Open source alternatives to Slack

What Slack does

Slack is a hosted messaging service for organizations, built around channels for teams and projects, with threads, direct messages and file sharing. Huddles add audio and video meetings with screen sharing, Slack Connect brings people from other organizations into shared channels, and canvases, clips, Workflow Builder, a large app directory and AI features round it out. It is offered on Free, Pro, Business+ and Enterprise plans.

Why people look for an alternative

• Short history on the free plan. On Free, messages and files are visible for 90 days, and content older than one year is permanently deleted. Paid plans keep data indefinitely by default.
• Free plan limits on calls and external chat. Huddles and Slack Connect are limited to one-to-one on Free.
• Data location. Without the data residency feature, Slack stores data in the US. Data residency requires Business+ or Enterprise, and some categories, such as membership information and usage and analytics data, may still be stored outside the chosen region.
• Limited exports. On Free and Pro, owners can export public channels; exporting private channels and direct messages requires applying for access, and Free workspaces only export file links from the last 90 days.
• Per-user subscription costs and lock-in. Full history and administration features are tied to per-user plans, and there is no way to run Slack on your own infrastructure.

What to compare in an alternative

1. License and editions. Is the server fully open source, or are SSO, guest accounts, compliance export and high availability in a paid edition? Do free editions cap users or message history?
2. Chat model. Slack-style channels and threads (Mattermost, Rocket.Chat), topic threads (Zulip), or a minimal room-based chat (Campfire).
3. Importing from Slack. Mattermost, Rocket.Chat and Zulip have documented importers for Slack exports; check what your Slack plan lets you export first.
4. Encryption. Transport encryption only, optional end-to-end encryption, or end-to-end encryption by default, and what it costs in search and compliance.
5. Self-hosting effort. A single container or binary with one database versus a multi-service Kubernetes stack.
6. Integrations and calls. Webhooks, bots and ready-made integrations, and whether calls are built in or handed to an external provider.

Which one to choose

Mattermost is the closest match to Slack’s layout and integrations and runs as one Go binary with PostgreSQL, but SSO and compliance features are paid and the free Entry edition caps history at 10,000 messages. Zulip is fully open source with every feature in the self-hosted server, imports from Slack, and organizes channels into topics, which helps busy and asynchronous teams. Rocket.Chat adds optional end-to-end encryption, Matrix federation and customer live chat, with tighter limits on its free editions. If end-to-end encryption by default matters most, choose Element for federation over the open Matrix standard or Wire for MLS-based encryption with an EU-hosted cloud. Organizations already running Nextcloud can add chat and calls with Nextcloud Talk. For a small team that wants the simplest possible setup, Campfire runs from one Docker container, and Tailchat offers a plugin-based chat for teams that want to build their own extensions.