ESLint "Could not find config file" - Flat Config (v9) Migration

By Daniel Zoghalchali·Latchkey

ESLint v9 defaults to flat config (eslint.config.js) and no longer auto-loads .eslintrc.*. After upgrading, a project still on the legacy format fails to find a config, or rejects options that flat config moved or removed.

What this error means

After an ESLint major bump, CI fails with Could not find config file, or errors about options/keys (env, extends, --ext) that flat config handles differently. The same repo linted fine on ESLint 8.

eslint output

Oops! Something went wrong! :(
ESLint: 9.0.0
Could not find config file.
# or
Invalid option '--ext' - perhaps you meant to use flat config's 'files'?

Common causes

Legacy .eslintrc on ESLint v9

ESLint 9 looks for eslint.config.js (flat config) by default and ignores .eslintrc.* unless compatibility is explicitly enabled, so it reports no config found.

Flat config restructures options

Flat config replaces env, top-level extends, and the --ext flag with languageOptions, config arrays/FlatCompat, and the files field. Old options error under the new format.

How to fix it

Migrate to eslint.config.js

Author a flat config; use FlatCompat to reuse legacy shareable configs during transition.

eslint.config.js

// eslint.config.js
import js from '@eslint/js'
import tseslint from 'typescript-eslint'
export default [
  js.configs.recommended,
  ...tseslint.configs.recommended,
  { files: ['**/*.{ts,tsx}'], rules: { /* ... */ } },
]

Replace removed CLI options

Drop --ext; specify extensions via files globs in the config instead.
Move env to languageOptions.globals (e.g. via the globals package).
Convert top-level extends to spreading config objects/arrays.

How to prevent it

Plan the flat-config migration before adopting ESLint v9 in CI.
Pin the ESLint major so an upgrade is a deliberate, tested change.
Keep one config format across local and CI to avoid drift.

Frequently asked questions

What causes "flat config migration"?

ESLint 9 looks for eslint.config.js (flat config) by default and ignores .eslintrc.* unless compatibility is explicitly enabled, so it reports no config found.

How do I fix flat config migration?