Mattermost

Open-source · self-hostable · replaces 3 SaaS tools on os-alt

mattermost/mattermost · alive · ★ 36.6k · last commit today · 857 open issues

License: AGPL-3.0 (Team Edition) — Team Edition core is AGPL-3.0; Enterprise adds SSO controls, compliance exports, and data retention under a commercial license.

Good fit for Engineering-heavy orgs that want Teams-shaped channel+thread chat with deeper integration hooks (Slack-compatible webhooks, slash commands, devops bots).

Weak at Files / co-edit story is bring-your-own — pair with Nextcloud + Collabora to recover the docx co-edit half of Teams.

In a terminal? npx -y github:SolvoHQ/os-alt-cli discord prints the Discord comparison table including Mattermost. how the CLI works →

Replaces these SaaS

Discord · Community / voice chat
Use bulkimport JSONL: write a script that reads Discord channels via the Discord API and converts to Mattermost's bulk format. Voice does not transfer (Mattermost calls is a different model).
Microsoft Teams · Team chat + video meetings
Use the official compose at docs.mattermost.com. Mattermost ships a Microsoft Teams importer (`mattermost import process`) that reads a Teams export JSON; in Teams admin: Compliance → eDiscovery → export channel content. Map M365 groups to Mattermost teams during the run. For voice/video, install the bundled Mattermost Calls plugin (WebRTC, no extra service required for small teams).
Slack · Team chat
Slack → Workspace settings → Import/Export → Export. Get the .zip. In Mattermost, run `mattermost import slack <team> <file.zip>` from the CLI. Preserves channels, users, message history, and most file uploads. Threads come over but emoji reactions are partial.

README badges for the SaaS this replaces

Maintainers and forks: drop a badge in your README to link readers from the SaaS-comparison page back to your repo.