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)

Proton publishes the Lumo clients under GPLv3, but the backend, routing and model-serving layer that does the actual work is closed, even though Proton markets Lumo as "fully open source." The iOS client is a thin SwiftUI shell around a web view pointing at Proton's hosted service, and the European Open Source AI Index reaches the same conclusion independently: only the clients are open.

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

  iOS client source under GPLv3

• https://isitreallyfoss.com/projects/lumo/ recorded 2026-09-29

  Independent review page (last reviewed 2026-04-13) stating "GPLv3 licenses." and "Source for the service itself, including where it directly interacts with large-language-models, could not be found in our review."

• https://proton.me/blog/lumo-1-1 recorded 2026-09-29

  Proton announcement that the web client is "already open source" and "today we are also releasing the" mobile app code, so the clients are public.

• https://proton.me/lumo recorded 2026-09-29

  Proton's Lumo page claiming "Lumo’s code is fully open source, so anyone can verify that it’s private and secure" in its marketing copy.

No Lumo-specific usage numbers are published, so adoption is read from the size of Proton's account base, more than 100 million accounts, across which Lumo is offered for free on web, iOS and Android without a separate sign-up. That account-base size is a proxy for Lumo's own use rather than a measured count of it, which is why confidence stays medium.

• https://proton.me/blog/lumo-ai recorded 2026-10-08

  "Lumo is a new service within the Proton privacy ecosystem, trusted by over 100 million people worldwide"; anyone can use it free, with no account required.

Lumo covers file upload, web search, fast and reasoning modes, image generation, memory and Projects, all on open-weight models that Proton hosts itself. It offers no choice of third-party models.

• https://proton.me/blog/lumo-1-1 recorded 2026-09-29

  Lumo 1.1 release post titled "Introducing Lumo 1.1 for faster, advanced reasoning", with "improved web search" for current events.

• https://proton.me/blog/lumo-2 recorded 2026-09-29

  Lumo 2.0 announcement: "Upload an image to analyze, create visuals from a prompt or a rough sketch, or edit existing images, all in the same conversation." plus "combines user-controlled memory and Projects" and "Administrative tools let you manage your team’s access", with fast and reasoning modes and web search.

• https://proton.me/blog/lumo-ai recorded 2026-09-29

  Launch post: "Lumo can understand and analyze files you upload" and "add end-to-end encrypted files from your Proton Drive to your chats with Lumo", with ghost mode for chats that disappear on close.

• https://proton.me/support/lumo-privacy recorded 2026-09-29

  Support page listing "The models we’re using currently are Qwen 3.5, GLM 5.3, Image-Turbo, FireRed-Image-Edit-1.1, and Apertus 1.5." and that users can "optionally enable web search".