Rules

A rule says where something applies: "only on Windows", "not on ARM", "only when this feature is on".

"rules": "allow.os.windows"

rules is one rule or a list. Absent means "always".

In a config

Vanilla Minecraft, with a rule in each of the three places a config can have one:

// opys.config.mjs
export default defineConfig({
  plugins: [
    minecraft({ version: '1.21.1' }),
    java({ version: '21' }),
    files({
      from: 'tools',
      to: (file) => '${game_directory}/tools/' + file.rel,
    }).addRule('**/*.dll', 'allow.os.windows'), // a file: installed only on Windows
  ],
  manifest: {
    command: '@minecraft.command',
    args: [
      // an argument: passed only on macOS
      { rules: 'allow.os.osx', value: ['-Xdock:name=My Pack'] },
      '@minecraft.jvmArgs',
      '@minecraft.mainClass',
      '@minecraft.gameArgs',
    ],
    workdir: '${game_directory}',
    vars: {
      // a variable: the last value whose rules pass wins
      pack_label: [
        { value: 'My Pack' },
        { value: 'My Pack (Mac)', rules: 'allow.os.osx' },
      ],
    },
  },
});

Two ways to write a rule

Both mean the same, and a reader must accept both.

Short

A string. This is what you write in a config.

Rule	Passes
allow.os.windows	On Windows.
allow.os.linux	On Linux.
allow.os.osx	On macOS.
allow.arch.aarch64	On ARM64.
allow.arch.x86_64	On 64-bit Intel and AMD.
allow.features.java_console	When that feature is switched on.
allow.os.osx@^14	On macOS whose version matches the pattern.
disallow.os.osx	Everywhere except macOS.
allow	Always.
disallow	Never.

disallow takes every form allow does.

Long

An object. This is Mojang's own form, and what the game's version files use.

{ "action": "allow", "os": { "name": "osx", "arch": "aarch64" } }

{ "action": "disallow", "features": { "is_demo_user": true } }

Field	What it is
action	allow or disallow. Required.
os	Any of name, arch, version. All given ones must match.
features	Feature names, each true (must be on) or false (must be off).

Use the long form when the short one cannot say it, such as an OS and an architecture in one rule. Give a rule os or features, not both.

How a ruleset is decided

1. A rule has a condition (os, features, or none). No condition always holds.
2. An allow rule passes when its condition holds. A disallow rule passes when it does not.
3. The ruleset passes when every rule passes.

["allow.os.linux", "disallow.features.demo"]

Passes on Linux, when demo is off.

Every rule, not any rule

Two allow rules for two operating systems pass nowhere, because no machine is both. For "Windows or Linux", write disallow.os.osx, or use two artifacts.

What a machine reports

Value	Possible values
name	linux, windows, osx, freebsd
arch	x86_64, aarch64
version	Empty, unless the launcher supplies it.

The opys runtime does not detect the OS version. A rule with a version pattern only works if the launcher passes a platform with one.

A rule may also name the architectures x86, arm, ppc64le and riscv64, which the runtime never reports, so such a rule passes only where a launcher passes that platform. x64, arm64 and arm32, as some version files write them, are read as x86_64, aarch64 and arm.

Features

A feature is a named switch, off by default. A launcher turns it on with features: ['java_console'], the CLI with --feature java_console.

The known ones are listed on Variables.