Slack alternatives that are 100% Open Source, no Open-Core
Of the 4 Slack alternatives in the catalog, only Huly and Zulip are 100% FOSS — Rocket.Chat and Mattermost are Open-Core (free core, but with features that can sit behind a paid tier).
Tools that meet this intent
Alternative to Linear, Notion, Slack
Projects + chat + docs in one, an alternative to Linear/Notion/Slack.
Alternative to Slack
Team chat organized by threads, an alternative to Slack.
Why being 100% FOSS matters here
With Huly or Zulip the code you run is exactly the code you can audit in their repository, with no separate "Enterprise" build with features locked away — if the project ever changes direction, you can always fork it.
What you gain and lose versus the SaaS
Huly and Zulip cover real team chat well, but with smaller communities than Rocket.Chat/Mattermost — fewer ready-made third-party plugins, in exchange for zero risk of a feature you use today moving behind a paywall tomorrow.