Classic Protocol Extension

Classic Protocol Extension (CPE) is a project to augment the Minecraft Classic network protocol with new and improved functionality.

Extensions are designed to keep extended clients and servers compatible with standard clients and servers. Standard clients and extended clients can play on the same server side-by-side. Extensions are designed to be modular: custom clients and servers can selectively implement any subset of extensions, and only mutually-supported extensions will be used.

This specification has not yet been finalized, and is subject to change. Last revision: 11 December 2015. Keep an eye on the history subpage for a chronology of changes.

Do you have an idea for an extension? Please post it on the Proposals subpage.

Support

See support subpage for a detailed table.

Custom servers that already support CPE: FemtoCraft, D3, LegendCraft, GemsCraft, Hypercube, MCSong

Custom servers that plan to add support: 800Craft, cloudBox, fCraft, GGS

Custom clients that already support CPE: ClassiCube Client, ClassicalSharp

Custom clients that plan to add support: Charged Miners, GemsClient

Negotiation

When CPE-capable client connects to a CPE-capable server, a brief negotiation needs to happen before any extensions are used. Client and server declare their capabilities and determine which extensions are mutually supported. All CPE-capable software is required to support this.

Client behavior: Extended clients must use magic number of 0x42 (66 decimal) for the padding byte of the ClientIdentification packet. It must then await a response. If server responds with any packet other than ExtInfo, client must assume that the server does NOT support CPE. If the server responds with an ExtInfo packet, client must parse it and any ExtEntry packets that follow. Client must 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. Client must now send ExtInfo packet of its own, followed by a list of zero or more client-supported extensions. After sending the last of ExtEntry packets, client should activate all mutually-supported extensions and resume normal login procedure.

Server behavior: When a client connects, server must read the ClientIdentification packet and check its padding byte. If this byte is set to 0x42 (66 decimal), assume that the client supports CPE. If this byte is set to any other value, assume that the client does NOT support CPE.

Server must immediately reply to CPE clients with an ExtInfo packet, followed by zero or more ExtEntry packets, and await a response from the client. Client will respond with one ExtInfo and zero or more ExtEntry packets. Server must then compare its locally-supported set of extensions with the list of extensions provided by the client, and find an intersection of these sets. These are the mutually-supported extensions. After receiving the last of ExtEntry packets, server should activate all mutually-supported extensions and resume normal login procedure.

Note 1: All standard/non-extended clients use 0x00 for the padding byte. All standard servers ignore this padding byte. Therefore, this negotiation process does not affect compatibility with standard software.

Note 2: Do not make any assumptions about supported functionality based on the AppName field of ExtInfo packet. It's for logging purposes only.

Note 3: Do not declare support for an extension until it is FULLY implemented, except for debugging.

ExtInfo Packet

Packet ID	Field Name	Field Type	Example	Notes
0x10 (16)	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 (17)	ExtName	string	MyExtension	Name of a supported extension
	Version	int	1	Only extensions with identical version numbers should be considered compatible.
Total Size:	69 bytes

Extensions

Note: The section heading is the name of the extension. Packet names are not same as extension names. For example, the first extension listed here is named "ClickDistance" and not "SetClickDistance".

Do you have an idea for an extension? Please post it on the Proposals subpage.

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). In Minecraft Classic, the default range is 160.

Motivation: This extension allows trusted players to have a wider or virtually-unlimited reach. It may simplify operation of certain bots. Restricting the reach may allow new games/mini-games.

Client Behavior: Upon receiving a SetClickDistance packet, client should immediately apply the change. Distance should persist between worlds/maps.

Server Behavior: Server should send SetClickDistance packet when the server connects, or whenever their permitted click distance changes. Server should allow for some margin-of-error (+/- 1 block) when enforcing click distance restrictions.

SetClickDistance packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x12 (18)	Distance	short	160
Total Size:	3 bytes

CustomBlocks

Used to add support for custom block types. Custom block IDs start at 50 (0x32). New block types will be added in batches, a few at a time. Both client and server declare which batch they support, and use the lower of the two versions. Claiming to support a batch implies fully implementing all the batches that came before it. If either server or client do not support this extension, only the standard 50 block types should be used.

Motivation: Adding new visually distinct blocks, to enhance Classic players' experience.

Client behavior: Client must expect a CustomBlockSupportLevel packet from a compatible server immediately after sending the last ExtEntry packet. It should then reply with its own CustomBlockSupportLevel packet, containing its actual support level. Client must then use the lower of the two levels in operation. Client must not send any block types that are not defined by the current support level. Client should expect ServerIdentification packet only AFTER sending its CustomBlockSupportLevel packet.

Server behavior: Server must send a CustomBlockSupportLevel packet to compatible clients immediately after receiving the last ExtEntry packet from the client. It should then wait to receive a CustomBlockSupportLevel packet from the client before sending the ServerIdentification packet. Server must then use the lower of the two levels in operation. If this level is lower than the server's, it has to filter data sent to the client, to make sure that the client never receives any block types that it does not support. Each level will define what substitutions to use.

CustomBlockSupportLevel packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x13 (19)	SupportLevel	byte	1
Total Size:	2

Blocks in support level 1

Client must be able to receive/render all 16 custom blocks to claim support. Server must be able to receive/store all 16 custom blocks to claim support.

Block name	Block ID	Fallback name	Fallback ID
CobblestoneSlab	0x32 (50)	Slab	0x2C (44)
Rope	0x33 (51)	BrownMushroom	0x27 (39)
Sandstone	0x34 (52)	Sand	0x0C (12)
Snow	0x35 (53)	Air	0x00 (0)
Fire	0x36 (54)	Lava	0x0A (10)
LightPinkWool	0x37 (55)	Pink	0x21 (33)
ForestGreenWool	0x38 (56)	Green	0x19 (25)
BrownWool	0x39 (57)	Dirt	0x03 (3)
DeepBlue	0x3A (58)	Blue	0x1d (29)
Turquoise	0x3B (59)	Cyan	0x1c (28)
Ice	0x3C (60)	Glass	0x14 (20)
CeramicTile	0x3D (61)	Iron	0x2a (42)
Magma	0x3E (62)	Obsidian	0x31 (49)
Pillar	0x3F (63)	White	0x24 (36)
Crate	0x40 (64)	WoodenPlanks	0x05 (5)
StoneBrick	0x41 (65)	Stone	0x01 (1)

Block IDs for future support levels are guaranteed to be assigned monotonically, incrementally, and permanently.

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.

Motivation: This allows server to know which block player is holding, for example for drawing commands, without needing to wait for the player's click. It also allows for features like /Spectate to show what block a spectated player is holding.

Client behavior: When this extension is mutually supported, client should use the PlayerID field of the PositionAndOrientation packet (currently unused) to indicate which blocktype the client is currently holding. It should be ready to accept HoldThis packets to change the block that the player is holding. If 0 is given for BlockToHold, client should hide the hand/block from the screen, and should not be able to click blocks, until they switch to a different blocktype. If an unrecognized blocktype is given, no action is needed.

Server behavior: The server can use HoldThis packet to force the client to hold the desired block type. It should be done sparingly.

HoldThis packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x14 (20)	BlockToHold	byte	49	Standard block type
	PreventChange	byte	0	0 = Allow player to change blocktype 1 = Prevent player from changing blocktype
Total Size:	3 bytes

EmoteFix

This extension indicates that the client can render emotes (ASCII control characters) in chat properly, without padding or suffixes that are required for vanilla client. This extension does not define any new packets.

Motivation: To improve appearance of emotes in chat.

Client behavior: Client should not emulate vanilla client's quirks.

Server behavior: Server should not pad or suffix emotes in chat.

TextHotKey

This extension allows the server to define "hotkeys" for certain commands.

Motivation: To speed up and simplify access to commonly-used commands and command macros by providing server-defined client-side hotkeys.

Client behavior: 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. Client may provide a way for the user to see a list of currently-defined hotkeys, and a way to notify the user when a hotkey was activated.

Server behavior: The server should send a definition of each hotkey (SetTextHotKey packet) once per connection.

SetTextHotKey packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x15 (21)	Label	string	Copy	Readable name of the hotkey
	Action	string	/Copy	Text to type in
	KeyCode	int	113	LWJGL keycode of the key
	KeyMods	byte	0	Key modifier flags, may be combined: 0 = None 1 = Ctrl 2 = Shift 4 = Alt
Total Size:	134 bytes

ExtPlayerList

Motivation: Provides more flexibility in naming of players and loading of skins, autocompletion, and player tab-list display. Separates tracking of in-game entities (spawned player models) and names on the player list. ExtAddPlayerName/ExtRemovePlayerName packets take over managing the player names list (tab-list), and ExtAddEntity2/DespawnPlayer packets are used only to manage in-game entities.

Version 1 (Deprecated since 28 August 2014)

This version of the extension has been deprecated and replaced with version 2. See old extensions subpage.

Version 2

Version 2 of this extension replaces ExtAddEntity packet with ExtAddEntity2, allows using full URLs for SkinName, and clarifies interaction with ChangeModel extension.

Client Behavior

When ExtAddPlayerName packet is received for an unrecognized NameID, a new name must be added to the player-name list. When receiving ExtAddPlayerName packet for an already-listed NameID, client must update its ListName, GroupName, and GroupRank. Player-name list must persist when client changes worlds/maps.

When an ExtAddEntity2 packet is received, it must be treated as the SpawnPlayer packet. A player model must be spawned in-game at the given location, with InGameName text drawn above it. Skin should be loaded using the given SkinName for a player name. When client receives ExtAddEntity2 packet for an already-spawned player, a duplicate entity must not be spawned and existing entity's position must not be changed. Instead their InGameName and SkinName must be updated. If a negative EntityID is given for ExtAddEntity2, client must update player's own spawn point, InGameName, and SkinName. The client must ignore regular SpawnPlayer packets, if any are received.

Player entity's skin should be loaded using the given SkinName. If SkinName appears to be a player name, skin should be downloaded from the default skin server. If SkinName appears to be a full URL to a PNG image (starts with http:// or https:// and ends with .png) then skin should be downloaded from that URL. If image is correctly sized/proportioned to use as a skin for the current model, it should be used. If a blank or unrecognized value is given for SkinName, or if given image could not be downloaded or used, then the default skin should be used.

Names on the player-name list should be grouped by GroupName in the player-name list. Names within a GroupName should be sorted by GroupRank (in ascending order). Names with the same GroupName and GroupRank should be sorted alphabetically by ListName. Color codes may be either drawn or stripped from ListName, GroupName, and InGameName.

When a standard DespawnPlayer packet is received for a recognized EntityID, player model must be removed from a world. When ExtRemovePlayerName packet is received for a recognized NameID, their name must be removed from player-name list. Packets with out-of-range or unrecognized NameIDs must be ignored.

In-game entities must never be affected by ExtAddPlayerName or ExtRemovePlayerName packets. Player name list must never be affected by ExtAddEntity2 or DespawnPlayer packets.

Server Behavior

Event	Packet to send
Player connects to server	ExtAddPlayerName
Player enters map	ExtAddEntity2
Player leaves map	DespawnPlayer
Player disconnects from server	ExtRemovePlayerName

Unique NameID between 0 and 255 should be assigned to every online player. When a new player connects to the server, ExtAddPlayerName must be sent. GroupName and GroupRank can be used in any way, for example to group players by map/world or rank/class/faction. Server must use ExtAddEntity2 in place of standard SpawnPlayer packet. Server should re-send ExtAddPlayerName packet, using the identical NameID, when player's ListName, GroupName, or GroupRank change. Server must reliably send an ExtRemovePlayerName packet when the player disconnects. Color codes are permitted in ListName, GroupName, and InGameName.

ExtAddPlayerName Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x16 (22)	NameID	short	5	Between 0 and 255
	PlayerName	string	Notch	Player name used for autocompletion. May be left empty (to exclude from autocompletion).
	ListName	string	&c[Op]Notch	Name displayed in the in-game list.
	GroupName	string	Staff	May be left blank.
	GroupRank	byte	0	Rank of a player within the group. Lower-number ranks are listed before higher-number ranks.
Total Size:	196 bytes

ExtAddEntity2 Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x21 (33)	EntityID	byte	5	Between 0 and 127
	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.
	SpawnX	short	1	X coordinate (32 units per block) of entity's spawn location.
	SpawnY	short	2	Y coordinate (32 units per block) of entity's spawn location.
	SpawnZ	short	3	Z coordinate (32 units per block) of entity's spawn location.
	SpawnYaw	byte	4	Orientation (left-right) at the entity's spawn location.
	SpawnPitch	byte	5	Orientation (up-down) at the entity's spawn location.
Total Size:	138 bytes

ExtRemovePlayerName Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x18 (24)	NameID	short	5	Between 0 and 255 Matches NameID of the ExtAddPlayerName packet
Total Size:	3 bytes

EnvColors

This extension allows server to alter some of the colors used by the client in environment rendering.

Motivation: To allow the server to give worlds/maps a unique feel: time-of-day, weather/climate, lighting effect, etc.

Client behavior: Client must check for EnvSetColor packets right before LevelFinalize packet, and apply these changes before the map is displayed. Client must be able to read this packet at other times, but it is not required to then apply the changes immediately. If an unrecognized or unsupported Variable field is given, no action is needed. If an out-of-range color is given by the server (i.e. if any of Red, Green, or Blue is less than 0 or greater than 255), then the specified Variable should be reset to its default value.

Server behavior: Server should normally only use EnvSetColor packets right before the LevelFinalize packet. To reset a variable to its default value, the server should send a packet with value -1 for Red, Green, and Blue.

EnvSetColor Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x19 (25)	Variable	byte	1	Enumeration of environmental variables 0 = sky color 1 = cloud color 2 = fog color 3 = ambient light (blocks in shadow) color 4 = diffuse light (sunlight) color
	Red	short	25	Between 0 and 255
	Green	short	128	Between 0 and 255
	Blue	short	0	Between 0 and 255
Total Size:	8 bytes

SelectionCuboid

Motivation: Allows the server to highlight parts of a world. Applications include zoning, previewing draw commands, previewing undo commands.

Coordinates: {StartX,StartY,StartZ} are coordinates of the block inside the selection that is closest to the map origin. {EndX,EndY,EndZ} are coordinates of the block inside the selection that is furthest from the map origin. Therefore, the resulting selection has dimensions {EndX-StartX+1, EndY-StartY+1, EndZ-StartZ+1).

Client behavior: Client should be ready to receive MakeSelection packets any time after LevelFinalize packet. Upon receiving the packet, a translucent cuboid should appear in the world. The cuboid may feature a plain or "grid" texture. Selections that extend outside the map may be either ignored or clipped to fit. Selections with inconsistent coordinates (e.g. where StartX<EndX) may either be ignored or re-ordered. Out-of-range values for Red, Green, Blue, and Opacity should be clipped to fit the valid range. Supporting Opacity is optional: the client may opt to provide fixed opacity instead. When map changes (i.e. when LevelInitialize packet is received), all existing selections should be removed. RemoveSelection packets that refer to non-existent SelectionIDs should be ignored.

Server behavior: All given coordinates must be contained within the map. End coordinates should be higher or equal than start coordinates.

MakeSelection packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x1A (26)	SelectionID	byte	0	Numeric ID of the selection. Between 0 and 127.
	Label	string	SomeZone	Text label associated with the selection
	StartX	short	1	X coordinate of the starting point
	StartY	short	2	Y coordinate of the starting point
	StartZ	short	3	Z coordinate of the starting point
	EndX	short	5	X coordinate of the ending point
	EndY	short	6	Y coordinate of the ending point
	EndZ	short	7	Z coordinate of the ending point
	Red	short	255	Between 0 and 255.
	Green	short	34	Between 0 and 255.
	Blue	short	128	Between 0 and 255.
	Opacity	short	255	0 = fully transparent 255 = fully opaque
Total Size:	86 bytes

RemoveSelection packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x1B (27)	SelectionID	byte	0
Total Size:	2 bytes

BlockPermissions

This extension allows the server to instruct the player that certain block types are allowed/disallowed to be placed or deleted.

Motivation: To prevent players from inadvertently placing or removing prohibited block types (e.g. water, lava, grass, admincrete), before it even reaches the server.

Client behavior: Client should prevent placement of prohibited block types (by graying out or hiding blocks in block-selection screen, or any other effective means). Client should prevent player from deleting prohibited block types. Client must be ready to receive SetBlockPermission packet after map load (LevelFinalize packet). Permission changes should take effect as soon as packet is received. Admincrete (solid block) permissions set by SetBlockPermission must always override permission set by ServerIdentification and UpdateUserType packets. If BlockType is set to 0, given permissions should affect ALL block types at once, overriding permissions set by any earlier packets. Permissions must persist between map changes. Client may optionally warn the player attempting to place/delete prohibited blocks via sound effect, visual effect, chat message, etc.

Server behavior: Server may send SetBlockPermission packets any time after map load (LevelFinalize packet). Any valid block ID may be specified for BlockType, including custom blocks (if CustomBlocks extension is mutually supported). BlockType may be set to 0 to set permissions for ALL block types at once. Server must not assume that client is compliant/obedient, and server must still verify each SetBlock packet coming from the client. What to do with non-complying clients (kick or warn) is up to you.

SetBlockPermission packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x1C (28)	BlockType	byte	8	Block's numeric ID (anything between 1 and max defined block).
	AllowPlacement	byte	0	0 = Prohibited 1 or any other value = Allowed
	AllowDeletion	byte	0	0 = Prohibited 1 or any other value = Allowed
	Total Size:	4 bytes

ChangeModel

Allows changing appearance of player models in supporting clients.

Client Behavior

The client will receive an EntityID and a string value containing the model name. The client will then change the model of the player whose ID is the same as the received AddEntity, ExtAddEntity, or ExtAddEntity2 packet. The model name will be parsed by the model manager and the model changed in game. If the model does not exists in the model manager or is 0-length, change the model back to humanoid. Alternatively, you can send the client an int converted to a string which represents a valid Block ID. An EntityID of -1 (255 unsigned) indicates the player's own model. If model is set to humanoid, player-specific skin should be used. For all other models, default model-specific textures should be used.

Server Behavior

The server may send an EntityID and then a ModelName to the client for a desired entity. The model name must be chosen from the list below. To reset the model to default, server should set ModelName to humanoid or to an empty string.

Interaction with ExtPlayerList

If an ExtAddEntity2 packet is received for a non-humanoid entity and SkinName specifies a full URL, that image should be downloaded and used as a texture for the currently model. Receiving another ExtAddEntity2 packet should either update this texture (if SkinName is a full URL) or reset it back to default (if SkinName is anything else). Receiving another ChangeModel packet with a non-humanoid ModelName should reset texture back to model-specific default.

ChangeModel Packet

Packet ID	Field Name	Field Type	Example	Notes
0x1D (29)	EntityID	byte	5	Between 0 and 127.
	ModelName	string	spider	The name of the model to be used OR a valid Block ID as a string.
Total Size:	66 bytes

Available models

Client can render any or none of the below, but it is down to the client to handle what can and cannot be rendered if the packet is received

Model Name	Model String
Chicken	chicken
Block Model	A valid block ID as a string
Creeper	creeper
Crocodile	croc
Humanoid	humanoid (or an invalid model name)
Pig	pig
Printer	printer
Sheep	sheep
Skeleton Archer	skeleton
Spider	spider
Zombie	zombie

EnvMapAppearance

This extension allows the server to specify custom terrain textures, and tweak appearance of map edges.

Motivation: To provide more ways to customize map appearance, including functionality that's currently provided by World of Minecraft's scheme.

Client behavior: Client must be able to receive EnvSetMapAppearance packets any time during level load (after the first LevelDataChunk packet is received and until the LevelFinalize packet is received). If the TextureURL field is blank or if the given file could not be loaded for any reason, then the texture pack should be reset to client's default. If an unsupported block ID is given for SideBlock or EdgeBlock, it should be ignored. Given SideLevel value should be adjusted to fit between 0 and MapDepth, if necessary. Client should keep using these appearance parameters for future maps, unless specified otherwise by the server.

Server behavior: Server may send EnvSetMapAppearance packets after the last LevelDataChunk packet is sent for a level. Server should not use any custom block IDs unless the client declared the appropriate CustomBlocks support level. To reset the texture pack to default value, server should send an EnvMapAppearance packet with empty string for TextureURL. To reset other fields, server should simply use the default values (listed below).

Block type restrictions: Only solid blocks are allowed to be used for SideBlock and EdgeBlock. Sprites (Sapling, Dandelion, Rose, BrownMushroom, RedMushroom, Rope, Fire) partial-height blocks (Slab, CobblestoneSlab, Snow), and transparent blocks (Air, Leaves, Glass) cannot be used for those fields.

EnvSetMapAppearance packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x1E (30)	TextureURL	string	http://somesite.net/terrain.png	Skin's full URL. Must be an HTTP URL, ending with .png, and served with image/png mime type.
	SideBlock	byte	7	Block ID. Default value is 7 (Admincrete).
	EdgeBlock	byte	8	Block ID. Default value is 8 (Water).
	SideLevel	short	31	Elevation from bottom of the map, in blocks. Value should be between 0 and MapDepth. Default value is MapDepth/2.
Total Size:	69 bytes

EnvWeatherType

This extension allows the server to trigger special weather conditions (like rain and snow) on demand.

Motivation: To allow the server to give worlds/maps a unique feel with weather effects, e.g. for adventure maps.

Client behavior: Client must be able to receive EnvWeather packets at any time after the first LevelDataChunk packet is received, and apply these changes immediately. If an unrecognized or unsupported WeatherType field is given, no action is needed.

Server behavior: Server may send EnvSetColor packets to connected clients at any time after the first LevelDataChunk packet is sent.

EnvSetWeatherType Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x1F (31)	WeatherType	byte	1	Enumeration of weather types 0 = sunny 1 = raining 2 = snowing
	Total Size:	2 bytes

HackControl

This extension allows the server to control cheats/hacks in the client.

Motivation: To allow fine-grained control over cheats/hacks on multi-world servers.

Client behavior: Client must be able to receive HackControl packets at any time after the first LevelDataChunk packet is received, and apply these changes immediately. Unrecognized or unsupported values for any field should be ignored. Clients may approximate JumpHeight by rounding down to the nearest half-block (i.e. nearest multiple of 16), if needed. If a negative value is given for JumpHeight, client should reset jump height to its default setting (around 40 in vanilla Minecraft).

Server behavior: Server may send HackControl packets to connected clients at any time after the first LevelDataChunk packet is sent.

HackControl Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x20 (32)	Flying	byte	0	0 = Prevent player from flying 1 = Allow flying
	NoClip	byte	0	0 = Prevent player from no-clipping (passing through solid blocks) 1 = Allow no-clipping
	Speeding	byte	0	0 = Only allow normal walking speed. 1 = Allow moving at any speeds.
	SpawnControl	byte	1	0 = Prevent player from pressing [R] to respawn, or using [Enter] to change spawn. 1 = Allow player to respawn / change spawn.
	ThirdPersonView	byte	1	0 = Disallow third-person view (only first-person view allowed). 1 = Allow third-person view.
	JumpHeight	short	40	Maximum height, in terms of player movement units (1/32nds of a block), to which the player is allowed to jump. Negative value (e.g. -1) means that the client should use its default jump height.
Total Size:	8 bytes

MessageTypes

This extension adds new ways of presenting messages in the client.

Motivation: To enhance the display of announcements and status information, and to reduce chat clutter.

Client behavior: When this extension is mutually supported, the PlayerID field of the standard server-to-client Message packet should be treated as a MessageType code. Unrecognized or unsupported codes may be ignored (in which case the message should be presented as a regular chat message). When connected to non-supporting servers, this field should be ignored.

Server behavior: Server may use the PlayerID field of the Message packet to set a MessageType. For non-supporting clients, this field should always be set to 0.

MessageType	Meaning	Suggested Implementation
0	Chat	Normal message, shown in the chat area.
1	Status1	Shown persistently in the top-right corner of the screen, in regular font.
2	Status2	Shown persistently just below Status1
3	Status3	Shown persistently just below Status2
11	BottomRight1	Shown persistently in the bottom-right corner of the screen, in regular font.
12	BottomRight2	Shown persistently just above BottomRight1
13	BottomRight3	Shown persistently just above BottomRight2
100	Announcement	Pops up in larger font near the top-center of the screen. Fades out after a few seconds.

This extension does not define any new packets.

PlayerClick

This extension lets the server receive details of every mouse click a player makes, including targeting information.

Motivation: To allow for the creation of more interactive maps: interactive buttons, cannons, ability to "tag" or "shoot" other players, ability to drag-to-draw at a distance, etc.

Client Behavior: When this extension is mutually supported, a PlayerClicked packet should be sent to the server, as soon as reasonably possible, every time the user presses or releases a mouse button. Mouse interactions that happen while player is interacting with chat, with GUI menus, or with any other application must NOT be sent. A matching "released" event must be sent for every "pressed" event.

High resolution up-to-date Yaw and Pitch for the player should be transmitted with every packet. If player is currently targeting another entity/player TargetEntityID fields should be set. If player is currently targeting a block (within permitted ClickDistance) TargetBlockX, TargetBlockY, TargetBlockZ, and TargetBlockFace should be set. An out-of-range value should be used for these fields when they are not in use. These packets should not be sent during level transitions (after receiving LevelInitialize and until receiving LevelFinalize packets).

Server Behavior: Server must be ready to accept any number of PlayerClicked packets at any time after level sending has completed. What to do with this information is entirely up to the server.

PlayerClicked packet

Client to Server

Packet ID	Field Name	Field Type	Example	Notes
0x22 (34)	Button	byte	0	0 = Left click, 1 = Right Click, 2 = Middle click
	Action	byte	0	0 = Pressed, 1 = Released
	Yaw	short	0	Yaw (heading) of the player. Same function as Yaw field of PositionAndOrientation packet, but with more precision.
	Pitch	short	0	Pitch (head angle) of the player. Same function as Pitch field of PositionAndOrientation packet, but with more precision.
	TargetEntityID	byte	0	ID of an entity currently targeted by the player, between 0 and 127. Any out-of-range value means "no entity is targeted."
	TargetBlockX	short	64	X coordinate of a targeted block, between 0 and 2048. Any out-of-range value for any of the coordinates means "no block is targeted."
	TargetBlockY	short	64	Y coordinate of a targeted block, between 0 and 2048.
	TargetBlockZ	short	64	Z coordinate of a targeted block, between 0 and 2048.
	TargetBlockFace	byte	1	Index of the block face that the player is targeting. Block faces are numbered as follows: 0. Away from X-origin 1. Towards X-origin 2. Away from Y-origin (up) 3. Towards Y-origin (down) 4. Away from Z-origin 5. Towards Z-origin Any out-of-range value for TargetBlockFace mean "no block is targeted."
Total Size:	15 bytes

LongerMessages

This extension allows clients to accept messages longer than 64 characters, and send them to the server in parts.

Motivation: To allow players to write longer messages!

Client Behavior: When a regular-length message is sent, a single Message packet is sent, with the unused PlayerID (aka MessageType) byte set to 0. When an extra-long message is sent, multiple Message packets are sent. All packets except the last one should have the unused byte set to 1, meaning "this message continues in the next Message packet".

Server Behavior: Server should check the unused PlayerID (aka MessageType) byte of every incoming Message packet. If the value is non-zero, it should treat it as a partial message. Partial messages should be stored and prepended to the next message received from this player, until a message with a 0 byte is received.

No new packets are defined, but treatment of Message packet is altered.

FullCP437

This extension allows players to send and receive chat with all characters in code page 437, rather than just the 0 to 127 characters.

Motivation: To allow players to use more characters for different purposes.

Client Behavior: When a message packet is received, clients must treat all characters in that message as unsigned bytes that represent indices into code page 437. Clients may optionally provide a way of directly entering code page 437 characters into chat.

Server Behavior: Servers must support being able to send and receive chat message packets which include unsigned byte values ranging from 0 to 255. Servers must also provide a fallback mechanism for converting unsigned bytes in the 128-255 range into the 0 to 127 range for clients that do not support this extension. (e.g converting all unsigned bytes in the 128 to 255 range to a '?' symbol)

Note: Ideally, servers should also provide some method of escaping that allows users to enter a token that is then converted into a code page 437 index, but that is not required to claim support for this extension.

As an example of an escaping mechanism, when "{infinity}" is entered by the player, it becomes a single unsigned byte with a value of '236' after being processed by the server. When the chat message packet is later received by the client, it renders the unsigned byte with the value of '236' as ∞.

BlockDefinitions

Proposed by Hetal, with contributions by fragmer.

This extension allows servers to define new block types with custom IDs and appearance, as well as the ability to redefine the properties of standard block types.

Motivation: To allow defining block types without client modifications. To make full use of custom textures.

Client Behavior: This extension depends on EnvMapAppearance extension, and must only be enabled if BOTH extensions are mutually supported. Client should be able receive any number of DefineBlock and RemoveBlockDefinition packets during the level loading process (after receiving LevelInitialize and until receiving LevelFinalize packets). However, dependent on client, this can be sent after level loading (Some clients might just find it more extensible than others). Client must keep track of all defined blocks between level changes or until they are individually removed with RemoveBlockDefinition.

Client must be able to accept and render blocks as specified in DefineBlock packets. Clients must accept any valid definitions with BlockID between 1 and 254, inclusive. If a new definition is accepted for an already-defined BlockID, the old definition should be replaced. If RemoveBlockDefinition is received for a non-defined BlockID, it should be ignored.

Texturing: New textures are defined by altering terrain.png and specifying it with EnvSetMapAppearance packet. DefineBlock fields TopTextureID, SideTextureID, and BottomTextureID refer to a tile number within terrain.png. Tiles are numbered 0 (top-left corner) through 255 (bottom-right corner), increasing left-to-right within each row. Refer to this image for a list of standard tiles, and a graphical list of tile indices.

Server Behavior: Server may send any number of DefineBlock packets when client expects it. Server should only use IDs of currently-defined blocks in block data sent in LevelDataChunk packets, SetBlock packets, or any other packets that expect block IDs.

Compatibility: For non-supporting clients, the server must convert custom block ids into fallback block ids (e.g. 'lamp' fallbacks to lava) that lie within the client's set of supported blocks. (TODO: Define how the client should store map files with block definitions. Maybe integrate this with ClassicWorld definition?) 0 is not allowed to be redefined, nor is 255, as 255 is typically used by servers to indicate 'outside map bounds'. Note that sending a RemoveBlockDefinition packet for a standard tile id is not defined, and may lead to undesired consequences.

DefineBlock Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
35	BlockID	byte	128	Between 1 and 254 (unsigned).
	Name	String	Cactus	Short, unique, player-friendly name for the block.
	Solidity	byte	1	Specified collision mode for this block type. Blocks may be walk-through (0), swim-through (1), or solid (2). Walk-through blocks do not collide with the player, and do not interfere with jumping. Players fall through these blocks. Swim-through blocks allow the player to descend or ascend slowly (at WalkSpeed), as if they were swimming. Solid blocks collide with the player. Players may walk on these.
	MovementSpeed	byte	1	Player movement speed modifier, defined relative to regular walk speed. Ratio to normal speed is computed with this formula: y = 2(x-128)/64 Thus a value of 0 translates to 0.25 ratio (25% speed), 128 to 1.00 ratio (100% speed), and 255 to 3.96 (396% speed). If Solidity is set to 0 or 1, speed is modified while player is inside (i.e. colliding with) the block. If Solidity is set to 2, speed is modified while player is standing on the block. If player is affected by several tiles, the lowest of all modifiers is used.
	TopTextureID	byte	1	Refers to a terrain.png tile. Between 0 and 255 (unsigned). Can be same as other texture IDs.
	SideTextureID	byte	2	Refers to a terrain.png tile. Between 0 and 255 (unsigned). Can be same as other texture IDs. This is the texture ID used for drawing sprite blocks (Shape is 0).
	BottomTextureID	byte	3	Refers to a terrain.png tile. Between 0 and 255 (unsigned). Can be same as other texture IDs.
	TransmitsLight	byte	0	Whether this block allows the sunlight to go through, for level-lighting purposes.
	WalkSound	byte	0	ID of a sound that client may play when player "steps" on, "walks" through, or "swims" through this tile. 0 = no sound, 1 = wood, 2 = gravel, 3 = grass, 4 = stone, 5 = metal, 6 = glass, 7 = wool, 8 = sand, 9 = snow.
	FullBright	byte	0	Whether or not the block/liquid is affected by shadows (such as lava). 0 = Affected (darkened when in shadow) 1 = Not affected (always bright).
	Shape	byte	0	Shape of the block model: Sprite = 0, Cube with height = 1-16 (e.g. slab would have a shape of '8', snow would have '2', dirt would have '16')
	BlockDraw	byte	0	Can be: 0 = fully opaque 1 = transparent (e.g. like glass) 2 = transparent but with no face culling of same neighbours (e.g. like leaves) 3 = translucent, where texture's alpha is blended (e.g. like ice or water) 4 = gas (e.g. like air)
	FogDensity	byte	0	Density of fog while client's camera is inside this block. Value of 0 means "do not change fog" -- level's default settings are used. Values between 1 and 255 are mapped to OpenGL's GL_FOG_DENSITY value for GL_EXP2 style fog with the following formula: GL_FOG_DENSITY = (x+1)/128 Thus value of 1 means density of ~0.0156, value of 127 means 1.0000, and 255 means 2.0000
	FogR	byte	255	Red component of the aforementioned fog. Only applies if FogDensity is above 0.
	FogG	byte	255	Green component of the aforementioned fog. Only applies if FogDensity is above 0.
	FogB	byte	255	Blue component of the aforementioned fog. Only applies if FogDensity is above 0.
Total Size:	80 bytes

RemoveBlockDefinition Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
36	BlockID	byte	128	Between 128 and 255
Total Size:	2 bytes

BlockDefinitionsExt

This extension allows servers to define new block types with custom IDs and appearance, as well as the ability to redefine the properties of standard block types.

Motivation: To allow defining custom block types that do not have minX/Y/Z values of 0, and maxX/Z values of 1.

Client Behavior: See the BlockDefinitions specification. Additionally, this extension depends on EnvMapAppearance extension and BlockDefinitions, and must only be enabled if BOTH extensions are mutually supported.

Texturing: See the BlockDefinitions specification. The MinX/Y/Z and MaxX/Y/Z fields also affect the location within the terrain tile for a given face that the block should sample texels from. (e.g. if minX and minZ was 8, and maxX and maxZ was 16, then for the terrain.png tile with pixels from (0, 0) to (16, 16), the top face would be drawn with the subset of pixels from (8, 8) to (16, 16))

Server Behavior: See the BlockDefinitions specification.

This packet is the same as DefineBlock packet, except the 'shape field' is replaced by 6 fields indicating the bounding box of the block in pixel coordinates. Note that this packet does not support sprites, you must send a regular DefineBlock packet for that.

DefineBlockExt Packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
37	Following fields from DefineBlock packet: BlockID, Name, Solidity, MovementSpeed, TopTextureID, SideTextureID, BottomTextureID, TransmitsLight, WalkSound, FullBright
	MinX	byte	0	Minimum X coordinate in pixels. Min allowed is 0, max allowed is 15.
	MinY	byte	0	Minimum Y coordinate in pixels. Min allowed is 0, max allowed is 15.
	MinZ	byte	0	Minimum Z coordinate in pixels. Min allowed is 0, max allowed is 15.
	MaxX	byte	16	Maximum X coordinate in pixels. Min allowed is 1, max allowed is 16.
	MaxY	byte	16	Maximum Y coordinate in pixels. Min allowed is 1, max allowed is 16.
	MaxZ	byte	16	Maximum Z coordinate in pixels. Min allowed is 1, max allowed is 16.
	Following fields from DefineBlock packet: BlockDraw, FogDensity, FogR, FogG, FogB
	Total Size:	85 bytes