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I am searching an open source alternative to Slack.

Requirements:

  • modern chat solution (public, private rooms, discussion threads, emoji-reactions, notification about unread messages, search, ...)
  • client needs to be open source
  • server needs to be open source
  • self-hosting should be possible
  • A web-based GUI should be available
  • Mobile usable web GUI (or native app).

Optional:

  • Most features of the software should be accessible via keyboard (without mouse).
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+25

Mattermost is the main alternative to Slack that covers most of your requirements. I have not looked at it in a while, but you will also find a few useful others here in previous questions:

Team chat with private rooms, file upload and link previews (alternative to Slack)

Team chat with native Android/iOS apps, all open source (alternative to Slack)

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  • Looks good. One drawback: It seems to miss discussion-threads. I don't need deep nested threads (like in mailing-lists), but a one-level-threading would be nice. This is roughly like Stackoverflow. Someone has a question and then answers to this questions are below this question (and not directly in channel). Would be nice to have. – guettli Jun 30 at 10:42
  • I thought it has threads? Please check here: docs.mattermost.com/help/getting-started/… to verify – Z Z Jun 30 at 13:58
  • also see comparison mattermost.com/mattermost-vs-slack – Z Z Jun 30 at 15:29
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Discord

Features

Discord is free and the most popular team communication app for gamers. The platform provides users with familiar Slack-like UI, Skype-like video, and voice chats. Discord is amazing to use while participating in a Dota 2 battle, but it also meets collaboration challenges many companies face.

The tool divides channels by text and voice. Nevertheless, users can seamlessly switch to more common private and public channels. It’s worth mentioning that Discord doesn’t offer integrations with apps you might use at work. So this tool will be a good choice for you if your team doesn’t need anything beyond the text, voice, or video communication. It‘s also the only chat in this list that offers push-to-talk features and the Instant Invite link that lets you share a chat server with anyone.

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Discord is free for a full set of features. But if you’d like to have custom emoji, larger file uploads, and GIF avatars in your team chat, join the paid plan for $4.99 per month.

Other Alternatives

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  • There was one requirement: "server needs to be open source". AFAIK Discord is not open source. – guettli Sep 15 at 10:37
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rocket.chat is open source.

I have never used it, but the feature list looks promising:

  • chat features
  • audio calls
  • video calls
  • screen sharing
  • ....
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0

According to this answer in a similar question, Zulip is modern and open source chat solution.

And it can handle threads (which mattermost and rocket-chat can't do at the moment).

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