ESLint hangs indefinitely when prettier is not installed (instead of throwing an error)

[isamu]

Description

When eslint-plugin-prettier is configured in eslint.config.js but prettier itself is not installed as a dependency, ESLint hangs indefinitely without producing any output or error message.

Expected behavior: ESLint should fail fast with a clear error message such as Cannot find module 'prettier'.

Actual behavior: ESLint process hangs forever (no output, no error, no exit). It must be killed manually (e.g., kill or Ctrl+C).

Reproduction Steps

1. Minimal package.json (note: no prettier dependency)

{
  "devDependencies": {
    "@eslint/js": "^10.0.1",
    "eslint": "^10.0.0",
    "eslint-config-prettier": "^10.1.8",
    "eslint-plugin-prettier": "^5.5.4",
    "typescript-eslint": "^8.54.0"
  }
}

2. eslint.config.js

import eslint from "@eslint/js";
import tseslint from "typescript-eslint";
import prettierConfig from "eslint-config-prettier";
import prettierPlugin from "eslint-plugin-prettier";

export default tseslint.config(
  eslint.configs.recommended,
  ...tseslint.configs.recommended,
  prettierConfig,
  {
    plugins: {
      prettier: prettierPlugin,
    },
    rules: {
      "prettier/prettier": "error",
    },
  },
  {
    ignores: ["node_modules/**", "lib/**"],
  }
);

3. Create any TypeScript file

echo 'const x = 1;' > src/test.ts

4. Run ESLint

npx eslint src/test.ts
# → Hangs indefinitely. No output. No error. Must be killed.

5. Install prettier and run again

npm install -D prettier
npx eslint src/test.ts
# → Works correctly. Reports prettier formatting errors and exits.

Debug Output

Running with --debug shows ESLint successfully parses and analyzes the file, but then hangs after scope analysis — presumably when the prettier/prettier rule tries to invoke Prettier:

eslint:languages:js Parsing successful: /path/to/src/test.ts
eslint:languages:js Scope analysis: /path/to/src/test.ts
eslint:languages:js Scope analysis successful: /path/to/src/test.ts
# ← Hangs here forever

Impact

This is particularly problematic because:

1. No indication of what's wrong — developers waste significant time debugging, as the hang gives no clue that prettier is simply missing.
2. CI pipelines hang indefinitely instead of failing with a useful error, potentially consuming runner minutes until timeout.
3. eslint-plugin-prettier imports successfully (since it's installed), so the config file loads without error — the failure only occurs at rule execution time.

We encountered this in a real project: isamu/movie-separate#2

Environment

• eslint-plugin-prettier: 5.5.4
• eslint-config-prettier: 10.1.8
• eslint: 10.0.0 (also reproduced with 9.39.1)
• typescript-eslint: 8.54.0
• Node.js: v24.12.0
• OS: macOS (Darwin 25.2.0 arm64)
• prettier: not installed (this is the trigger)

Suggested Fix

The plugin should check for prettier availability at rule creation time and throw a descriptive error if it cannot be resolved, e.g.:

Error: eslint-plugin-prettier requires prettier to be installed.
Please run: npm install -D prettier