disallow-stylelint-custom-syntax

Disallow configuring Stylelint's top-level customSyntax option inside authored Stylelint config files.

Targeted pattern scope

This rule targets Stylelint config modules such as stylelint.config.ts, stylelint.config.mjs, and .stylelintrc.js.

It focuses on top-level exported config objects, including configs wrapped in defineConfig(...).

What this rule reports

This rule reports Stylelint config objects that include a top-level customSyntax property.

Why this rule exists

Top-level customSyntax applies globally and can accidentally affect unrelated file types. Syntax selection is safer when scoped to overrides entries with explicit files matchers.

❌ Incorrect

export default {
    customSyntax: "postcss-scss",
    rules: {},
};

✅ Correct

export default {
    overrides: [
        {
            customSyntax: "postcss-scss",
            files: ["**/*.scss"],
            rules: {},
        },
    ],
    rules: {},
};

Behavior and migration notes

• This rule removes top-level customSyntax.
• It preserves the rest of the config object.
• Move syntax declarations into override entries that include explicit file patterns.

ESLint flat config example

import stylelint2 from "eslint-plugin-stylelint-2";

export default [stylelint2.configs.configuration];

When not to use it

Do not use this rule if your repository intentionally uses one global syntax parser for every linted file.

Package documentation

Stylelint package documentation:

Rule catalog ID: R014

Further reading

• Stylelint customSyntax
• Stylelint overrides