It get worse, and the model weights is a bit inaccurate with the Sept update:

The only open source code we have found is for the Lumo mobile and web apps. Proton calling the Lumo AI assistant open source based on that is a bit like Microsoft calling Windows open source just because there’s a github repository for Windows Terminal.

The models listed on Lumo’s privacy policy page are “Nemo, OpenHands 32B, OLMO 2 32B, and Mistral Small 3”. OpenHands is a QWEN fine-tune, and Nemo and Mistral Small are both Mistral models. Since Proton has open-sourced neither the Lumo system prompt nor the mysterious routing methods that decide which model will handle your query, you never know what you are going to get.

So if the server isn’t open source, and the server does all the work, this system is simply not Open Source.