Why we ship two stems and not six

Every stem separation service faces the same question: how many stems do you offer? Six looks better on a pricing page than two. We ship two, and the reason is not technical.

What we could not ship

Demucs is the model most people mean when they say four or six stems. We went looking for a commercial licence grant covering its weights and did not find one — what we found instead was the weights described as provided "for scientific purposes" only, a research-only use, not a commercial one. It is excellent, and without a commercial grant we cannot sell access to it.

MDX23C is the other obvious candidate. Its code may be open source, but when we went looking for a licence covering the weights themselves, we found none — the licence is unstated, and an absent licence is not a permissive one. Our rule is that ambiguous or missing licence terms are treated as do-not-ship, because the alternative is betting a business on a silence.

What we could

UVR's MDX-NET vocals and instrumental weights are covered by the same MIT licence as the GUI code: the README asks third-party developers who use their models to honour the MIT licence, and we read that as extending the grant to the weights themselves. It comes with an attribution requirement, which is why UVR (Anjok07 & aufr33) is credited on the screen where you separate a track and in the site footer — not as a courtesy, as a licence condition.

Why we say this out loud

Two stems shipped under a licence we can point to beats six stems advertised and not defensible. We would rather tell you what we cannot do than have you discover the limit after paying for it.

The full licence status of every model we considered, including the ones we rejected and why, is in docs/model-licensing.md in our API repository.