# 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](https://www.gpui.rs),
Zed's UI framework; it has its own Cargo workspace and is not part of the game's builds.

![Play](docs/play.png)

The opening, then Play ([clip](docs/opening.webm)):

![Opening](docs/opening-1.png)

## What it does

- **Versions**: lists the published releases of horizon-rust/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. 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.
- **Download and prepare**: for a signed-in account that owns the game (asked again first),
  the launcher downloads the pinned version from Mojang itself (piston-meta, piston-data,
  libraries.minecraft.net, resources.download.minecraft.net; never a mirror) into `minecraft/`
  in its data folder, in the official launcher's layout: the version file, the client jar, this
  system's libraries, the asset index and the assets. Every file must have the size and sha1
  Mojang lists. The Play page shows how far it is and has Cancel; files already whole are kept,
  so the next try goes on from there. **From my install** prepares from the player's own launcher
  instead, and an install that lacks the version falls back to the download. Without an owner,
  the Play page asks to sign in first. `horizon-launcher prepare --download`.
- **Mods**: search Modrinth (CurseForge support is coming soon) for resource
  packs and modpacks, each marked Works, Partly or Doesn't run in
  Horizon; Java mod jars are never offered. Install puts a pack and what it needs in
  `resourcepacks/` only after its SHA-512 matches, and a
  modpack goes to the Modpacks page; installed packs show their source and checksum, with Update,
  **Update all** and Remove ([docs/mod-browser.md](../docs/mod-browser.md)).
- **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](../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 come from your
  installed instance (CurseForge downloads are coming soon). 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.
- **Servers**: host a world for friends with buttons. **Create a server** asks for a name, a world
  (a new one, with an optional seed, or a copy of one of your single-player worlds), the game mode,
  difficulty, most players, who can join (friends only, the default, or anyone with the address)
  and your agreement to the Minecraft EULA; with the defaults (tick the EULA, press Create) you get
  a friends-only survival world with you as its operator. Each server then has **Start**, **Stop** and **Restart**, its state (also
  when it crashed and was restarted), players online with their heads, TPS, MSPT and RAM, the
  address friends on your network type with **Copy**, a console that takes commands, **Open the
  web panel**, and four tabs: **Overview**; **Players** (online players with Make operator, Kick
  and Ban; who may join, with Friends only, Let in and Remove; Unban); **Settings** (the message
  in the server list, game mode, difficulty, PvP, most players, view and simulation distance,
  port, account checks, **Run in the background and at login**); **Backups** (Back up now, the
  list with dates and sizes, Restore with a confirmation, Back up automatically, and **Updates**:
  Check for updates, Update now with its automatic rollback, and how many older versions to
  keep). Changes the running game can take apply at once, the others say "Restart to apply".
  Everything goes through `horizon-server` itself ([docs/hosting.md](../docs/hosting.md)):
  the launcher copies a prepared version's programs and game files into `server-runtime/` laid
  out as `install.sh` does, runs `horizon-server init`, `config`, `start`, `backup`, `restore`,
  `update` and `service` there, and talks to a running server through its control API. Problems
  are said in plain words, with a button when there's a fix (another port, the game files, the
  EULA).
  **Friends over the internet** (on Overview) uses Tailscale: it says whether Tailscale is
  installed and signed in (with Install, Sign in and Check again; signing in is always yours, in
  Tailscale's own app or page), shows this computer's Tailscale name and address with **Copy**,
  and invites a friend to this one computer: in the admin console, or here with an access token
  you make there (kept in the system keychain), which also lists invitations with Withdraw /
  Stop sharing and offers **Limit them to Minecraft**: when your tailnet has Tailscale's default
  access controls, one click changes them so friends you share with reach only the server's
  port (Remove the limit undoes it); custom access controls are never rewritten, the page gives
  the rule to add instead. Horizon never turns on Funnel.
- **Instances**: separate game folders, each with its own worlds, options and packs. The game's
  folder stays the first one, "Horizon", exactly where it was; imported and registry modpacks are
  instances too, in their own folders as before; new ones go to `instances/<name>/` in the game's
  folder, empty but for the player's options and name, or a copy of another instance (never its
  sign-in sessions, logs or data collection state). Names, icons (a block or item texture from the
  player's prepared files, or a colour) and groups are kept in `instances.json` in the launcher's
  data folder, so renaming moves nothing. Every instance follows the launcher's data collection
  answer and No AI. In the window, **+** beside Instances makes one, the sidebar lists them by
  group (click one to play it), and Home's Instance row has Edit (name, group, icon), Copy and
  Open folder. The Instance row shows its play time and when it was last played: every run started
  from the window or `horizon-launcher play` adds to it (`instance list` shows it too). Each
  instance plays the launcher's version, which follows updates, unless the Play page's version
  picker keeps it on another installed one (to keep an old world on the version it was made
  with); picking the version marked "follows updates" lets it follow again.
- **Export**: Home's Instance row has **Export…**: the selected instance (or a port) as a `.mrpack`
  (Modrinth's modpack format: resource packs the browser installed from Modrinth are listed by
  their download, the rest is in `overrides/`) or a `.zip` of the game folder's layout, with the
  worlds switched on. It is written to `exports/` in the launcher's data folder (**Show in
  folder**). Only settings (options.txt, horizon-*.txt, servers.dat), resource and shader packs,
  `horizon-mods/` and `config/` go in: never logs, screenshots, backups, caches or account files;
  a world goes without `playerdata/`, `stats/` and `advancements/`, and its level.dat without the
  player's UUID. `horizon-launcher instance export [<instance>] [--format mrpack|zip] [--world
  <w>]… [--out <file>]`.
- **Import by dropping a file**: a file dropped anywhere on the window is imported for what it is
  (`horizon_launcher::import`). A shared instance (a `.mrpack` that needs no mod loader, such as
  Export writes, or a game folder `.zip`) becomes a new instance, opened on Home. Its `.mrpack`
  downloads come from Modrinth with their SHA-512 checked, and only what an instance keeps is
  taken, inside its folder. A Java modpack goes to the Modpacks import, and a resource or shader
  pack into the selected instance. A data pack is pointed at the world rows, and the Packs section
  and the world rows keep taking their own drops. A note in the corner says what happened.
  `horizon-launcher instance import <file>`.
- **Launch options**: Home's Instance row has **Launch…**: per instance, a shell command run in its
  folder before the game (the game starts only if it succeeds), a wrapper the game runs through
  (`nice -n 5`), and environment variables (`NAME=value OTHER="two words"`), kept in
  `instances.json` (`horizon_launcher::launch`). The game stays one process the launcher follows:
  on Unix `/bin/sh` runs the command and then `exec`s the wrapper and the game, whose words pass
  through `"$@"` untouched; on Windows `cmd /C` runs `<command> && <game>`. Both the window's Play
  and `play` use them. `horizon-launcher instance launch <instance> [--pre-launch …]
  [--wrapper …] [--env …]` (an MCP agent needs `confirm: true`: it sets commands Play runs).
  On Linux the panel also has switches for what the computer has: **GameMode** (`gamemoderun`
  on the PATH), **MangoHud** (`mangohud`), and, with two graphics cards under
  `/sys/class/drm`, the **dedicated graphics card** (`DRI_PRIME=1`, or NVIDIA's
  `__NV_PRIME_RENDER_OFFLOAD`, `__VK_LAYER_NV_optimus` and `__GLX_VENDOR_LIBRARY_NAME` when its
  driver is loaded). A switch whose tool is gone does nothing. `instance launch --gamemode
  on|off --mangohud on|off --dedicated-gpu on|off`; `HORIZON_LAUNCHER_LINUX_TOOLS` makes any
  system show them, for tests.
- **Desktop shortcut**: Home's Instance row has **Shortcut**: a shortcut on the desktop that plays
  the selected instance without opening the launcher (`horizon-launcher play --instance <id>`).
  A `.desktop` file on Linux (also put in the applications menu, and marked trusted with `gio`
  where it exists), a small app with Horizon's icon on macOS, a `.lnk` on Windows
  (`horizon_launcher::shortcut`). **Remove shortcut** moves it to the trash; it is found by the
  instance id inside it, so a rename doesn't lose it. `horizon-launcher instance shortcut
  [instance] [--remove]`; `HORIZON_LAUNCHER_DESKTOP` points it at another folder.
- **Accounts and quick switch**: the top bar's account chip opens a menu of everyone who can play:
  each signed-in Microsoft account, the offline names the player keeps, and the name the game
  made up (`horizon-player.txt`). Picking one plays as it from the next Play. Offline names play
  only while an account that owns the game is signed in ([Microsoft accounts](#microsoft-accounts));
  signing the last one out keeps them, unplayable until one is back. Settings > Account
  adds offline names (3 to 16 letters, digits or `_`; the game gets `--name`) and forgets them;
  the Console's Share log takes them out of the log too. `horizon-launcher account offline
  [NAME]`, `account select <name>` (an account or an offline name), `account forget-offline
  <name>`.
- **Accent and Home background**: on top of Dark, Light or System, Settings > Launcher picks an
  accent (Horizon's violet, blue, teal, green, amber, red or pink) and a picture of the player's
  own behind Home (PNG, JPEG or WebP, cropped to fill; **Landscape** brings Horizon's back). An
  accent turns the violet tokens to its hue, then darkens or lightens them just enough to keep
  white on the Play button at 4.5:1 and the accent at 3:1 on every background (a test holds every
  accent in both looks to it). The picture is copied into the data folder.
  `horizon-launcher settings accent NAME`, `settings background PATH|default`.
- **Language**: English, French and Russian (the languages Horizon's own game menus have).
  The window follows the system's language unless the player picks one in Settings > Launcher
  (each named in itself); a string a table lacks shows in English. The tables are
  `launcher/lang/<code>.json`, English text to its translation, Horizon's own; a test fails when
  the code asks `tr("…")` for a string a table lacks, or a table keeps one the code dropped.
  `horizon-launcher settings language system|en|fr|ru`.
- **Skin and cape**: each offline name's **Skin…** in Settings > Account shows the front of its
  skin and cape, takes a skin PNG (64×64, or the older 64×32) and a cape PNG (64×32), and picks
  classic or slim arms. The launcher keeps copies in its `skins/` folder and gives the game
  `HORIZON_OFFLINE_SKIN`, `HORIZON_OFFLINE_SKIN_MODEL` and `HORIZON_OFFLINE_CAPE` when that name
  plays; the game draws the default skins until it reads them. A Microsoft account's skin and
  cape: coming soon, with sign-in. `horizon-launcher account skin <name> [FILE] [--model
  classic|slim] [--cape FILE] [--no-skin] [--no-cape]`.
- **Console**: Home's Console shows the game's output of this run as it comes: a search (every
  word, any case), Follow to keep the newest line in view, Copy, and Open logs folder. Warnings
  and errors stand out. **Share log…** uploads the log to [mclo.gs](https://mclo.gs), a paste
  service for Minecraft logs, only after the player confirms; the home folder, the user name and
  the account names are taken out first (`horizon_launcher::game_log`), and **Delete it** removes
  the paste. `horizon-launcher share-log [--instance]` and `unshare-log <id:token>`;
  `HORIZON_LAUNCHER_PASTE_URL` points it at a test server.
- **Worlds**: Home lists the selected instance's worlds (picture, mode, version, last played,
  size). **Back up** zips one into the instance's `backups/` as the game's Make Backup does
  (`<date>_<time>_<folder>.zip`); **Copy** puts "<name> (1)" beside it; **Delete** moves it to the
  system's trash after a confirmation; **Open folder**. The same from the command line:
  `horizon-launcher worlds [list|backup|copy|delete] [--instance <name>]`.
  **Play** on a world starts the game straight into it. **Data packs** opens the world's
  `datapacks/` under its row: what is there (and which the game has turned off), **Add data pack…**,
  and **Remove** (to the trash); a data pack `.zip` dropped on a world goes into it. The game turns
  a new one on when the world opens. From the command line: `horizon-launcher worlds
  datapack-list|datapack-add|datapack-remove <world> …`. The Mods page's **Data packs** tab finds
  them on Modrinth and installs one into the world picked there (`mods search --kind datapack`,
  `mods install <pack> --world <world>`), saying what of it runs in Horizon.
- **Packs**: Home lists the selected instance's resource packs and shader packs, each with a switch.
  The choice is the game's own: `resourcePacks` in options.txt (a pack turned on goes on top, as
  the game's Resource Packs screen does) and `pickedShaderPack` in `horizon-graphics.txt` (one shader
  pack at a time). **Add pack…** or a `.zip` / `.hshader` dropped on the section copies it into the
  right folder; **Remove** turns it off and moves it to the trash after a confirmation. From the
  command line: `horizon-launcher packs [list|add|on|off|remove] [--kind resource|shader]
  [--instance <name>]`.
- **Screenshots**: Home shows the selected instance's newest screenshots (what F2 saves in its
  `screenshots/` folder) and all of them on request. Clicking one opens it over the window (← and →
  step through) with **Copy** (the picture, to the clipboard), **Open**, **Show in folder** and
  **Delete** (to the trash, after a confirmation). From the command line:
  `horizon-launcher screenshots [list|delete] [--instance <name>]`.
- **Multiplayer**: Home lists the selected instance's servers, the game's own Multiplayer list
  (`servers.dat`, each with the picture the game last saw). **Join** starts the game straight into
  one; **Add server** and **Remove** (after a confirmation) change the file the game reads,
  keeping every field the launcher doesn't show and the servers joined with Direct Connection. The
  same from the command line: `horizon-launcher multiplayer [list|add|remove] [--instance <name>]`
  and `horizon-launcher play --server <address or name>`.
- **Patch notes**: each release's notes, newest first, with a link to GitHub Sponsors. Quiet "Sponsor", "Discord" and "Website" links sit in the sidebar 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/`, `settings.json`, and for the Servers
page `servers.json` (the list), `servers/<name>/` (the servers created there) and
`server-runtime/` (the programs and game files they run).

- **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,
  what the window does while you play (stays by default; Minimise, or Hide on macOS, puts it away
  when the game starts and brings it back to the front when the game closes; `settings
  while-playing stay|minimize|hide`), the game's extra command-line arguments, an HTTP proxy for
  downloads, and the launcher's own version and update state.
- **Updates**: there is one channel. Every release is published as the release, GitHub's latest,
  once its smoke test passed (docs/release-flow.md), and the launcher installs it. The game
  pre-releases of the old Alpha channel (v0.2.1 on) stay on the Versions page to install
  by hand, but are never installed as an update. The `update_channel` and `beta_builds` keys
  older launchers wrote in `settings.json` are ignored and dropped on the next save.
- **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.
- **Proxy**: when the internet here needs one (a school's, an office's, a country where a host is
  unreachable), Settings' Proxy takes the `http://host:port` address your proxy app listens on, and
  every download goes through it: Horizon's releases and the launcher's own update, Minecraft's
  files and Java from Mojang, Modrinth, CurseForge, Horizon's mod registry, pictures, Microsoft
  sign-in, and the jars a modpack import fetches. It is kept as `proxy` in `settings.json`, and
  `horizon-launcher settings proxy http://127.0.0.1:7890` sets it too ("default" removes it).
  Without one, `HTTPS_PROXY` and `HTTP_PROXY` are used when the launcher is started with them, and
  `NO_PROXY` names the hosts to reach directly, a modpack import's jars included. A server on this
  computer never goes through it.
- **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.

Long pages, dialogs, the server console and data previews have a visible scrollbar when
their content overflows. Drag the thumb or click the track to navigate without a mouse wheel.

Pages and Home's landscape fill the window as it resizes. Windows uses a matching title
bar with native dragging, resizing, Snap,
minimize, maximize/restore and close; macOS and Linux keep their system title bars.

## Updates

The launcher looks for a newer Horizon release when it starts and every 30 minutes while it
is open, and shows a small "available, Install" line on Play.

It also updates itself from GitHub releases tagged `launcher@X.Y.Z` (Horizon's own are `vX.Y.Z`;
neither is ever taken for the other). Before 0.2 they were `launcher-vX.Y.Z` and `vYYYY.MM.DD`:
every 0.2 tag counts as newer than all of those (docs/versioning.md). 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.

On Linux a release build that starts writes its applications-menu entry only when there is
none, or when the program the entry starts (its `TryExec`, else its `Exec`) no longer exists.
An entry pointing at another launcher that works is left alone, so a test or development build
never takes it over. Turning the Settings switch on, or `horizon-launcher desktop-entry`,
points the entry at this launcher whatever is there.

Linux packagers can set `HORIZON_LAUNCHER_DESKTOP_EXEC` to a wrapper's absolute path.
Menu entries the launcher writes 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. On Windows it
is a windowed program, so a desktop start opens no terminal; a subcommand writes to the terminal
it was typed in, or to the pipes and files it was given (MCP keeps its stdio). Interactive `cmd`
and PowerShell don't wait for a windowed program: they show the prompt again before its output
and don't set `%ERRORLEVEL%` / `$LASTEXITCODE`. Scripts that need the exit code use
`start /wait horizon-launcher …` or `Start-Process -Wait`, or read its output through a pipe.

    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>] [--instance <name>] [--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 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 instance [list]                the instances and their game folders (* = Play starts it)
    horizon-launcher instance create <name> [--copy <instance>]   empty, or a copy of one's worlds, options and packs
    horizon-launcher instance rename|icon|group <instance> <value>  icon: a block or item name, #rrggbb or none
    horizon-launcher instance select <instance>     make Play start it
    horizon-launcher instance version <instance> <tag|latest>   keep it on one installed version, or follow updates
    horizon-launcher worlds [list] [--instance <name>]        an instance's worlds
    horizon-launcher worlds backup|copy|delete <world> [--instance <name>]   delete moves it to the trash
    horizon-launcher packs [list] [--kind resource|shader] [--instance <name>]   resource and shader packs, on or off
    horizon-launcher packs add <file> | on|off|remove <pack> [--kind shader]
    horizon-launcher screenshots [list] [--instance <name>]   an instance's screenshots, the newest first
    horizon-launcher screenshots delete <name> [--instance <name>]   moves it to the trash
    horizon-launcher multiplayer [list] [--instance <name>]   the servers in an instance's Multiplayer list
    horizon-launcher multiplayer add <address> [--name <name>] | remove <server>
    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 mods [list]                    what the mod browser installed (docs/mod-browser.md)
    horizon-launcher mods search <text> [--kind resource|shader|modpack] [--everything] [--source modrinth]
    horizon-launcher mods install <id or slug>      checksum checked, with what it needs; a modpack is imported
    horizon-launcher mods update [<pack>] [--check]
    horizon-launcher mods 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`, `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). Test runs also set `HORIZON_LAUNCHER_TRASH=<folder>` and
`HORIZON_LAUNCHER_CLIPBOARD=<file>` inside their scratch folders: what Delete would move to the
system's trash goes into that folder, and what Copy would put on the clipboard is written to that
file, so a test never touches the real trash or clipboard.

## AI agents (MCP)

`horizon-launcher mcp` is a Model Context Protocol server over stdio. `horizon-launcher
agent-config` prints the config for Claude Code, Codex, Cursor and any MCP client, and the
command that adds it; for Claude Code:

    claude mcp add --scope user horizon -- horizon-launcher mcp

(with the full path to the binary if it is not on your `PATH`). Every CLI command is a tool of the
same name and arguments, generated from the CLI's definition (`src/cli_tools.rs`): `versions`,
`install`, `remove`, `prepare`, `settings`, `mods_search`, `mods_install`, `modpack_import`,
`servers_create`, `data_status`... A call runs that command with `--json` and returns its
output. Tools that delete something (`remove`, `mods_remove`, `modpack_remove`,
`servers_forget`, `account_sign_out`, `data_delete`) need `confirm: true`. The session runs a few
itself, under the CLI's names: `play`, `server` with `server_command`, `server_log` and
`server_stop`, `account_list` and `account_sign_in` (returns the link and code for the player;
the sign-in finishes in the background). 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: `play` starts it with a local control socket (`world`,
`creative`, `join` 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](../docs/horizon-ctl-commands.md). 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 `play`.

`horizon-launcher no-ai on` (the Settings page's No AI switch) hides and turns off every AI
feature: `mcp` and `agent-config` refuse, the game opens no control socket and `horizon-ctl`
refuses. Every other command keeps working.

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

## Microsoft accounts

The launcher starts the game only while a Microsoft account that owns Minecraft: Java Edition is
signed in, as other launchers do: without one, Play (Home, instances, `horizon-launcher play`, the
MCP's `play`) says "Sign in with a Microsoft account that owns Minecraft: Java Edition to play"
and starts nothing. Settings → Accounts signs in: the launcher shows a code, you enter it at
microsoft.com/link, and the launcher checks that the account owns the game. It asks again at every
Play, with the token renewal; with no network the last answer stands, and a "doesn't own" blocks
at once. The account also plays online-mode servers under your own name and skin; offline names
play LAN and offline-mode servers beside it. The game binary started directly (`horizon-client`,
`horizon-ctl launch`), the harnesses and the servers don't go through this check. 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](../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](../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](../docs/benchmark-report.md): 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 (read on a thread after the first frame: on Windows those queries take seconds). 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](../docs/telemetry.md)).

Long lists on Mods (results, installed entries, coverage reports and project versions), Versions
and Patch notes build only nearby rows, including on their first frame. Variable row heights
are measured as they enter view; hidden ranges share spacers in the existing page scrollbar.
On Windows, `cargo test --bin horizon-launcher desktop_list_renders_the_viewport_after_scrollbar_jumps
-- --ignored` checks 10,000 rows of different heights in a real GPUI window (requires an awake desktop/GPU).

## 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

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


## Screenshots

![Servers](docs/servers.png)
![Friends over the internet](docs/servers-tailscale.png)
![Modpacks](docs/modpacks.png)
![Versions](docs/versions.png)
![Patch notes](docs/patch-notes.png)
![Settings](docs/settings.png)