horizon-airdef/launcher

Horizon launcher

A small desktop app that installs Horizon builds, prepares them from your own copy of Minecraft, and starts the client or a dedicated server. Built with GPUI, Zed's UI framework; it has its own Cargo workspace and is not part of the game's builds.

The opening, then Play (clip):

What it does

• Versions: lists the published releases of ludovic111/horizon, installs the build for this platform into its own folder, and removes it again.
• Play: prepares resources as PLAY.txt describes (your launcher's 26.3 install, the 26.3 server jar from that version's own json, and Java 25) and copies its sounds from your launcher's asset store (horizon-prepare sounds; a launcher install without them only leaves the game silent), then starts the client, or a server (--dedicated --nogui) in a folder you choose, with a live console. Preparing is horizon-prepare: linked into the launcher, used in-process when a version ships the same preparer sources, else that version's own horizon-prepare; versions released before it still prepare with their Python script and need Python 3.10+. Before importing, preparation reuses checksum-verified game jars and libraries from the current or another installed Horizon version, so the original Minecraft install is only required when no valid imported copy exists.
• Modpacks: import a Java modpack (a Modrinth .mrpack, a CurseForge zip, or a Prism, MultiMC or CurseForge instance folder) and see its coverage report: per mod, ported, partial, not yet or not needed, with the reason (docs/modpacks.md). Port gets the jars of what this computer can port today (content mods with data and assets only) the player's own way: copied from a folder they have, or downloaded from the pack's own links or the same file on Modrinth's CDN, hash checked; files only CurseForge has need its API key, so they come from your installed instance. It carries them over for the selected version, and Play starts the game as the pack's own profile (ports/<pack>/home/ in the game's home: its own saves and options) with those mods. Nothing is uploaded or shared: mod jars and ports stay on this computer.
• Patch notes: each release's notes, newest first, with a link to GitHub Sponsors. Quiet "Discord" and "Website" links sit in the footer.

Data lives in the platform's data directory (~/.local/share/horizon-launcher on Linux, ~/Library/Application Support/horizon-launcher on macOS, %APPDATA%\horizon-launcher on Windows): versions/<tag>/, server-jars/, server/ and settings.json.

• Settings: install new Horizon versions automatically (on unless you turn it off; the selected version only moves to the new one if you were on the newest), the opening animation, the game's menu style, the game's extra command-line arguments, and the launcher's own version and update state.
• Game arguments: extra arguments added to the game's command line (for example --server-thread), one line where quotes group a word with spaces. They are appended after the launcher's own arguments, kept as game_args in settings.json, and apply the next time the game starts. horizon-launcher game-args "<line>" sets them too, and play --arg <ARG> passes one for a single run.
• Menu style: Horizon's own menus or Classic (vanilla's screens). Classic has no way back inside the game, so the launcher is where you return to Horizon's. It is the menuStyle: line of horizon.txt in the game's home folder (HORIZON_HOME, else horizon in the data directory), which the game reads when it starts.
• Links: the Discord invite, the Sponsors page and the website come from launcher/links.json on main ({"discord": "...", "sponsor": "...", "website": "..."}), so they change without a launcher release. Offline, the launcher uses the same values built in; an empty address hides its button.

Updates

The launcher looks for a newer Horizon release when it starts and every four hours while it is open, and shows a small "available, Install" line on Play.

It also updates itself from GitHub releases tagged launcher-vX.Y.Z (Horizon's own are vYYYY.MM.DD; neither is ever taken for the other). It downloads the new build, replaces its own binary, and offers "Restart" in the footer. Development builds and HORIZON_LAUNCHER_NO_SELF_UPDATE=1 never replace themselves.

Linux packagers can set HORIZON_LAUNCHER_DESKTOP_EXEC to a wrapper's absolute path. Menu entries created by Settings or desktop-entry then start that wrapper, which can set HORIZON_LAUNCHER_NO_SELF_UPDATE=1 to keep launcher updates with the package manager.

Launcher releases are automatic (docs/release-flow.md in the repository): when main changes the launcher, horizon-prepare or horizon-theme, the launcher workflow builds Linux, macOS and Windows and publishes launcher-vX.Y.Z (the next patch, or the next minor after a pull request labelled feature) with horizon-launcher-linux-x86_64.tar.gz, horizon-launcher-macos-arm64.zip and horizon-launcher-windows-x64.zip, never marked latest. The version is built in from the tag (HORIZON_LAUNCHER_VERSION); Cargo.toml's version is only what local builds report.

Command line

The same binary is a command-line tool; without a subcommand it opens the window.

horizon-launcher versions [--installed]       releases on GitHub, or what is installed (* = selected)
horizon-launcher install <tag|latest>
horizon-launcher remove <tag>
horizon-launcher prepare [--minecraft <dir>] [--version <tag>]
horizon-launcher play [--version <tag>] [--arg <ARG>]...   start the client; --arg passes an extra argument through (repeatable)
horizon-launcher server [--dir <dir>] [--version <tag>] [--accept-eula]
horizon-launcher notes [<tag>]                  patch notes as plain text
horizon-launcher links                          the Discord, sponsor and website links
horizon-launcher menu-style [horizon|classic]   show or set the game's menu style
horizon-launcher game-args [<line>]             show or set the extra arguments the game starts with
horizon-launcher update [--check]               Horizon and the launcher
horizon-launcher logs [--lines N] [--crash]     the end of the game's log, or its newest crash report
horizon-launcher account [list]                 the Microsoft accounts signed in (* = plays)
horizon-launcher account sign-in                sign in in your browser with a code
horizon-launcher account sign-out <name>
horizon-launcher account select <name>          play as this account
horizon-launcher account offline                play without an account
horizon-launcher modpack [list]                 the imported packs and their coverage
horizon-launcher modpack import <pack>          a .mrpack, CurseForge zip or instance folder
horizon-launcher modpack port <pack> [--from <dir>] [--version <tag>]   get the jars and carry them over
horizon-launcher modpack play <pack> [--version <tag>]                 play it as its own profile
horizon-launcher modpack remove <pack>
horizon-launcher servers [list]                 the servers hosted from the launcher, and whether they run
horizon-launcher servers create [--name N] [--seed S | --copy-world DIR] [--mode M] [--difficulty D]
                 [--max-players N] [--open] [--check-accounts] [--player NAME]... --accept-eula
horizon-launcher servers set-up [--version <tag>]   copy a prepared version's programs and game files for servers
horizon-launcher servers run <server> -- <horizon-server command>   for example: servers run "My server" -- start
horizon-launcher servers forget <server>        take it off the list (its folder stays)
horizon-launcher benchmark [--repeats N]        measure the launcher (see Performance log)
horizon-launcher data [status]                  what data collection is allowed, the install ID, what waits
horizon-launcher data allow|deny <kinds|all>    performance, crashes, bugs, benchmarks
horizon-launcher data show                      exactly what would be sent
horizon-launcher data delete                    delete what was sent and what waits; new install ID
horizon-launcher data bug "<text>" [--log]      a bug report, printed for you to confirm first
horizon-launcher data benchmark <report.json>   upload a benchmark report, printed first

--json prints JSON where there is something to report (versions, install, notes, next, links, menu-style, game-args, update --check, logs, account, benchmark, data, modpack). Errors go to stderr with exit code 1. server runs in the terminal, so lines you type reach its console. HORIZON_LAUNCHER_HOME replaces the data folder (for scripts and tests).

AI agents (MCP)

horizon-launcher mcp is a Model Context Protocol server over stdio. To use it from Claude Code:

claude mcp add horizon -- horizon-launcher mcp

(with the full path to the binary if it is not on your PATH). Its tools: list_versions, install_version, remove_version, prepare_resources, launch_client, start_server, server_command, server_log, stop_server, patch_notes, links, menu_style, logs, check_updates, accounts, sign_in (returns the link and code for the player; the sign-in finishes in the background), sign_out, select_account, and for data collection data_collection, set_data_collection, data_preview, delete_my_data and report_bug (only with the player's say-so). A server started through MCP stops (cleanly, with stop) when the session ends. Starting one needs accept_eula, which an agent should only pass with the player's consent.

The same server also plays the game: launch_client starts it with a local control socket (world, creative and offscreen are optional), and every horizon-ctl tool of the selected version is offered as game_<name>: game_state, game_ui (every widget of the open screen), game_click, game_goto, game_break, game_inventory, game_screenshot (an image) and the rest of horizon-ctl's commands. The launcher runs that version's horizon-ctl mcp and passes the calls through; while no game runs, they answer with an error saying to call launch_client.

The window, the command line and the MCP server all call the same core, src/backend.rs.

Microsoft accounts

Settings → Accounts signs in a Microsoft account to play on online-mode servers under your own name and skin: the launcher shows a code, you enter it at microsoft.com/link, and the launcher checks that the account owns Minecraft: Java Edition. The Play page says who plays ("Playing as"); without an account the game plays offline, as before. Tokens live in the system keychain (Keychain, Credential Manager, or the Secret Service on Linux), never in a file, a log or the command line; accounts.json in the data folder keeps only names and UUIDs. Without a keychain you stay signed in until the launcher closes. When the game starts, it gets the session in a private file it deletes as soon as it has read it. Sign out removes the tokens.

Sign-in needs Horizon's own Azure app registration: its client ID is built in from HORIZON_MSA_CLIENT_ID (and HORIZON_MSA_CLIENT_ID at run time overrides it; empty = not configured). How it works: docs/microsoft-account.md.

Logs and crash reports

The game writes its log to logs/latest.log in the game folder (HORIZON_HOME, else ~/.local/share/horizon, ~/Library/Application Support/horizon or %APPDATA%\horizon), and archives the previous one at start as logs/<date>-<n>.log.gz, as Minecraft does. When it crashes it writes crash-reports/crash-<date>_<time>-client.txt beside them. When a game the launcher started crashes, the Play page says so, with buttons to open or copy the report, open the logs folder and report it on Discord; Settings has the logs folder too. A crash report leaves the machine only if you turn on crash reports (below).

Data collection

The first time it opens, the launcher asks whether you want to send performance statistics, crash reports, bug reports and benchmark uploads, each on its own switch and all off until you turn one on ("Send nothing" is one click). Settings → Data collection changes them any time, shows exactly what would be sent (the JSON), and deletes everything sent from your install. It asks again only when what is collected changes. The game obeys the same answer, kept in telemetry/consent.json in the game folder. What waits is sent when the launcher starts and closes. Every field, the caps and the format: docs/telemetry.md.

Performance log

Each session writes its own measurements to benchmarks/launcher-<time>-session.json in the launcher's data folder, in the benchmark report format: start to the first frame, the release list fetch, installs and updates (download and unpack), preparing, Play to the game's first frame, memory, UI frame times and the binary's size, with the machine's CPU, RAM and OS. Attach it to an issue about the launcher being slow. horizon-launcher benchmark measures on purpose: it starts the launcher --repeats times (5; the first start is the cold one, as far as the OS's file cache allows), switches pages for a few seconds in each, rests 3 seconds to measure idle CPU, fetches the release list 3 times, and writes one report there with means and 95% confidence intervals. Its window has to be on a screen that is awake. Session logs stay on your machine; a benchmark report is uploaded, dated to the day, only if you allowed benchmark uploads (data collection).

Building

cargo build --release

On NixOS, nix-shell in this folder provides the libraries GPUI links to (Wayland, X11, xkbcommon, Vulkan, fontconfig). macOS needs Xcode's command line tools. The launcher workflow builds all three platforms on every push to the launcher branch (or a launcher-* one), then installs the latest Horizon into a temporary folder and checks that the client starts (--help) and that the binaries import only system libraries (Windows).

cargo test --lib    # tag separation, version order, update selection, notes as text,
                    # the auto-install default, the links fallback, the menu style file

Geist and Geist Mono (SIL Open Font License, crates/horizon-theme/fonts/OFL-*.txt, shared with the game's menus) are bundled so the type is the same on every platform.

Screenshots