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Ludovic Marie 6774b2b3e7 Docs: "How Horizon is built", a page on who writes it and why it's safe to run

New players ask on Discord whether Horizon is legit and whether it is vibecoded. One plain page answers
both, every claim checked against the repository today: agents write most of the code, what checks it
instead of a human reading every line (harnesses, verify, parity, the contribution gate, the release
smoke test), signed checksums, the macOS and Windows signing status as it is (not notarized, not
code-signed), the mod sandbox, opt-in telemetry, no Mojang files and no other project's code.

On the website (docs nav under Start here, a card in the home page's community section, the footers) and
as docs/how-horizon-is-built.md. CONTRIBUTING.md gets the public "Only original code" rule the page links.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2026-10-25 00:47:49 +02:00

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Contributing to Horizon

Thanks for helping. Horizon is Minecraft: Java Edition 26.3 rewritten in Rust, and the one rule that shapes everything is that it plays exactly like vanilla. This page is the short version of how we work; questions are welcome on Discord.

This repository holds the game, the launcher and the website, plus the tools, tests and docs that build, check and release them. Anything else (videos, Discord and other community material) lives elsewhere.

Build it

You need Rust (stable), Python 3.10+, JDK 25, and on Linux the ALSA and udev headers (libasound2-dev libudev-dev). Horizon never downloads the game: it prepares its resources from your own copy of Minecraft 26.3, as described in Build from source.

cargo run --release -p horizon-prepare -- --prepare --server-jar ~/Downloads/server.jar
cargo run --release -p horizon-client

The launcher is its own Cargo workspace in launcher/ (cd launcher && cargo run; on NixOS, nix-shell first).

Vanilla first

Wherever vanilla has a feature, vanilla is the specification, down to the order of random calls. "Parity" here means Horizon gives the same results as the pinned 26.3 client and server for the same inputs, checked by a harness: a tools/test_*_parity.py script that runs a small Java oracle against the real game and a Rust probe against Horizon, and compares their outputs. There are about 290 of them.

Read the vanilla code before writing Horizon's. tools/rea_vanilla.py queries the pinned game bytecode: python3 tools/rea_vanilla.py method net.minecraft.world.level.block.DispenserBlock dispenseFrom. Say in your pull request which vanilla classes and methods you followed.
Gameplay changes come with a harness, new or extended, so the next change can't silently break them. The parity workflow shows how harnesses run; copy a nearby one.
Horizon's own features (the launcher, the new menus) follow the design system and are off by default wherever they would change the vanilla look.
Everything players run is Rust. Python and Java are fine for dev tools and harness drivers in tools/, never for player-facing code.

Never commit game files, decompiled source or prepared resources (.local/): tools/check_public_tree.py rejects them (git config core.hooksPath .githooks runs it before every commit).

Only original code

Horizon's code is written from Minecraft 26.3 itself and from public documentation, never from another mod, loader or renderer. Don't read, copy, translate or paraphrase the source of Iris, OptiFine, Sodium or any other project, and don't send code an AI tool wrote from it. Their user documentation and file formats are fine to read, and their packs and mods are fine to run and observe. Shader packs were removed on 24 October 2026 (#1561) because their code had been written from another project's source; they come back only as a clean-room rewrite from a written description of the pack format.

Before asking for review

Run the self-check and paste the block it prints into your pull request:

python3 tools/pr_ready.py --base upstream/main   # or origin/main if you work on the main repository

It looks at what your branch changes and runs what matters: rustfmt, clippy with warnings denied, the tests of the crates you touched (and the launcher's), generated files, Cargo.lock, the Python tests, and the vanilla harnesses your change can affect. Without the vanilla data on your machine it lists those harnesses instead; CI runs them. --quick skips the harnesses. It exits non-zero when something fails.

For the game workspace, Clippy checks all targets of the touched crates and their transitive workspace dependents, including examples. The dependency scope includes optional, platform, build and development dependencies. Tests run library, binary, integration and doc tests without building unrelated example executables. Shared Cargo configuration and toolchain changes select the whole workspace; CI also keeps its workspace-wide checks.

Pull requests

Small. One change per pull request, under about 500 lines of code where you can; split bigger work into pieces that each make sense alone. Small pull requests get reviewed and merged the same day.
Fill in the template: what and why, the vanilla reference, the pr_ready block, screenshots for anything you can see.
Tick "Allow edits by maintainers". When something small is left (a lint, a rebase, a nit) we push the fixup ourselves instead of sending it back to you; only design questions come back.
Labels (we set them if you can't): feature when players get something new (it appears under "New" in the release notes), skip-release when players see no change (tooling, docs, tests), nothing for a fix. release-now is for crash fixes that shouldn't wait. pr_ready suggests one.

Every merge to main is released automatically (release flow), so main always stays playable. How agents, checks, releases and signed updates keep this safe: security.

Parity checks on forks

The verify check (formatting, lints, tests) runs on every pull request. The parity check runs the affected harnesses on a maintainer's machine, which holds the game files, so it never runs a fork's code on its own: a maintainer reads your change, then adds the run-parity label, which runs it once on your current commit. After new pushes, we add it again.

Talk to us

Discord for questions and ideas, issues for bugs. If you want to take something on, say so in its issue so nobody else starts the same work.

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