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Slack does 5 jobs: channels and direct messages, threaded conversations, voice and video huddles, integrations, search and message history. Most projects replace one or two of them, so pick the job you actually need.
Open-source alternatives to Slack that run locally, allow commercial use and need no paid API include Campfire (ONCE) (channels and direct messages, integrations and search and message history; MIT), Tuwunel (channels and direct messages, threaded conversations and integrations; Apache-2.0) and Jitsi Meet (voice and video huddles; Apache-2.0; runs on CPU). Mattermost and Synapse (Matrix) with Element cover all 5 Slack jobs.
Block's self-hostable workspace where people and AI agents share channels, threads, DMs, canvases, workflows and git, built on a Nostr relay.
Basecamp's simple MIT-licensed group chat with rooms, DMs, file attachments, search and bot API, shipped as one Docker image.
AGPL-3.0 instant messaging and VoIP app for groups and communities, with desktop and mobile apps and a free Docker self-host path.
Apache-2.0 browser video conferencing with screen sharing, in-call chat and polls; self-host it for calls alongside a team chat.
Open core, self-hosted team chat with channels, threads, calls, playbooks and integrations; a single Go binary backed by PostgreSQL.
Chat, audio and video calls and screen sharing as a Nextcloud app, with bots, webhooks, federation and Nextcloud Files integration.
Open core, self-hosted team chat with channels, threads, DMs, apps and federation; free Starter (50 users) and MIT Community editions.
Discord-style chat platform (renamed from Revolt) with servers, channels, DMs and voice/video, self-hostable with Docker.
Reference Matrix homeserver from Element; pair it with Element apps for federated, end-to-end encrypted rooms, DMs, threads and calls.
Lightweight Matrix homeserver in Rust, successor to conduwuit; a Synapse alternative to use with Element or other Matrix clients.
Backend for Wire, an end-to-end encrypted business messenger with teams, conversations and calls, self-hostable on Kubernetes.
Apache-2.0 team chat built around topic-based threads in channels, self-hosted on Ubuntu or Debian or used as hosted Zulip Cloud.
No project meets all of your requirements. The list below shows what you'd need to relax.
We hid them because something is missing, unknown or conflicting. Here's what.
Yes. Mattermost, Synapse (Matrix) with Element, Rocket.Chat, Zulip and Buzz are free to download and run on your own computer without paid API keys. Mattermost and Synapse (Matrix) with Element cover every job we list for Slack; the others cover only part of it, so check which job you need.
Yes, with Rocket.Chat (MIT), Mattermost (AGPL-3.0-only) and Zulip (Apache-2.0). Check the hardware notes and licence conditions on each page before you start.
Yes, with Rocket.Chat (MIT), Mattermost (AGPL-3.0-only) and Zulip (Apache-2.0). Check the hardware notes and licence conditions on each page before you start.
Confirmed: Buzz, Campfire (ONCE), Jitsi Meet, Tuwunel and Zulip. With conditions: Fluxer, Mattermost, Nextcloud Talk, Rocket.Chat, Stoat (formerly Revolt), Synapse (Matrix) with Element and Wire. This is not legal advice; read each licence.
Mattermost and Synapse (Matrix) with Element cover every job we list for Slack.