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fastnbt project

fastnbt-shield fastnbt-version-shield fastnbt-docs-shield build-status-shield

fastsnbt-shield fastsnbt-version-shield fastsnbt-docs-shield

fastanvil-shield fastanvil-version-shield fastanvil-docs-shield

FastNBT is a serde serializer and deserializer for Minecraft: Java Edition's NBT format, including Value type and nbt! macro. For stringified NBT (sNBT) see FastSNBT.

FastAnvil allows rendering maps of worlds, and a Region for using the Region file format. Includes partial support for 1.12 worlds.

An in-browser Rust-to-WASM powered Minecraft map renderer demo is below. Supports 1.19 down to 1.15 inclusive.


Demo of Hermitcraft season 8 and more at

alt rendered map

The anvil binary from fastnbt-tools can render your world leveraging all of your CPU.


A bunch of examples can be found in fastnbt/examples, fastanvil/examples and tools/src. Some examples are recreated below.

Example: editing level.dat

The following edits the world spawn to 250, 200, 250 (probably not a good idea!). Full example in fastnbt/examples directory.

#[derive(Serialize, Deserialize)]
struct LevelDat {
    #[serde(rename = "Data")]
    data: Data,

#[derive(Serialize, Deserialize)]
#[serde(rename_all = "PascalCase")]
struct Data {
    spawn_x: i32,
    spawn_y: i32,
    spawn_z: i32,

    other: HashMap<String, Value>,

fn main() {
    let args: Vec<_> = std::env::args_os().collect();
    let file = std::fs::File::open(&args[1]).unwrap();
    let mut decoder = GzDecoder::new(file);
    let mut bytes = vec![];
    decoder.read_to_end(&mut bytes).unwrap();

    let mut leveldat: LevelDat = fastnbt::from_bytes(&bytes).unwrap(); = 250; = 200; = 250;

    let new_bytes = fastnbt::to_bytes(&leveldat).unwrap();
    let outfile = std::fs::File::create("level.dat").unwrap();
    let mut encoder = GzEncoder::new(outfile, Compression::fast());

Example: print player inventory

This example demonstrates printing out a players inventory and ender chest contents from the player dat files found in worlds. We

  • use serde's renaming attribute to have rustfmt conformant field names,
  • use lifetimes to save on string allocations, and
  • use the Value type to deserialize a field we don't specify the exact structure of.
#[derive(Deserialize, Debug)]
#[serde(rename_all = "PascalCase")]
struct PlayerDat<'a> {
    data_version: i32,

    inventory: Vec<InventorySlot<'a>>,
    ender_items: Vec<InventorySlot<'a>>,

#[derive(Deserialize, Debug)]
struct InventorySlot<'a> {
    id: &'a str,        // We avoid allocating a string here.
    tag: Option<Value>, // Also get the less structured properties of the object.

    // We need to rename fields a lot.
    #[serde(rename = "Count")]
    count: i8,

fn main() {
    let args: Vec<_> = std::env::args().skip(1).collect();
    let file = std::fs::File::open(args[0].clone()).unwrap();

    // Player dat files are compressed with GZip.
    let mut decoder = GzDecoder::new(file);
    let mut data = vec![];
    decoder.read_to_end(&mut data).unwrap();

    let player: Result<PlayerDat> = from_bytes(data.as_slice());

    println!("{:#?}", player);


For the libraries

fastnbt = "2"
fastanvil = "0.30"

For the anvil executable

cargo install fastnbt-tools