Classic Protocol Extension

Negotiation

Extended clients should use magic number of 0x42 for the padding byte of the Player Identification packet. All standard/non-extended clients pad this packet with 0x00, so extended servers should be able to easily notice extended clients. All standard/non-extended servers will ignore the modified padding byte, preserving compatibility.

When the server sees that padding byte is 0x42, it should immediately reply with an ExtInfo packet, followed by zero or more ExtEntry packets, and await a response (ExtInfo packet) from the client.

After the client has sent its identification packet, it should await a response. If the server responds with an ExtInfo packet, it should parse it and any ExtEntry packets that follow. Client should then compare its locally-supported set of extensions with the list of extensions provided by the server, and find an intersection of these sets. These are the mutually-supported extensions.

At this time, client should send ExtInfo packet of its own, followed by the list of mutually-supported extensions. After receiving the last of ExtEntry packets, server should activate the mutually-supported extensions, and resume normal login procedure.

ExtInfo Packet

Packet ID	Field Name	Field Type	Example	Notes
0x10	AppName	string	MyServer	Client or server software name
	ExtensionCount	short	1	Between 0 and 32767
Total Size:	67 bytes

ExtEntry Packet

Packet ID	Field Name	Field Type	Example	Notes
0x11	ExtName	string	MyExtension	Name of a supported extension
	Version	int	1
Total Size:	69 bytes

Extensions

ClickDistance

Used to extend or restrict the distance at which client may click blocks, controlled by the server. Click range is given in player-space units (32 units per block). Upon receiving a SetClickDistance packet, client should immediately apply the change.

SetClickDistance packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x20	Distance	short	160
Total Size:	3 bytes

HeldBlock

Provides a way for the client to notify the server about the blocktype that it is currently holding, and for the server to change the currently-held block type. When this extension is mutually supported, client should use the Player ID field of the Position and Orientation packet (currently unused) to indicate which blocktype the client is currently holding.

The server can use HoldThis packet to force the client to hold the desired block type.

HoldThis packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x21	BlockToHold	byte	Obsidian
Total Size:	2 bytes

EmoteFix

This extension indicates that the client can render emotes in chat properly, without padding or suffixes that are required for vanilla client. When this extension is mutually supported, server should not pad or suffix emotes in chat. This extension does not define any new packets.

TextHotKey

This extension allows the server to define "hotkeys" for certain commands. The server should send a definition of each hotkey (SetTextHotKey packet) once per connection. Client should not try to persist previously-defined hotkeys between sessions. When a defined hotkey is activated by the user, client should open up a text prompt and type in contents of the "Action" field. A newline character ("\n") in the Action field indicates that whatever is currently typed-in should be sent to the server. If Action does not end with a newline, text prompt should be left open, for the user to complete.

SetTextHotKey packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x22	Action	string	/Copy	Text to type in
	KeyCode	int	113	SDL keycode of the key
	KeyMods	byte	0	Key modifier flags, may be combined: 0 = None 1 = Ctrl 2 = Shift 4 = Alt
Total Size:	70 bytes

ExtAddEntity

This extension augments the Spawn Player packet to support more players and provide more flexibility in naming of players and loading of skins.

This extension turns PlayerID byte from a signed to an unsigned byte. It allows up to 250 players/entities to be uniquely numbered on one map (with IDs between 0 and 249). ID 250-254 are reserved for future use. ID 255 ("-1" signed) should still refer to "self."

ExtAddEntity Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x23	PlayerID	unsigned byte	5	Between 0 and 249
	PlayerName	string	Notch	Raw player name, without any formatting, to be used for autocompletion.
	ListName	string	&c[Op]Notch	Player name to be shown in the player list (tab-list).
	InGameName	string	&cNotch	Player name to be shown in-game, hovering above player model.
	SkinName	string	Notch	Player name whose skin should be used by the client.
Total Size:	258 bytes

EnvColors

This extension allows server to change some of the colors used in world rendering, to give worlds unique feel (time-of-day, climate, etc).

EnvSetColor Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x24	VariableToSet	byte	1	Enumeration of environmental variables 0 = reset everything 1 = sky color 2 = cloud color 3 = fog color 4 = diffuse light (blocks in shadow) color 5 = sunlight (blocks in sun) color
	Red	unsigned byte	0xF9
	Green	unsigned byte	0x3C
	Blue	unsigned byte	0x50
Total Size:	5 bytes