"Open weights" usually means the weights and nothing else. NVIDIA published the training data and the recipe as well, which is a different word.

Nemotron 3.5 Lightning is a 30 billion parameter mixture of experts activating roughly 3 billion per token, on a hybrid Mamba and transformer architecture. It scores 24 on the Artificial Analysis Intelligence Index, up from 15 for Nemotron 3 Nano, and NVIDIA claim performance close to gpt-oss-120b at about a quarter of the parameters. Weights, training data and recipes all ship under NVIDIA's OpenMDW-1.1 licence, alongside NeMo Switchyard, an open router for systems that use more than one model.

Why this one is different

Almost nobody publishes the data. Weights let you run a model; the recipe lets you rebuild and modify it, and the data lets you audit what went into it. It is also worth naming the incentive plainly: NVIDIA sells the hardware. A company whose revenue comes from the compute has a straightforward reason to make good models cheap and reproducible, and that is a structurally different motive from every lab selling tokens.

A company that sells the compute has a reason to make the models cheap.

How we got here

1. 2024 to 2025Open weights become normal. Open data does not, and reproducing a released model from scratch remains impossible for outsiders.
2. Early 2026The Nemotron 3 line establishes small hybrid Mamba and transformer models aimed at agents rather than chat.
3. 11 Aug 2026Lightning ships weights, data and recipe, plus Switchyard for routing between models.

What it does and does not mean

OpenMDW-1.1 is NVIDIA's own licence, not an OSI approved one, so read it before assuming it behaves like Apache. Publishing training data also does not settle where that data came from, which is the question currently in front of several courts. And the Intelligence Index is one aggregate. What is genuinely unusual is the completeness: this is the closest a major vendor has come to letting somebody else rebuild the model rather than merely run it.