NBT

The Named Binary Tag (NBT) file format is an extremely simple, albeit annoying (did we really need yet another format?)^{[See Discussion]} structured binary format used by the Minecraft game for a variety of things. Due to this, several third-party utilities now also utilize the format. You may find example files at the bottom of this article.

Mojang has also released a reference implementation along with their Anvil conversion tool, available from https://mojang.com/2012/02/new-minecraft-map-format-anvil/

Current Uses

The NBT format is currently used in several places, chiefly:

In the Protocol as part of Slot Data
Multiplayer saved server list (servers.dat).
Player data (both single player and multiplayer, one file per player). This includes such things as inventory and location.
Saved worlds (both single player and multiplayer).
- World index file (level.dat) that contains general information (spawn point, time of day, etc...)
- Chunk data (see Region Files)

Unfortunately, the NBT files you can encounter as a developer will be stored in three different ways, just to make things interesting.

Uncompressed,
gzip'd,
zlib'd (aka DEFLATE with a few bytes extra)

Libraries

There are many, many libraries for manipulating NBT, written in several languages, and often several per language. For example,

C,
C#,
D,
Go,
Java,
Javascript,
PHP,
Python,
Ruby,
Rust,
Scala,
Kotlin (Work in progress),
You get the idea…

Unless you have specific goals or licence requirements, it is extremely recommended to go with one of the existing libraries.

Utilities

Almost every 3rd-party Minecraft application uses NBT on some level. There also exist several dedicated NBT editors, which will likely be useful to you if you are developing an NBT library of your own. These include:

NBTEdit (C#, Mono-capable), one of the very first NBT editors.
NEINedit (Obj-C), an OS X specific editor.
nbt2yaml (Python), provides command-line editing of NBT via the YAML format, as well as a fast and minimalist NBT parsing/rendering API.
nbted (Rust), provides command-line editing of NBT files via your $EDITOR

Specification

The NBT file format is extremely simple, and writing a library capable of reading/writing it is a simple affair. There are 11 datatypes supported by this format, and one type used to close compound tags. It is strongly advised to read this entire section or you may run into issues.

Type ID	Type Name	Payload Size (Bytes)	Description
0	TAG_End	0	Signifies the end of a TAG_Compound. It is only ever used inside a TAG_Compound, and is not named despite being in a TAG_Compound
1	TAG_Byte	1	A single signed byte
2	TAG_Short	2	A single signed, big endian 16 bit integer
3	TAG_Int	4	A single signed, big endian 32 bit integer
4	TAG_Long	8	A single signed, big endian 64 bit integer
5	TAG_Float	4	A single, big endian IEEE-754 single-precision floating point number
6	TAG_Double	8	A single, big endian IEEE-754 double-precision floating point number
7	TAG_Byte_Array	...	A length-prefixed array of signed bytes. The prefix is a signed integer (thus 4 bytes)
8	TAG_String	...	A length-prefixed UTF-8 string. The prefix is an unsigned short (thus 2 bytes) signifying the length of the string in bytes
9	TAG_List	...	A list of nameless tags, all of the same type. The list is prefixed with the `Type ID` of the items it contains (thus 1 byte), and the length of the list as a signed integer (a further 4 bytes). If the length of the list is 0 or negative, the type may be 0 (TAG_End) but otherwise it must be any other type. (The notchian implementation uses TAG_End in that situation, but another reference implementation by Mojang uses 1 instead; parsers should accept any type if the length is <= 0).
10	TAG_Compound	...	Effectively a list of a named tags. Order is not guaranteed.
11	TAG_Int_Array	...	A length-prefixed array of signed integers. The prefix is a signed integer (thus 4 bytes) and indicates the number of 4 byte integers.
12	TAG_Long_Array	...	A length-prefixed array of signed longs. The prefix is a signed integer (thus 4 bytes) and indicates the number of 8 byte longs.

There are a couple of simple things to remember:

The datatypes representing numbers are in big-endian in the PC version, but Pocket Version is in little-endian. Unless you're using Java, you will most likely have to swap it to little-endian. See the Wikipedia article on Endianness.
Every NBT file will always implicitly be inside a tag compound, and also begin with a TAG_Compound
The structure of a NBT file is defined by the TAG_List and TAG_Compound types, as such a tag itself will only contain the payload, but depending on what the tag is contained within may contain additional headers. I.e. if it's inside a Compound, then each tag will begin with the TAG_id, and then a string (the tag's name), and finally the payload. While in a list it will be only the payload, as there is no name and the tag type is given in the beginning of the list.

For example, here's the example layout of a TAG_Short on disk:

	Type ID	Length of Name	Name	Payload
Decoded	2	9	`shortTest`	`32767`
On Disk (in hex)	`02`	`00 09`	`73 68 6F 72 74 54 65 73 74`	`7F FF`

If this TAG_Short had been in a TAG_List, it would have been nothing more than the payload, since the type is implied and tags within the first level of a list are nameless.

Examples

There are two defacto example files used for testing your implementation (test.nbt & bigtest.nbt), originally provided by Markus. The example output provided below was generated using PyNBT's debug-nbt tool.

test.nbt

This first example is an uncompressed "Hello World" NBT example. Should you parse it correctly, you will get a structure similar to the following:

  TAG_Compound('hello world'): 1 entry
  {
    TAG_String('name'): 'Bananrama'
  }

Here is the example explained:

(The entire thing is implicitly inside a compound)	Type ID (first element in the implicit compound)	Length of name of the root compound	Name of the root compound	Type ID of first element in root compound	Length of name of first element in root	Name of first element	Length of string	String	Tag end (of root compound)
Decoded	Compound	11	hello world	String	4	name	9	Bananrama
On Disk (in hex)	`0a`	`00 0b`	`68 65 6c 6c 6f 20 77 6f 72 6c 64`	`08`	`00 04`	`6e 61 6d 65`	`00 09`	`42 61 6e 61 6e 72 61 6d 61`	`00`

bigtest.nbt

This second example is a gzip compressed test of every available tag. If your program can successfully parse this file, then you've done well. Note that the tags under TAG_List do not have a name, as mentioned above.

  TAG_Compound('Level'): 11 entries
  {
    TAG_Compound('nested compound test'): 2 entries
    {
      TAG_Compound('egg'): 2 entries
      {
        TAG_String('name'): 'Eggbert'
        TAG_Float('value'): 0.5
      }
      TAG_Compound('ham'): 2 entries
      {
        TAG_String('name'): 'Hampus'
        TAG_Float('value'): 0.75
      }
    }
    TAG_Int('intTest'): 2147483647
    TAG_Byte('byteTest'): 127
    TAG_String('stringTest'): 'HELLO WORLD THIS IS A TEST STRING \xc3\x85\xc3\x84\xc3\x96!'
    TAG_List('listTest (long)'): 5 entries
    {
      TAG_Long(None): 11
      TAG_Long(None): 12
      TAG_Long(None): 13
      TAG_Long(None): 14
      TAG_Long(None): 15
    }
    TAG_Double('doubleTest'): 0.49312871321823148
    TAG_Float('floatTest'): 0.49823147058486938
    TAG_Long('longTest'): 9223372036854775807L
    TAG_List('listTest (compound)'): 2 entries
    {
      TAG_Compound(None): 2 entries
      {
        TAG_Long('created-on'): 1264099775885L
        TAG_String('name'): 'Compound tag #0'
      }
      TAG_Compound(None): 2 entries
      {
        TAG_Long('created-on'): 1264099775885L
        TAG_String('name'): 'Compound tag #1'
      }
    }
    TAG_Byte_Array('byteArrayTest (the first 1000 values of (n*n*255+n*7)%100, starting with n=0 (0, 62, 34, 16, 8, ...))'): [1000 bytes]
    TAG_Short('shortTest'): 32767
  }

servers.dat

The servers.dat file contains a list of multiplayer servers you've added to the game. To mix things up a bit, this file will always be uncompressed. Below is an example of the structure seen in servers.dat.

  TAG_Compound(''): 1 entry
  {
    TAG_List('servers'): 2 entries
    {
      TAG_Compound(None): 3 entries
      {
        TAG_Byte('acceptTextures'): 1 (Automatically accept resourcepacks from this server)
        TAG_String('ip'): '199.167.132.229:25620'
        TAG_String('name'): 'Dainz1 - Creative'
        
      }
      TAG_Compound(None): 3 entries
      {
        TAG_String('icon'): 'iVBORw0KGgoAAAANUhEUgAAAEAAAABACA...' (The base64-encoded server icon. Trimmed here for the example's sake)
        TAG_String('ip'): '76.127.122.65:25565'
        TAG_String('name'): 'minstarmin4'
        
      }
    }
  }

level.dat

This final example is of a single player level.dat, which is compressed using gzip. Notice the player's inventory and general world details such as spawn position, world name, and the game seed.

  TAG_Compound(''): 1 entry
  {
    TAG_Compound('Data'): 17 entries
    {
      TAG_Byte('raining'): 0
      TAG_Long('RandomSeed'): 3142388825013346304L
      TAG_Int('SpawnX'): 0
      TAG_Int('SpawnZ'): 0
      TAG_Long('LastPlayed'): 1323133681772L
      TAG_Int('GameType'): 1
      TAG_Int('SpawnY'): 63
      TAG_Byte('MapFeatures'): 1
      TAG_Compound('Player'): 24 entries
      {
        TAG_Int('XpTotal'): 0
        TAG_Compound('abilities'): 4 entries
        {
          TAG_Byte('instabuild'): 1
          TAG_Byte('flying'): 1
          TAG_Byte('mayfly'): 1
          TAG_Byte('invulnerable'): 1
        }
        TAG_Int('XpLevel'): 0
        TAG_Int('Score'): 0
        TAG_Short('Health'): 20
        TAG_List('Inventory'): 13 entries
        {
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 1
            TAG_Byte('Slot'): 0
            TAG_Short('id'): 24
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 1
            TAG_Byte('Slot'): 1
            TAG_Short('id'): 25
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 1
            TAG_Byte('Slot'): 2
            TAG_Short('id'): 326
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 1
            TAG_Byte('Slot'): 3
            TAG_Short('id'): 29
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 10
            TAG_Byte('Slot'): 4
            TAG_Short('id'): 69
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 3
            TAG_Byte('Slot'): 5
            TAG_Short('id'): 33
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 43
            TAG_Byte('Slot'): 6
            TAG_Short('id'): 356
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 64
            TAG_Byte('Slot'): 7
            TAG_Short('id'): 331
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 20
            TAG_Byte('Slot'): 8
            TAG_Short('id'): 76
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 64
            TAG_Byte('Slot'): 9
            TAG_Short('id'): 331
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 1
            TAG_Byte('Slot'): 10
            TAG_Short('id'): 323
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 16
            TAG_Byte('Slot'): 11
            TAG_Short('id'): 331
            TAG_Short('Damage'): 0
          }
          TAG_Compound(None): 4 entries
          {
            TAG_Byte('Count'): 1
            TAG_Byte('Slot'): 12
            TAG_Short('id'): 110
            TAG_Short('Damage'): 0
          }
        }
        TAG_Short('HurtTime'): 0
        TAG_Short('Fire'): -20
        TAG_Float('foodExhaustionLevel'): 0.0
        TAG_Float('foodSaturationLevel'): 5.0
        TAG_Int('foodTickTimer'): 0
        TAG_Short('SleepTimer'): 0
        TAG_Short('DeathTime'): 0
        TAG_List('Rotation'): 2 entries
        {
          TAG_Float(None): 1151.9342041015625
          TAG_Float(None): 32.249679565429688
        }
        TAG_Float('XpP'): 0.0
        TAG_Float('FallDistance'): 0.0
        TAG_Short('Air'): 300
        TAG_List('Motion'): 3 entries
        {
          TAG_Double(None): -2.9778325794951344e-11
          TAG_Double(None): -0.078400001525878907
          TAG_Double(None): 1.1763942772801152e-11
        }
        TAG_Int('Dimension'): 0
        TAG_Byte('OnGround'): 1
        TAG_List('Pos'): 3 entries
        {
          TAG_Double(None): 256.87499499518492
          TAG_Double(None): 112.62000000476837
          TAG_Double(None): -34.578128612797634
        }
        TAG_Byte('Sleeping'): 0
        TAG_Short('AttackTime'): 0
        TAG_Int('foodLevel'): 20
      }
      TAG_Int('thunderTime'): 2724
      TAG_Int('version'): 19132
      TAG_Int('rainTime'): 5476
      TAG_Long('Time'): 128763
      TAG_Byte('thundering'): 1
      TAG_Byte('hardcore'): 0
      TAG_Long('SizeOnDisk'): 0
      TAG_String('LevelName'): 'Sandstone Test World'
    }
  }

Download

test.nbt/hello_world.nbt (uncompressed),
bigtest.nbt (gzip compressed)

NBT

Contents

Current Uses

Libraries

Utilities

Specification

Examples

test.nbt

bigtest.nbt

servers.dat

level.dat

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