Pre-release protocol

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This page documents the changes from the last stable Minecraft release (currently 1.15.2, protocol 578) to the current pre-release (currently 20w21a, protocol 718). Note that this page contains bleeding-edge information that may not be completely or correctly documented.

One who wishes to commandeer the merging of this into Protocol when an update is made must be sure to respect any changes that may have occurred to the respective packets there.

Contents

Data types

The UUID type is now also sometimes encoded by the Notchian server as an array of four 32-bit integers, from most significant to least significant (matching the new format used on entity NBT). This representation is identical to using 16 bytes, two 64-bit integers, or one 128-bit integers.

Packets

ID Packet name Documentation
Handshaking serverbound
0x00 Handshake Current Pre
Play clientbound
0x0F Chat Message (clientbound) Current Pre
0x22 Chunk Data Current Pre
0x26 Join Game Current Pre
0x3B Respawn Current Pre
0x4E 0x43 Spawn Position Current (unchanged)
0x43 0x44 Display Scoreboard Current (unchanged)
0x44 0x45 Entity Metadata Current (unchanged)
0x45 0x46 Attach Entity Current (unchanged)
0x46 0x47 Entity Velocity Current (unchanged)
0x47 0x48 Entity Equipment Current (unchanged)
0x48 0x49 Set Experience Current (unchanged)
0x49 0x4A Update Health Current (unchanged)
0x4A 0x4B Scoreboard Objective Current (unchanged)
0x4B 0x4C Set Passengers Current (unchanged)
0x4C 0x4D Teams Current (unchanged)
0x4D 0x4E Update Score Current (unchanged)
0x59 Entity Properties Current Pre
Play serverbound
0x0F Generate Structure Pre
0x0F 0x10 Keep Alive (serverbound) Current (unchanged)
0x10 0x11 Lock Difficulty Current (unchanged)
0x11 0x12 Player Position Current (unchanged)
0x12 0x13 Player Position And Rotation (serverbound) Current (unchanged)
0x13 0x14 Player Rotation Current (unchanged)
0x14 0x15 Player Movement Current (unchanged)
0x15 0x16 Vehicle Move (serverbound) Current (unchanged)
0x16 0x17 Steer Boat Current (unchanged)
0x17 0x18 Pick Item Current (unchanged)
0x18 0x19 Craft Recipe Request Current (unchanged)
0x19 0x1A Player Abilities (serverbound) Current (unchanged)
0x1A 0x1B Player Digging Current (unchanged)
0x1B 0x1C Entity Action Current (unchanged)
0x1C 0x1D Steer Vehicle Current (unchanged)
0x1D 0x1E Recipe Book Data Current (unchanged)
0x1E 0x1F Name Item Current (unchanged)
0x1F 0x20 Resource Pack Status Current (unchanged)
0x20 0x21 Advancement Tab Current (unchanged)
0x21 0x22 Select Trade Current (unchanged)
0x22 0x23 Set Beacon Effect Current (unchanged)
0x23 0x24 Held Item Change (serverbound) Current (unchanged)
0x24 0x25 Update Command Block Current (unchanged)
0x25 0x26 Update Command Block Minecart Current (unchanged)
0x26 0x27 Creative Inventory Action Current (unchanged)
0x27 0x28 Update Jigsaw Block Current Pre
0x28 0x29 Update Structure Block Current (unchanged)
0x29 0x2A Update Sign Current (unchanged)
0x2A 0x2B Animation (serverbound) Current (unchanged)
0x2B 0x2C Spectate Current (unchanged)
0x2C 0x2D Player Block Placement Current (unchanged)
0x2D 0x2E Use Item Current (unchanged)
Login clientbound
0x02 Login Success Current Pre

New/modified data types

No changes so far.

Entity Metadata

FishingHook

Extends Entity.

Index Type Meaning Default
7 VarInt Hooked entity id + 1, or 0 if there is no hooked entity 0
8 Boolean Is catchable False

Abstract Arrow

Extends Entity.

Abstract base class for Tipped Arrow (which is used for regular arrows as well as tipped ones) and Spectral Arrow.

Index Type Meaning Default
7 Byte Bit mask Meaning 0
0x01 Is critical
0x02 Is noclip (used by loyalty tridents when returning)
8 OptUUID Unused Empty
9 8 Byte Peircing level 0

Tipped Arrow

Extends Abstract Arrow.

Used for both tipped and regular arrows. If not tipped, then color is set to -1 and no tipped arrow particles are used.

Index Type Meaning Default
10 9 VarInt Color (-1 for no particles) -1

Spectral Arrow

Extends Abstract Arrow.

No additional metadata.

Trident

Extends Abstract Arrow.

Index Type Meaning Default
10 9 VarInt Loyalty level (enchantment) 0
11 10 Boolean Has enchantment glint False

Strider

Extends Animal.

Index Type Meaning Default
16 VarInt Total time to "boost" with warped fungus on a stick for 0
17 Boolean Should display name tag (true unless riding a vehicle or on or in a block tagged with strider_warm_blocks (default: lava)) false
18 Boolean Has saddle false

Hoglin

Extends Animal.

Index Type Meaning Default
16 Boolean Is immune to zombification false

Zoglin

Extends Monster.

Index Type Meaning Default
15 Boolean Is baby false

Piglin

Extends Monster.

Index Type Meaning Default
15 Boolean Is baby false
16 Boolean Is immune to zombification false
17 Boolean Is charging crossbow false

Zombie Pigman Zombified Piglin

Extends Zombie.

No additional metadata.

Block Actions

No changes so far.

Inventories

No changes so far.

Plugin Channels

No changes so far.

Play

Clientbound

Chat Message (clientbound)

Identifying the difference between Chat/System Message is important as it helps respect the user's chat visibility options. See processing chat for more info about these positions.

Warning.png Game info accepts json formatting but does not display it, although the deprecated §-based formatting works. This is not an issue when using the Title packet, so prefer that packet for displaying information in that slot. See MC-119145 for more information.

Packet ID State Bound To Field Name Field Type Notes
0x0F Play Client JSON Data Chat Limited to 32767 bytes
Position Byte 0: chat (chat box), 1: system message (chat box), 2: game info (above hotbar).
Sender UUID Used by the Notchian client for the disableChat launch option. Setting both longs to 0 will always display the message regardless of the setting.

Chunk Data

Main article: Chunk Format
See also: #Unload Chunk
Huh.png The following information needs to be added to this page:
How do biomes work now? The biome change happened at the same time as the seed change, but it's not clear how/if biomes could be computed given that it's not the actual seed... (/r/mojira discussion which notes that it seems to be some kind of interpolation, and 3D biomes are only used in the nether)

The server only sends skylight information for chunk pillars in the Overworld, it's up to the client to know in which dimenison the player is currently located. You can also infer this information from the primary bitmask and the amount of uncompressed bytes sent. This packet also sends all block entities in the chunk (though sending them is not required; it is still legal to send them with Update Block Entity later).

Packet ID State Bound To Field Name Field Type Notes
0x22 Play Client Chunk X Int Chunk coordinate (block coordinate divided by 16, rounded down)
Chunk Z Int Chunk coordinate (block coordinate divided by 16, rounded down)
Full chunk Boolean See Chunk Format
Primary Bit Mask VarInt Bitmask with bits set to 1 for every 16×16×16 chunk section whose data is included in Data. The least significant bit represents the chunk section at the bottom of the chunk column (from y=0 to y=15).
Heightmaps NBT Compound containing one long array named MOTION_BLOCKING, which is a heightmap for the highest solid block at each position in the chunk (as a compacted long array with 256 entries at 9 bits per entry totaling 36 longs). The Notchian server also adds a WORLD_SURFACE long array, the purpose of which is unknown, but it's not required for the chunk to be accepted.
Biomes Optional array of Integer 1024 biome IDs, ordered by x then z then y, in 4×4×4 blocks. Not present if full chunk is false.
Size VarInt Size of Data in bytes
Data Byte array See data structure in Chunk Format
Number of block entities VarInt Number of elements in the following array
Block entities Array of NBT Tag All block entities in the chunk. Use the x, y, and z tags in the NBT to determine their positions.

Note that the Notchian client requires an Update View Position packet when it crosses a chunk border, otherwise it'll only display render distance + 2 chunks around the chunk it spawned in.

The compacted array format has been adjusted so that individual entries no longer span across multiple longs, affecting the main data array and heightmaps.

Join Game

See Protocol Encryption for information on logging in.

Packet ID State Bound To Field Name Field Type Notes
0x26 Play Client Entity ID Int The player's Entity ID (EID)
Gamemode Unsigned Byte 0: Survival, 1: Creative, 2: Adventure, 3: Spectator. Bit 3 (0x8) is the hardcore flag.
Dimension Codec NBT Tag Compound This CompoundTag contains a ListTag named "dimension". The ListTag should contain CompoundTags with named StringTags "key" and "element". The full extent of these is still unknown, but the tag represents a dimension registry. See below for the vanilla default.
Dimension Int Enum Identifier -1: The Nether, 0: The Overworld, 1: The End Valid dimensions are defined per dimension registry sent before this
Hashed seed Long First 8 bytes of the SHA-256 hash of the world's seed.
Max Players Unsigned Byte Was once used by the client to draw the player list, but now is ignored
Level Type String Enum (16) default, flat, largeBiomes, amplified, customized, buffet, default_1_1
View Distance VarInt Render distance (2-32)
Reduced Debug Info Boolean If true, a Notchian client shows reduced information on the debug screen. For servers in development, this should almost always be false.
Enable respawn screen Boolean Set to false when the doImmediateRespawn gamerule is true
Is Debug Boolean True if the world is a debug mode world; debug mode worlds cannot be modified and have predefined blocks
Is Flat Boolean True if the world is a superflat world; flat worlds have different void fog and a horizon at y=0 instead of y=63

Example of Dimension codec CompoundTag sent by the server:

   ListTag (Type: CompoundTag) "dimension":
       CompoundTag: 
           StringTag "key" value: 'minecraft:overworld'
           StringTag "element" value: 'minecraft:overworld'
       CompoundTag: 
           StringTag "key" value: 'minecraft:the_nether'
           StringTag "element" value: 'minecraft:the_nether'
       CompoundTag: 
           StringTag "key" value: 'minecraft:the_end'
           StringTag "element" value: 'minecraft:the_end'

Respawn

To change the player's dimension (overworld/nether/end), send them a respawn packet with the appropriate dimension, followed by prechunks/chunks for the new dimension, and finally a position and look packet. You do not need to unload chunks, the client will do it automatically.

Packet ID State Bound To Field Name Field Type Notes
0x3B Play Client Dimension Int Enum Identifier -1: The Nether, 0: The Overworld, 1: The End Valid dimensions are defined per dimension registry sent in Join Game
Hashed seed Long First 8 bytes of the SHA-256 hash of the world's seed.
Gamemode Unsigned Byte 0: survival, 1: creative, 2: adventure, 3: spectator. The hardcore flag is not included
Level Type String (16) Same as Join Game
Is Debug Boolean True if the world is a debug mode world; debug mode worlds cannot be modified and have predefined blocks
Is Flat Boolean True if the world is a superflat world; flat worlds have different void fog and a horizon at y=0 instead of y=63
Copy metadata Boolean If false, metadata is reset on the respawned player entity. Set to true for dimension changes (including the dimension change triggered by sending client status perform respawn to exit the end poem/credits), and false for normal respawns.

Warning.png Avoid changing player's dimension to same dimension they were already in unless they are dead. If you change the dimension to one they are already in, weird bugs can occur, such as the player being unable to attack other players in new world (until they die and respawn).

If you must respawn a player in the same dimension without killing them, send two respawn packets, one to a different world and then another to the world you want. You do not need to complete the first respawn; it only matters that you send two packets.

Entity Properties

Sets attributes on the given entity.

Packet ID State Bound To Field Name Field Type Notes
0x59 Play Client Entity ID VarInt
Number Of Properties Int Number of elements in the following array
Property Key Array String (64) Identifier See below
Value Double See below
Number Of Modifiers VarInt Number of elements in the following array
Modifiers Array of Modifier Data See Attribute#Modifiers. Modifier Data defined below.

Known Key values (see also Attribute#Modifiers):

Key Default Min Max Label
generic.maxHealth generic.max_health 20.0 0.0 1024.0 Max Health
generic.followRange generic.follow_range 32.0 0.0 2048.0 Follow Range
generic.knockbackResistance generic.knockback_resistance 0.0 0.0 1.0 Knockback Resistance
generic.movementSpeed generic.movement_speed 0.7 0.0 1024.0 Movement Speed
generic.attackDamage generic.attack_damage 2.0 0.0 2048.0 Attack Damage
generic.attackSpeed generic.attack_speed 4.0 0.0 1024.0 Attack Speed
generic.flyingSpeed generic.flying_speed 0.4 0.0 1024.0 Flying Speed
generic.armor 0.0 0.0 30.0 Armor
generic.armorToughness generic.armor_toughness 0.0 0.0 20.0 Armor Toughness
generic.attackKnockback generic.attack_knockback 0.0 0.0 5.0
generic.luck 0.0 -1024.0 1024.0 Luck
horse.jumpStrength horse.jump_strength 0.7 0.0 2.0 Jump Strength
zombie.spawnReinforcements zombie.spawn_reinforcements 0.0 0.0 1.0 Spawn Reinforcements Chance
generic.reachDistance 5.0 0.0 1024.0 Player Reach Distance (Forge only)
forge.swimSpeed 1.0 0.0 1024.0 Swimming Speed (Forge only)

Unknown attributes will cause a game crash (MC-150405) due to the default minimum being larger than the default value.

Modifier Data structure:

Field Name Field Type Notes
UUID UUID
Amount Double May be positive or negative
Operation Byte See below

The operation controls how the base value of the modifier is changed.

  • 0: Add/subtract amount
  • 1: Add/subtract amount percent of the current value
  • 2: Multiply by amount percent

All of the 0's are applied first, and then the 1's, and then the 2's.

Serverbound

Generate Structure

Sent when Generate is pressed on the Jigsaw Block interface.

Packet ID State Bound To Field Name Field Type Notes
0x0F Play Server Location Position Block entity location
Levels VarInt Value of the levels slider/max depth to generate

Update Jigsaw Block

Sent when Done is pressed on the Jigsaw Block interface.

Packet ID State Bound To Field Name Field Type Notes
0x27 0x28 Play Server Location Position Block entity location
Attachment type Identifier
Name Identifier
Target Identifier
Target pool Pool Identifier
Final state String "Turns into" on the GUI, final_state in NBT
Joint type String rollable if the attached piece can be rotated, else aligned

Handshaking

Clientbound

There are no clientbound packets in the Handshaking state, since the protocol immediately switches to a different state after the client sends the first packet.

Serverbound

Handshake

This causes the server to switch into the target state.

Packet ID State Bound To Field Name Field Type Notes
0x00 Handshaking Server Protocol Version VarInt See protocol version numbers (currently 578 718)
Server Address String Hostname or IP, e.g. localhost or 127.0.0.1, that was used to connect. The Notchian server does not use this information. Note that SRV records are a complete redirect, e.g. if _minecraft._tcp.example.com points to mc.example.org, users connecting to example.com will provide mc.example.org as server address in addition to connecting to it.
Server Port Unsigned Short Default is 25565. The Notchian server does not use this information.
Next State VarInt Enum 1 for status, 2 for login

Status

Clientbound

No changes so far.

Serverbound

No changes so far.

Login

Clientbound

Login Success

Packet ID State Bound To Field Name Field Type Notes
0x02 Login Client UUID String (36) UUID Unlike in other packets, this field contains the UUID as a string with hyphens.
Username String (16)

This packet switches the connection state to play.

Serverbound

No changes so far.