Is Linwood Butterfly Free and Open Source?

Updated

Yes. Linwood Butterfly is free and open source software, released under the GNU AGPL-3.0 license. You can download it and use it without paying, and because the source code is open, you can inspect, modify, and redistribute it under the terms of that license. The project also keeps your documents in documented .bfly and .tbfly formats rather than a closed, proprietary container.

That combination matters if you care about lock-in: the license covers the software, and the documented file formats cover your work.

What "free and open source" means here

Two separate things are usually bundled into that phrase, and Butterfly satisfies both:

  • Free as in cost — the site presents Butterfly as free and open source, with no pricing signals, subscription tiers, or payment links on the page. There is nothing to buy to use the app.
  • Free as in freedom — the GNU AGPL-3.0 license is a strong copyleft license. It guarantees you the right to run, study, share, and modify the software. If you distribute a modified version, the AGPL requires you to release your source under the same license, including when the software is offered over a network.

The AGPL is stricter than permissive licenses like MIT. If you plan to build Butterfly into a hosted product, read the license terms first — the network-use clause is the part that catches people off guard.

Your files stay in open, documented formats

Butterfly stores documents in .bfly and .tbfly formats, and these are documented rather than secret. Practically, that means:

  • Your notes and drawings are not trapped in a format only one company can read.
  • You can back up, move, or archive files yourself.
  • If the project ever stalled, the format is documented well enough for other tools to work with it.

This is the file-format half of "your work belongs to you." The license is the software half.

Local-first by default

Butterfly keeps documents on your device unless you choose to save them elsewhere, and it works without an internet connection. That design reinforces the ownership point: nothing forces your work onto someone else's server.

If you do want sync, you connect your own cloud rather than a mandatory account. The site specifically mentions WebDAV services like Nextcloud for syncing, backing up, and editing notes across devices. That is a bring-your-own-storage model, not a vendor-hosted one.

Where it runs

Butterfly runs on Android, Windows, macOS, Linux, and in the browser, with an iOS preview also available. The same free, open-source app covers all of these, so choosing it does not lock you to one operating system.

What to check before you commit

  • License fit — if you only use Butterfly for personal notes and sketches, AGPL-3.0 is a non-issue. If you want to embed or host it commercially, review the AGPL obligations.
  • Sync setup — syncing is not automatic out of the box; you bring a WebDAV service such as Nextcloud. If you do not already have one, factor that in.
  • Collaboration is experimental — end-to-end encrypted sessions work through Swamp or a WebSocket server you host yourself, and the site labels this experimental. Do not rely on it for critical shared work yet.

For most people asking whether Butterfly is free and open source, the answer is straightforward: yes on both counts, with documented file formats and local-first storage as the practical payoff.