Troubleshooting and FAQ
"Failed to log in: Invalid session" when joining a server
The server has online mode turned on, which requires a real Microsoft account. MCL accounts are offline by default, so if everyone in your group is using an offline account, turn online mode off on the server, either through Server Config if it is an existing server, or the settings panel in Host Server if MCL is running it.
My skin does not show up for anyone else
Check which loader the profile uses. Custom skins only sync on Fabric, Forge, NeoForge, and Quilt profiles, a Vanilla profile has no way to resolve or publish a custom skin at all, see Skin Studio for why. If the profile already uses one of those loaders, make sure Share My Skin is turned on in Settings.
I can ping a server in MCL but cannot actually join
Pinging only needs the server to respond, joining needs authentication to succeed too, so this usually points back to the online mode issue above rather than a connectivity problem.
Friends on the same network cannot connect to a server I am hosting
Check that the port shown in the Host Server tab, 25565 by default, is not blocked by Windows Firewall or your router. If you are connecting over something like Radmin VPN, use the address shown in the Radmin adapter, not the one MCL displays for local hosting.
Which loaders can I host a server with?
Vanilla, Fabric, Quilt, Forge, and NeoForge, both locally and through MCL Agent.
The MCL Agent will not start, something about a crypto provider
This means the build is missing its TLS backend. Rebuild with cargo build --release --bin mcl-agent --no-default-features --features agent, the agent feature pulls in everything the certificate and TLS handling needs.
I lost the certificate or token for a remote host
Reading the agent's console output again will not show the token or certificate a second time by design in some setups, but both are saved to files in the agent's data directory (agent-token.txt and agent-cert.pem), open those directly on the VPS if you need to see them again.
Where do I report a bug or ask something not covered here?
Open an issue on the repository.