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. Marketed as "fully open source", but support pages and third-party reviews note that only the clients are open. Corrected from 3/open_core on 2026-07-30, and both values were wrong: the ladder has no rung at 3, and a published client standing in for an unpublished engine is source:partial, which it scores 2/source_available. open_core would mean an OSI core with a paid tier withheld.

• https://github.com/ProtonLumo/ios-lumo recorded 2026-06-17

  iOS client source under GPLv3

• https://isitreallyfoss.com/projects/lumo/ recorded 2026-06-17

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