonstrate provenance, for example under regulatory or procurement rules that ask where a model came from.

It is also a different kind of trust. With a fully open model, you are not trusting a lab’s description of its data; you can look. For most writers that is more assurance than they need, but for a public-sector body, a regulated institution, or a researcher, it can be the deciding factor.

Which One You Actually Need

For private rewriting, open weights is enough. The privacy you care about, your text never leaving the machine, comes from local execution, and any open-weights model under a permissive license delivers it. We covered choosing among them in best open weights models for writing.

Reach for fully open when you must demonstrate provenance, audit training data, or meet a sovereignty or reproducibility requirement. That is a compliance and research need, not a privacy one, and most individuals never hit it.

A Wrivio Context works the same on either kind of local model:

Rewrite this in a professional tone. Keep every fact exact. Do not add information not in the original.

Press Ctrl+Shift+Space, paste the draft, and the model runs locally regardless of how open its recipe is.

Read The License Separately

Openness of weights and openness of license are different axes too. A model can publish its weights under a restrictive license, or its data under a permissive one. Always check the actual license for your use, especially commercial use. We laid out the common licenses in open weights model licenses.

Common Questions

What is the difference between open weights and fully open?

Open weights means you can download and run the trained model, but the training data and full recipe stay private. Fully open means weights, training data, and documentation are all published, so the model can in principle be audited and reproduced.

Which do I need for private writing?

Open weights is enough. The privacy you care about comes from running the model locally, so your text never leaves the machine. You do not need the training data published to get that guarantee.

When does fully open matter?

When you must demonstrate where a model came from, audit its training data, or meet a sovereignty or reproducibility requirement. That is a compliance and research need, common in public-sector and regulated settings, rare for individuals.

Does open weights mean free for commercial use?

Not automatically. Openness of weights and openness of license are separate. Always check the specific license, especially for commercial use, because some open-weights models carry real restrictions.

Download Wrivio for Windows to run a permissively licensed open-weights model locally, with your text never leaving the machine.