horizon/docs/api/launch-reference.md

Launch API reference

What horizon-client promises a launcher that starts it, Launch API 0.1.0. The guide: launch.md; stability and versions: stability.md.

Options

Any other option is refused with exit code 2, so ask --launch-api before passing one a build may not know.

Option	What it does
--resources DIR	The prepared resources (horizon-prepare's output). Default: resources beside the executable, else .local/horizon-client/26.3.
--game-dir DIR	The game folder: saves/, options.txt, resourcepacks/, horizon-mods/, screenshots/. Created when missing. Default: the working directory. --gameDir is the same.
--home DIR	Logs (logs/latest.log) and crash reports (crash-reports/). Default: HORIZON_HOME, else horizon in the platform data folder.
--width N	The window's starting width (default 1280).
--height N	The window's starting height (default 720).
--fullscreen	Opens the window fullscreen, whatever options.txt says; options.txt is not changed.
--session-file PATH	The signed-in account, as the session JSON below. Read once, then deleted.
--session-stdin	The same JSON on standard input, so it never touches the disk.
--quickPlaySingleplayer WORLD	Opens this world of saves/ at start, only if it exists (else the title screen). WORLD is one folder name: a path is refused.
--quickPlayMultiplayer HOST[:PORT]	Joins this server at start.
--launch-api	Prints the reply below and exits 0, before anything else runs.

--launch-api

Prints one JSON object on stdout and exits 0. horizon is the build's version; readers ignore fields they don't know.

{
  "crash_report_line": "Horizon: crash report saved to ",
  "exit_codes": [
    0,
    1,
    2,
    101
  ],
  "horizon": "<version>",
  "launch_api": "0.1.0",
  "minecraft": "26.3",
  "options": [
    {
      "name": "--resources",
      "value": "DIR"
    },
    {
      "name": "--game-dir",
      "value": "DIR"
    },
    {
      "name": "--home",
      "value": "DIR"
    },
    {
      "name": "--width",
      "value": "N"
    },
    {
      "name": "--height",
      "value": "N"
    },
    {
      "name": "--fullscreen",
      "value": null
    },
    {
      "name": "--session-file",
      "value": "PATH"
    },
    {
      "name": "--session-stdin",
      "value": null
    },
    {
      "name": "--quickPlaySingleplayer",
      "value": "WORLD"
    },
    {
      "name": "--quickPlayMultiplayer",
      "value": "HOST[:PORT]"
    },
    {
      "name": "--launch-api",
      "value": null
    }
  ]
}

Exit codes

Code	Meaning
0	The player quit, or --help or --launch-api printed.
1	The game stopped on an error. A crash report was saved.
2	The command line was refused: an unknown option, a missing or bad value. No crash report.
101	The game panicked. A crash report was saved.

When a crash report is saved, the last line on stderr is Horizon: crash report saved to followed by the report's path.