otheropen-coreRuns locallyWriting & Office

Mattermost: Open-Core Slack Alternative

Open-core self-hosted team collaboration. Mixed LICENSE.txt (compiled MIT, source AGPL).

Mattermost web UI with a team channel, member list, and message pane (2016 screenshot) — Official project image. Source · License: Apache-2.0.

License: other
Stars: 39,410 (2026-10-23)
Platforms: Linux, Docker, Kubernetes, Web, Windows, macOS, Android, iOS
First seen: 2015-06-15

TL;DR. Mattermost is Open-core self-hosted team collaboration. Mixed LICENSE.txt (compiled MIT, source AGPL). License: other (verified). Platforms: Linux, Docker, Kubernetes, Web, Windows, macOS, Android, iOS. Price: open-core. Runs locally: yes. It is an alternative to Slack.

Name	Mattermost
Language	Go / TypeScript
License	other
Pricing	open-core
Platforms	Linux, Docker, Kubernetes, Web, Windows, macOS, Android, iOS
Runs locally	Yes
Stats	39,410 stars · 9,068 forks (as of 2026-10-23)
First seen	2015-06-15
Repository	https://github.com/mattermost/mattermost

Mattermost is an open-core, self-hosted team collaboration platform. Official hosts we opened on 2026-10-23: mattermost/mattermost (39,410 stars, 9,068 forks, created 2015-06-15, last push 2026-10-22T23:57:58Z), LICENSE.txt, release v12.0.1 (2026-10-22), mattermost.com, and docs.mattermost.com. GitHub’s license field is NOASSERTION.

It is listed on Slack alternatives as the Slack-shaped channels option. That mapping is editorial. We have no sourced quality comparison against Slack or Zulip.

The README (opened 2026-10-23): “open core, self-hosted collaboration platform that offers chat, workflow automation, voice calling, screen sharing, and AI integration,” written in Go and React, “runs as a single Linux binary,” PostgreSQL. “A new compiled version is released under an MIT license every month on the 16th.” Those sentences stay vendor. v12.0.1’s GitHub notes say the release “contains Medium severity level security fixes.”

Key facts (2026-10-23)

Field	Value	Source
Stars / forks	39,410 / 9,068	GitHub API
Created	2015-06-15T06:50:02Z	same
Last push	2026-10-22T23:57:58Z	same
Latest release	v12.0.1, 2026-10-22T12:26:41Z	GitHub Releases
License file	Mixed (see below)	LICENSE.txt
GitHub license API	Other / NOASSERTION	GitHub API
Language (API / README)	TypeScript (API); Go + React (README)	same
Job	Self-hosted team chat / collaboration	README

License (read the file)

Copied from LICENSE.txt (2026-10-23):

Compiled Mattermost, Inc. builds: MIT (MIT-COMPILED-LICENSE.md inside those builds).
Source you compile: GNU AGPL v3.0, or a commercial license from Mattermost.
Named admin/config trees: Apache-2.0.
server/enterprise: Mattermost Source Available license (E20 subscription for production use).

Not legal advice. This is why the Slack hub does not treat Mattermost as a simple OSI-MIT row the way it treats Zulip.

Install (docs, not tested)

README / docs paths: Docker, Ubuntu, tar, Omnibus, Kubernetes/Helm. Developer setup is a separate guide. forkalt has not deployed v12.0.1.

Slack alternatives
Zulip (Apache-2.0 topic-threaded row)
Writing & Office
Workspace docs: Notion alternatives
Chat models (not team chat): ChatGPT alternatives

FAQ

Is Mattermost open source?

The repo is public (39,410 stars, last push 2026-10-22). LICENSE.txt is mixed: compiled vendor binaries MIT; source you build is AGPL v3.0 or commercial; some admin/config trees Apache-2.0; server/enterprise is source-available. GitHub’s license field is NOASSERTION. We copy the file. Not legal advice.

What version did you check?

GitHub latest release v12.0.1 (published 2026-10-22T12:26:41Z). API 2026-10-23: 39,410 stars, 9,068 forks, created 2015-06-15, last push 2026-10-22T23:57:58Z. Homepage mattermost.com.

Does it replace Slack?

The README calls Mattermost an open-core self-hosted collaboration platform (chat, workflow automation, voice, screen sharing, AI integration). That is vendor copy. We have no sourced bake-off. See Slack alternatives. Zulip is the Apache-2.0 topic-threaded row on that hub.

Sources & verification

Last verified 2026-10-23 · Last updated 2026-10-23· Edited by forkalt editors · Editorial policy

License notes are not legal advice. Read the original LICENSE. Trademark & disclaimer.