Zulip is an open-source team chat application with a threading model built around topics within channels, rather than flat message streams. This makes it unusually suited to async-heavy teams: conversations are easy to follow days later without reading everything in between. It can be self-hosted on any Linux server or used via Zulip Cloud, including a free tier for open-source projects.
Zulip
zulip.com
Our take
Zulip’s topic threading is a genuine architectural difference from Slack, not just a feature flag, and teams that work across time zones often find it significantly less exhausting to stay current. The trade-off is a higher learning curve: the channel-plus-topic model confuses people used to Slack’s flat channels, and onboarding needs more upfront explanation. The self-hosted path gives you full data ownership with no ongoing vendor dependency. If your team already communicates well in Slack, switching takes real effort; if you are starting fresh or currently drowning in missed messages, Zulip is worth the adjustment.
GitHub at a glance
zulip/zulip
Stars
25,387
Last commit
today
healthy
License
Apache-2.0
Latest release
12.0
2mo ago
Listed in
Zulip alternatives
Zulip license, in plain English
Apache-2.0 Permissive
Permissive like MIT, with an explicit patent grant and a requirement to flag any changes you make.
Permits
- Commercial use
- Modification
- Distribution
- Patent use
- Private use
Requires
- License and copyright notice
- State changes
Does not provide
- Trademark use
- Liability cover
- Warranty
Why it matters: Permissive licensing lets anyone reuse this, including inside closed products. That is freedom to build on, but no guarantee that downstream copies stay open.
Plain-language summary of the project's license, not legal advice. Read the full text for the exact terms.