Plugin channels

Plugin channels allow client mods and server plugins to communicate without cluttering up chat. This post by Dinnerbone is a good introduction and basic documentation.

Reserved channels

minecraft:register

Two-way

Allows the client or server to register for one or more custom channels, indicating that data should be sent on those channels if the receiving end supports it too. Payload is a null (0x00) separated list of strings.

This channel was renamed from REGISTER in 1.13.

minecraft:unregister

Two-way

Allows the client or server to unregister from one or more custom channels, indicating that the receiving end should stop sending data on those channels. Payload is a null-separated list of strings. This is only useful if plugins are disabled/unloaded while the client is connected.

This channel was renamed from UNREGISTER in 1.13.

Channels internal to Minecraft

Since 1.3, Minecraft itself uses several plugin channels to implement new features. These internal channel names are prefixed by MC|. They are not formally registered using the register channel. The vanilla Minecraft server will send these packets regardless, and the vanilla client will accept them.

Note that there were originally more channels included, but most of these were removed in 1.13. See this revision of the article for the original list.

minecraft:brand

Two-way

Announces the server and client implementation name right after a player has logged in. For the Notchian client and server server, this is "vanilla" (which is chosen using the Function net.minecraft.client.ClientBrandRetriever.getClientModName()) (encoded as a protocol string: a UTF-8 string with the length prefixed as a VarInt).

These brands are used in crash reports and a few other locations; it's recommended that custom clients and servers use changed brands for the purpose of identification (for the Notchian client, the class used to get the brand is one of the few non-obfuscated classes). The brand is not processed in any other way, and Notchian clients will connect to servers with changed brands with no issue (the brand is not used to validate).

The Notchian server sends a MC|Brand packet right after it sends a Join Game packet, and the Notchian client sends it right after receiving a Join Game packet. However, some modified clients and servers will not send this packet (or will take longer to send it than normal), so it is important to not crash if the brand has not been sent. Additionally, the brand may change at any time (for instance, if connected through a BungeeCord instance, you may switch from a server with one brand to a server with another brand without receiving a Join Game packet).

minecraft:book_open

Server to client

When a player right clicks with a signed book. This tells the client to open the book GUI.

The payload is a VarInt enum for the selected hand: 0 for the main hand, 1 for the offhand.

minecraft:trader_list

Server to client

The list of trades a villager NPC is offering.

Field Name		Field Type		Notes
Window ID		Int		The ID of the window that is open; this is an int rather than a byte.
Size		Byte		The number of trades in the following array
Trades	Input item 1	Array	Slot	The first item the villager is buying
	Output item		Slot	The item the villager is selling
	Has second item		Boolean	Whether there is a second item
	Input item 2		Optional slot	The second item the villager is buying; only present if they have a second item.
	Trade disabled		Boolean	True if the trade is disabled; false if the trade is enabled.
	Number of tool uses		Int	May actually be the number of times this trade has been used; MCP may be misleading me
	Maximum number of trade uses		Int	Number of times this trade can be used

minecraft:debug/paths

Server to client

What appears to be the purpose of MC|DebugPath as seen in the snapshot 16w14a announcement. This is not found in the normal game.

Never sent, but does something with pathfinding debugging. The client reads the data and stores it, but its renderer cannot be enabled without modifications.

Name	Type	Notes
Unknown 1	Int	Used as a key in the mapping that stores this data; might be an entity ID?
Unknown 2	Float	Appears to be the "radius" of the squares for each pathpoint
Entity	PathEntity	See below

PathEntity structure:

Name	Type	Notes
Current path index	Int	Index in the points array that the entity is currently targeting. That point is rendered red; the others are rendered blue.
Target	PathPoint	Rendered as a green cube
Number of points	Int	Number of elements in the following array
Points array	Array of PathPoint	The actual points on the path
Open set length	Int	Number of elements in the following array
Open set	Array of PathPoint	Displayed in blue
Closed set length	Int	Number of elements in the following array
Closed set	Array of PathPoint	Displayed in red

PathPoint structure:

Name	Type	Notes
X	Int
Y	Int
Z	Int
Distance from origin	Float
Cost	Float
Cost malus	Float	Number that is rendered
Has been visited	Boolean
Node type	Int enum	See below
Distance to target	Float

Values for node type:

• 0: BLOCKED
• 1: OPEN
• 2: WALKABLE
• 3: TRAPDOOR
• 4: FENCE
• 5: LAVA
• 6: WATER
• 7: RAIL
• 8: DANGER_FIRE
• 9: DAMAGE_FIRE
• 10: DANGER_CACTUS
• 11: DAMAGE_CACTUS
• 12: DANGER_OTHER
• 13: DAMAGE_OTHER
• 14: DOOR_OPEN
• 15: DOOR_WOOD_CLOSED
• 16: DOOR_IRON_CLOSED

minecraft:debug/neighbors_update

Server to client

Never sent, but used to debug block updates. Does not render without modifying the client.

This channel is invalid, as its name is longer than the max length (23 > 20).

Name	Type	Notes
Time	VarLong	World timestamp at which the update occurred. 200 ticks after this timestamp, the given update stops rendering.
Location	Position	Location of the block that updated.

minecraft:debug/caves

Never sent, but (presumably) used to debug caves. Does not render without modifying the client.

Field name		Field type		Notes
Cave position		Position		Rendered as a white cube; also used to determine the color of the other parts.
Count		Int		Number of elements in the following array
Data	Sub position	Array	Position
	Radius		Float	Size of the area, as half of the side-length

minecraft:debug/structures

Never sent, but (presumably) used to debug structures. Does not render without modifying the client.

Adds a single new structure, which will always be rendered if the player is in the same dimension.

Field name		Field type		Notes
Dimension		Int		The dimension the structure is in.
Bounding box minX		Int		Main box for the structure (rendered in white). min must be less than max
Bounding box minY		Int
Bounding box minZ		Int
Bounding box maxX		Int
Bounding box maxY		Int
Bounding box maxZ		Int
Count		Int		Number of elements in the following array
Sub-boxes	Bounding box minX	Array	Int	min must be less than max
	Bounding box minY		Int
	Bounding box minZ		Int
	Bounding box maxX		Int
	Bounding box maxY		Int
	Bounding box maxZ		Int
	Flag		Boolean	If true, the sub-box is rendered in green, otherwise in blue.

minecraft:debug/worldgen_attempt

Never sent, but (presumably) used to debug something with world generation. Does not render without modifying the client.

Adds a colored cube of the list of things to render. This cube is never removed.

This channel is invalid, as its name is longer than the max length (23 > 20).

Field name	Field type	Notes
Location	Position	The center of the location to render.
Size	Float	Diameter/side length of a cube to render.
Red	Float	Red value to render, from 0.0 to 1.0.
Green	Float	Green value to render, from 0.0 to 1.0.
Blue	Float	Blue value to render, from 0.0 to 1.0.
Alpha	Float	Alpha value to render, from 0.0 to 1.0.

MC|PingHost

Client to server

Sent after a Server list ping in Minecraft 1.6. More information on Server List Ping#1.6. In 1.7 and above, the Request (Status, 0x00, serverbound) packet is instead sent before the ping.

Since this plugin channel is only sent for the legacy server list ping, it uses the older packet structure.

Notable community plugin channels

Channels listed in this section are not used by the vanilla Minecraft client or server. This is just a likely-incomplete list of channels used by mods/plugins popular within the Minecraft community.

bungeecord:main

Formerly BungeeCord; additionally, note that the channel name is remapped by spigot so that the old name can still be used in plugins.

See here

FML|HS, FML

Main article: Minecraft Forge Handshake

Used by Minecraft Forge to negotiate required mods, among other things. FML|HS FML

ML|OpenTE

Server to client

Used by ModLoader to support custom GUI windows. Does not use the REGISTER channel.

WECUI

Main article: /WorldEditCUI

Used by the server-side WorldEdit and the client-side WorldEditCUI to coordinate selections.

wdl:init, wdl:control, wdl:request

Main article: /World downloader

Used by the client-side World Downloader mod to get permission information and perform permission requests.