1. Is the server up?
A first start downloads about 570 MB before the world loads. On Flux, the Overview tab says whether the server is running and shows its Game ID once the world is up.
2. The Game ID
Copy it again: it is case-sensitive. If it was changed (it can be, when it got out), the old one no longer works.
3. The game version
A server on an older build than the players refuses them. Ours checks Steam every 30 minutes and restarts onto a new build once nobody is playing. A player on an older build than the server needs to update their game.
4. The password and the platform (joining by IP)
Use the whole join line, address;port;;password. If the server is limited to one store (-allowonlyplatform), players from the others are refused. Console players cannot join: the dedicated server accepts the PC stores only (Steam, Epic, Microsoft Store and GOG).
5. Mods
Players need the same mods as the server. A player missing one, or with a different version, cannot play on it.
Frequently asked questions
Why does my Core Keeper Game ID not work?
Usually a typo, a server still starting, or a Game ID that was changed. Copy it again from the dashboard.
Why can my Epic friend not join my Core Keeper server?
They join by IP with the address, port and password, not the Game ID; and the server must allow every PC store.