GlossaryFloor 1 · The Modela solid block on its own: the prediction machineFloor 1 · The Model
open weight
No. 055 · v2026-08FR: open weight (poids ouverts)An open weight model is a model whose parameters can be downloaded: you can run it on your own machines, adapt it, integrate it. Like an engine delivered to you that you fit wherever you want, without the plans of the factory that built it.
What it is not
Open weights do not make free software. The term open source presupposes defined freedoms with no discrimination as to use, whereas most released models impose restrictions: user thresholds, forbidden uses, an obligation to credit the origin. Nor is it transparency: the training data, the recipe and the training code most often remain secret, so that one can run the model without being able to reproduce it or to audit it.
In depth
What open weights bring
What open weights actually bring comes down to three things, and they are often enough to settle the decision. You can host the model wherever you want, and therefore process data that must not leave a perimeter. You control the version: it will not change under the feet of a system you have evaluated, which is a constant problem with models served online. And you can fine-tune the model to a domain, which is not always possible otherwise.
Costs displaced
Costs change in nature rather than disappearing. A downloaded model is free to acquire but not to run: it takes accelerators, memory, a team able to serve it, to watch over it and to update it. Depending on volumes, self-hosting can work out cheaper than a service billed per token, or a great deal more expensive. The calculation is made case by case, and it includes human time, which comparisons regularly forget.
Reading the licence
The landscape of licences deserves to be read before it is invoked. Some releases really are under a recognised free licence, others under a bespoke licence that restricts commercial use beyond a threshold or forbids certain domains. These conditions have concrete effects: they can forbid an intended use, or oblige you to display a notice. One should therefore speak of “open weight” models by default, and keep “open source” for the cases where the licence really justifies it.
Relations where the neighbours live
- Related comparisons
- Open source or Open weight
Check 3 questions · click your answer
Level 1 · Recognise
What can you do with an open weight model that you cannot do otherwise?
Level 2 · Distinguish
Why are “open weight” and “open source” not interchangeable?
Level 2 · Distinguish
Is an open weight model cheaper than a service billed per token?
Lexigraph, "Open weight", v2026-08, https://www.lexigraph.org/en/open-weight/, CC BY 4.0.