Proton

source

partial(GPLv3 clients only

clients

open(GPLv3 - web in ProtonMail/WebClients, iOS/Android in ProtonLumo)

service

closed(backend, routing, model-serving not public)

license

GPLv3(clients)

Open clients (GPLv3) over a proprietary hosted service: the backend, routing and model-serving layer are not published. It is marketed as "fully open source", but support pages and third-party reviews note that only the clients are open. A published client standing in for an unpublished engine makes the source partial rather than public, which scores as source-available; open core would mean an OSI-licensed core with a paid tier withheld, which this is not. The published iOS client is a SwiftUI shell around a web view pointed at lumo.proton.me, with native payment and speech handling - exactly that split, a client published and the service behind it not. The European Open Source AI Index reaches the same conclusion independently.

• https://github.com/ProtonLumo/ios-lumo recorded 2026-08-13

  iOS client source under GPLv3

• https://isitreallyfoss.com/projects/lumo/ recorded 2026-08-13

  only the Lumo mobile/web apps are open; service + model interaction code not public