If a friend cannot join your Core Keeper world, first update every installation, restart the game and session, and verify that both players are on a compatible platform combination. Most connection failures come from version differences, unsupported cross-play, lobby discovery, mod mismatches, or a dedicated server that is not reachable—not from a router that needs random ports opened.
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Use the exact error message to choose the right fix. Steam host-and-play uses Steam networking and does not require port forwarding; a dedicated server follows different rules and needs a configured, reachable port for direct connections.
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Write down the complete message shown by Core Keeper, not just the fact that joining failed. Also record who is hosting, each player’s platform and storefront, the game version shown on each installation, whether cross-play is enabled, and whether either side uses mods. This separates an unsupported platform combination from a missing lobby, a transport timeout, or a mod-loading failure.
| Error or symptom | What it usually indicates | First action |
|---|---|---|
| “Game cannot be found” | Lobby discovery or platform-compatibility problem | Update both games, recreate the session, and confirm the platform combination is supported |
| “Connection timed out” | The client cannot complete the connection to the host or direct-connect server | Determine whether this is Steam host-and-play or a dedicated server, then use the networking rules for that path |
| “Game version mismatch” | The game build or mod set differs between participants | Match the game version and every enabled mod and version, then test without mods |
| The world is unavailable when the host leaves | Normal host-and-play is tied to that host’s session | Use a dedicated server if the world must remain available without the host |
Keep this information while troubleshooting. If you eventually contact support, it lets the support team reproduce the same path instead of guessing at the cause.
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1. Update every player and restart the session
Make version alignment the first technical check. Bring every installation to the same current Core Keeper version, fully exit the game, relaunch it, and have the host create a new session. The joining player should then use the in-game Join Game flow rather than trying to connect to an old lobby.
This matters even when nobody intentionally changed a setting. Fireshine’s patch notes document fixes affecting lobby finding, join errors, and cross-play networking. A session created before an update can therefore fail after one player has patched and another has not. Updating both sides removes that variable before you investigate firewalls, ports, or mods.
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- Close Core Keeper on every participating device.
- Install all available updates on each device and storefront.
- Restart the game after the update completes; do not leave it suspended in the background.
- Have the host create a fresh multiplayer session.
- Have the client open Join Game and search again.
If the error changes after this sequence, keep the new wording. A change from “Game cannot be found” to “Game version mismatch,” for example, points to a different branch of the diagnosis.
2. Check whether the platform combination is supported
Do this before changing router settings. The current FAQ describes cross-play between PC storefronts including Steam, Epic, and GOG. It does not support cross-platform play between PC and console. Cross-generation console play is supported, so console players on different generations can play together within the console ecosystem.
Supported PC storefront combinations
Steam, Epic, and GOG PC players can join one another under the game’s current PC compatibility rules. Each installation still needs to be updated to a compatible build, and mods must agree if the session is modded.
PC and console boundaries
A PC player cannot join a console-only session, and a console player cannot join a PC-only session. If one player is on Xbox or PlayStation and the other is on PC, the failed join is a product boundary rather than evidence that the host’s router is misconfigured.
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Cross-generation console play
The FAQ describes cross-generation console play as supported. When two console players still cannot connect, continue with the update, restart, session-discovery, and mod checks instead of assuming that the console generations themselves are incompatible.
3. Recreate a Steam host-and-play session
For ordinary player-hosted multiplayer through Steam, use the in-game flow and let Steam handle the connection. The official Steam announcement says that player-hosted multiplayer uses Steam’s networking back end and that port forwarding is no longer required for this path. Opening random router ports because a friend cannot see a Steam lobby can add risk without fixing the underlying problem.
- Exit Core Keeper on both machines.
- Restart the Steam client on both machines so each account has a fresh connection.
- Launch the game again and make sure both installations report the same game version.
- Have the host create the multiplayer world or session.
- Have the other player select Join Game and try the new lobby.
- If discovery still fails, repeat the test with mods disabled on both sides.
The no-port-forwarding statement applies to Steam’s in-game host-and-play route. It does not describe a separately installed dedicated server that accepts direct connections. Keep those two setups separate when deciding what to change on the network.
4. Remove mod uncertainty from the diagnosis
A “Game version mismatch” can refer to the base game or to the mod set. For a modded world, every client and the server should have exactly the same mods installed, with matching versions and load results. One missing, outdated, or broken mod can prevent a join even when the underlying game build is identical.
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Compare the complete mod set
- List every enabled mod on each client and, if applicable, on the dedicated server.
- Compare the version of each mod, not just its name.
- Confirm that each mod reports a successful load rather than an error.
- Check logs for a failed, outdated, or missing dependency.
Use an unmodded control test
Temporarily disable the mods on all participating clients and start a clean test session. If the unmodded session connects, the base multiplayer path is working and the mismatch is inside the mod set. Re-enable mods only after you have identified a common, working set. If the unmodded session also fails, continue with platform and networking checks instead of repeatedly reinstalling mods.
When reporting a mod problem, state whether the failure occurs in single-player, normal multiplayer, or a dedicated server, and include the relevant logs. That context changes which component needs attention.
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5. Decide whether you need a dedicated server
Normal host-and-play is the simplest option, but the world is tied to the host’s active session. If the host leaves, the world is not intended to remain available as an always-on service. The dedicated-server tool exists for the opposite requirement: the world can remain available without the normal host being online.
Install the dedicated-server tool from Steam’s Tools menu. Setup takes more work because you must maintain the server package and define its connection settings, but it avoids making one player’s game session the availability requirement.
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|---|---|---|
| Host availability | The world is tied to the host session | Intended to remain available without the normal host |
| Setup effort | Lowest; use the in-game flow and Steam networking | Higher; install and maintain the server tool and settings |
| Port configuration | Steam’s official announcement says port forwarding is not required for this path | Direct connections require a configured port that is open for incoming traffic |
| Cross-play scope | Controlled by the current platform-compatibility rules | The -allowonlyplatform setting can restrict direct-connection platforms |
| Mod risk | Clients still need compatible mods | Server and clients need matching mods and versions |
6. Configure a dedicated server carefully
The current server documentation identifies several command-line controls. The exact launch syntax can vary with the server package you installed, so verify the argument names against that package before starting it. Third-party documentation can lag behind a game update.
| Argument | Purpose | Documented behavior |
|---|---|---|
-port |
Bind a port for direct connections | A configured port enables direct connections, and that port must be open for incoming traffic |
-ip |
Select the local network interface | Defaults to 0.0.0.0 when unset |
-password |
Set the password used for IP connections | Applies to direct IP connections |
-maxplayers |
Set the player cap | Defaults to eight players when omitted |
-allowonlyplatform |
Restrict direct-connection platforms | Can limit access to Steam, Epic, Microsoft, or GOG |
Choose the listening interface
Leave -ip unset when the server should listen on the default all-interface address documented as 0.0.0.0. If you deliberately bind to a particular local interface, make sure that interface is the one reachable by the clients you intend to support. Binding the wrong interface can make a running server appear offline.
Set the direct-connection port
Use -port to select the port on which the dedicated server accepts direct connections. The documentation states that this port must be open for incoming traffic. If the server process is running but clients receive a timeout, verify that the chosen port is reachable through the host machine’s network and any firewall policy before changing unrelated game settings.
Protect the server with a password
Set -password when you want direct IP connections to require a password. Give the value to intended players through a private channel. A password does not correct a blocked port or an incompatible platform; it only controls authentication after a direct connection reaches the server.
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Limit the player count
Use -maxplayers to define the server cap. If you omit it, the documented default is eight players. Set the value deliberately so a full server is not mistaken for a discovery or timeout failure.
Restrict direct-connection platforms
The -allowonlyplatform option can restrict a direct-connection server to Steam, Epic, Microsoft, or GOG. Use a platform restriction only when it matches the clients you actually want to admit. A player rejected by this setting may be able to run the game normally but still fail to join this particular server.
7. Troubleshoot a dedicated-server timeout
A timeout on a dedicated server is different from a missing Steam lobby. Work from the server outward:
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- Check the launch arguments, especially
-port,-ip, and any platform restriction. - Confirm that the configured port is open for incoming traffic as required for direct connections.
- Verify that clients are using the address and port belonging to that server, not an old session or a Steam host-and-play lobby.
- Make sure the server password, if set, is entered exactly.
- Align the game version and complete mod set between the server and every client.
- Test with an unmodded client and a minimal player group before adding more variables.
If the server works for one platform but not another, inspect -allowonlyplatform before changing the port. If no platform can connect, focus on the listening interface, incoming-port rule, and server version.
8. Read the symptom after each change
Do not apply every possible fix at once. Change one category, recreate the session, and record what happened.
When the lobby remains invisible
Repeat the update and restart sequence, then verify that the host and client are using a supported platform combination. On Steam, recreate the lobby after restarting Steam and the game. A persistent “Game cannot be found” message after those checks is evidence to capture for support, not a reason to open arbitrary router ports.
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When the message is a version mismatch
Compare the base game build first, then compare every mod and version. Confirm successful mod loading, remove broken or outdated mods, and run an unmodded test. A dedicated server must match its clients too.
When the client times out
Identify the connection path. Steam host-and-play does not require port forwarding according to the official announcement; a dedicated server does require its configured direct-connection port to be open for incoming traffic. Applying the dedicated-server port rule to a normal Steam lobby, or vice versa, sends troubleshooting in the wrong direction.
When only one player cannot join
Compare that player’s platform, storefront, game version, mod list, and credentials with a player who can connect. If the working player is on a different platform category, check the cross-play boundary or a server platform restriction before blaming the host machine.
9. Prepare an effective support request
Steam’s support guidance directs Core Keeper players to Fireshine Games customer support. Escalate after you have completed the update, platform, host-path, and mod checks, and include reproducible details rather than a one-line “multiplayer is broken” report.
- The exact error text, including whether it says “Game cannot be found,” “Connection timed out,” or “Game version mismatch.”
- The game and dedicated-server versions.
- Which player is hosting, and whether the session is Steam host-and-play or a dedicated server.
- Each player’s platform and storefront.
- Whether cross-play is enabled.
- Whether mods are enabled, the complete mod list and versions, and relevant logs.
- The steps already attempted and the result of an unmodded test.
For a mod-related report, explicitly state whether the failure occurs in single-player, multiplayer, or a dedicated server. Include logs when available and describe the smallest sequence that reproduces the failure. This gives support a clear path to separate a game-build problem from a mod, lobby, or direct-network issue.
Quick Recap
Use the shortest safe fix path
- Record the exact error, roles, platforms, versions, cross-play state, and mod state.
- Update every installation and restart the game and session.
- Confirm that the platform combination is supported: PC storefronts can play together, PC-to-console play is unavailable, and cross-generation console play is supported.
- For Steam host-and-play, recreate the lobby and rely on Steam networking instead of forwarding ports.
- For a modded session, make the complete mod set and versions identical, then test without mods.
- If the world must stay online without its normal host, install and configure the dedicated-server tool, including its port, interface, password, player cap, and platform restriction.
- Send Fireshine support the exact error and reproducible configuration if the failure remains.
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