🎮 Game Error Codes What it means & how to fix it

Core Keeper Co-op Not Working? Here's the Fix

When Core Keeper multiplayer fails — the game ID won't connect, the invite does nothing, or you get stuck on "joining" — it's usually a version mismatch, a Steam invite/overlay problem, or a strict NAT. Fastest fix: both players update to the same build, add each other on Steam, and have the host share the correct game ID while the world is loaded.

Exact details for Core Keeper

Default port (UDP)
27016
Gotcha
-port is command-line only; if you omit it the server uses the Steam relay instead of a direct port
Bind IP flag
-ip only applies when -port is set
Common cause of 'can't join'
Relying on Steam relay behind strict NAT — set -port 27016 explicitly and forward it

Error codes

Game ID won't connect The joiner's code doesn't reach the host's session

Cause: Wrong/expired game ID, or the host isn't in the world yet.

Fix:
  1. Host must be inside the world before the joiner enters the game ID.
  2. Copy the game ID exactly (no spaces) from the host's pause menu.
  3. Generate a fresh game ID if the old session was closed.
  4. Prefer the Steam invite/overlay over manual IDs where possible.
Stuck on joining / can't join friend The connection starts but never completes

Cause: Steam friends/overlay issue or strict NAT.

Fix:
  1. Both players must be Steam friends and have the Steam overlay enabled.
  2. Allow Core Keeper through Windows Firewall (Private + Public).
  3. Set router NAT to Open or enable UPnP; use a wired connection.
  4. Restart Steam on both machines and try the invite again.
Version mismatch Client and host are on different Core Keeper builds

Cause: One player updated or is on a beta/experimental branch.

Fix:
  1. Update Core Keeper on both machines so builds match.
  2. Both opt out of experimental/beta branches in Steam > Betas.
  3. Verify integrity of game files in Steam.
  4. Restart both games after updating.
Can't connect to dedicated server The dedicated server is running but joiners never reach it

Cause: UDP port 27015 isn't forwarded, or players are on a different build than the server.

Fix:
  1. Forward UDP port 27015 (default) on the host's router to the server machine.
  2. Make sure the server and all clients are on the exact same Core Keeper build.
  3. Join by Direct Connect using the host's public IP and port if the server list is empty.
  4. Restart the dedicated server after a game update so it re-registers.
Connection timed out / stuck loading world The join starts, the world begins loading, then it hangs and drops

Cause: Large world save transfer is being blocked by firewall/NAT, or a slow upload on the host.

Fix:
  1. Allow Core Keeper through Windows Firewall on BOTH Private and Public profiles.
  2. Set the host's router NAT to Open or enable UPnP; prefer a wired connection.
  3. Have the host lower background upload (close cloud sync / streaming) during the join.
  4. If it still hangs, host restarts the world and the joiner reconnects with a fresh game ID.
Steam and Game Pass players cant connect A Steam player and an Xbox/Windows (Game Pass) player cannot join each other

Cause: The Steam build and the Microsoft Store/Game Pass build are separate versions with no cross-store co-op.

Fix:
  1. Both players must own and launch Core Keeper on the SAME store (both Steam, or both Game Pass).
  2. There is no crossplay between Steam and the Microsoft Store version — buy the matching edition if you want to play together.
  3. If one of you is on Game Pass, the other can play via Game Pass on PC too (same Microsoft account ecosystem).
Dedicated server not showing in server browser Your dedicated server is running but never appears in the in-game list

Cause: The in-game server browser is unreliable and often fails to list LAN/remote dedicated servers.

Fix:
  1. Join by Direct IP instead of the browser: use the host public IP (or LAN IP) and the forwarded UDP port.
  2. Confirm UDP port 27015 is forwarded on the host router and allowed through the firewall.
  3. Check the server console/log shows "Game ID" / "listening" before joiners try to connect.

Fixes that work for most Core Keeper errors

  1. Update Core Keeper on both machines to the same build.
  2. Host loads the world first, then shares the game ID or invites via Steam.
  3. Both players Steam friends with the overlay enabled.
  4. Allow the game through Windows Firewall (Private + Public).
  5. Set NAT to Open / enable UPnP; use a wired connection.
  6. Verify integrity of game files if joining stalls.

FAQ

Why won't the game ID work in Core Keeper?

The host must already be inside the world before you enter the ID, and the ID has to be copied exactly. If the session was closed, generate a fresh game ID. When possible, use the Steam overlay invite instead of a manual code.

How do I fix "can't join friend" in Core Keeper?

Make sure both players are Steam friends with the overlay enabled, allow the game through Windows Firewall, and set NAT to Open or enable UPnP. A wired connection and a Steam restart on both sides usually clears a stuck "joining".

Does Core Keeper have dedicated servers?

Yes — Core Keeper supports dedicated servers, but most co-op is host-based via Steam. For a dedicated server, forward the configured port and keep the host online; for host-based play, UPnP/Open NAT is enough.

How do I connect to a Core Keeper dedicated server?

Forward UDP port 27015 on the host router, keep the server and every client on the same build, and use Direct Connect with the host's public IP if the in-game list is empty. Restart the dedicated server after any update so it re-registers.

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