Geyser: io.netty.handler.codec.DecoderException with a Forge server

[Leeo97one]

Describe the bug
I can't join a Forge server running without any mod:

Leeo97one has disconnected from remote java server on address 127.0.0.1 because of io.netty.handler.codec.DecoderException: java.lang.IllegalArgumentException: Value minecraft: has no mapping for key class com.github.steveice10.mc.protocol.data.game.command.CommandParser.

To Reproduce
Steps to reproduce the behavior:

1. Run a Forge server without any mod
2. Connect via Geyser
3. See error

Expected behavior
Successful connection.

Screenshots

Server version
forge-1.15.2-31.1.0-universal

Geyser version
Revision: 03e11df58beafbc91b414f8e50a020a76e8e2e7c

Bedrock version
1.14.30

Additional context
No mods.

[ldare373]

My understanding is that geyser will not work with forge.

[RealTriassic]

Geyser Proxy doesn't work with Modded Forge Servers, or basically any Forge.

[Leeo97one]

OK and no support is planned? At least for server which doesn't require any client-side mod.

[RealTriassic]

OK and no support is planned? At least for server which doesn't require any client-side mod.

Most likely no.

[Heath123]

Geyser should probably work in the same way as a vanilla client- can a vanilla client join the server?

[Leeo97one]

Geyser should probably work in the same way as a vanilla client- can a vanilla client join the server?

Yes of course.

[ldare373]

Wait what

[LogicismDev]

You will not be able to join a forge server basically since Geyser doesn't support custom blocks and GUIs. So basically if a vanilla client can't join, it'll make sense why a bedrock user can't join through Geyser.

[Leeo97one]

A vanilla client can join a Forge server with server-side only mods (or no mod at all). Easy to test if you want.
Exemple of mod: https://www.curseforge.com/minecraft/mc-mods/fast-leaf-decay

[RednedEpic]

Ideally, Geyser should be able to join Forge and Fabric servers that don't require the client to have any mods without any problems. Likely what is happening is the Forge server is sending a packet that Geyser does not recognize, thus causing them to be kicked. On a normal Notchian client, the packet is ignored.

[Loganh4005]

I also remember something like this error when trying to get bungeecord working with forge before I went to spigot + vanilla

[DoctorMacc]

I'll keep this open, but it appears the problem is in MCProtocolLib and not us. I'll be filing a pull request shortly.

[DoctorMacc]

This should be fixed next build, when the MCProtocolLib changes can register.

[DoctorMacc]

This should be now be fixed.