Zulip

Zulip is an open source team chat application with a unique topic-based threading model. Instead of a single continuous channel stream, every message has a topic, which makes it easy to follow multiple conversations at once and stay focused. It's designed for teams that want structured, organized discussion.

Discord organizes communication into channels where messages flow as one long stream. Zulip organizes each channel into topics, so related messages stay grouped together. This makes catching up on a busy channel far faster — you can skim topics and only read the conversations relevant to you, a big advantage for large teams.

Yes. Zulip is fully open source and self-hostable. It ships with a built-in installation script for Linux and is also available as a Docker image. Thousands of organizations, including many open source communities, run Zulip on their own servers.

Zulip offers topic-based threading, direct and group messages, file sharing, emoji reactions, inline image/video previews, full-text search, integrations with over 100 tools (GitHub, GitLab, Jira, and more), and a comprehensive API. It also supports email integration, so users can read and reply to messages by email.

Zulip is free and open source (Apache 2.0 for the server, MIT for clients). You can self-host it at no cost with unlimited users and history. The company behind Zulip also offers paid hosting and enterprise support, but self-hosting is completely free.