ESLint Flat Config "Could not find plugin" / "Key plugins" - Fix in CI

By Daniel Zoghalchali·Latchkey

In ESLint flat config, plugins are imported as modules and registered in a plugins object on each config entry - there is no extends/string resolution like eslintrc. A rule referencing a plugin namespace that is not registered fails because flat config never resolves plugin names by string.

What this error means

ESLint fails with Could not find plugin "<name>", Key "rules": Key "<plugin>/<rule>": Could not find plugin, or TypeError: Cannot read properties of undefined when a rule's plugin namespace is not registered in eslint.config.js.

eslint output

Error: Key "rules": Key "@typescript-eslint/no-unused-vars":
Could not find plugin "@typescript-eslint".
  at eslint.config.js

Common causes

Plugin not registered in the plugins object

Flat config does not auto-resolve plugin names. A rule like @typescript-eslint/... only works if the config entry registers '@typescript-eslint': tseslint in its plugins map.

eslintrc-style string extends in flat config

Copying extends: ['plugin:...'] or plugins: ['react'] (string form) from eslintrc into flat config does not work - flat config needs imported plugin objects.

How to fix it

Import and register the plugin object

Import the plugin and register it under plugins, then reference its rules.

eslint.config.js

// eslint.config.js
import tseslint from '@typescript-eslint/eslint-plugin'
import tsParser from '@typescript-eslint/parser'
export default [
  {
    files: ['**/*.ts', '**/*.tsx'],
    languageOptions: { parser: tsParser },
    plugins: { '@typescript-eslint': tseslint },
    rules: { '@typescript-eslint/no-unused-vars': 'error' },
  },
]

Replace eslintrc string extends

Drop extends: ["plugin:..."]; spread the plugin's exported flat config instead.
Register every plugin namespace your rules reference in plugins.
Use @eslint/eslintrc's FlatCompat only as a temporary bridge for legacy shareable configs.

How to prevent it

Register every referenced plugin in the flat-config plugins object.
Prefer plugins' exported flat configs over hand-wiring.
Do not copy eslintrc string extends/plugins into flat config.

Frequently asked questions

What causes "flat config plugin error"?

Flat config does not auto-resolve plugin names. A rule like @typescript-eslint/... only works if the config entry registers '@typescript-eslint': tseslint in its plugins map.

How do I fix flat config plugin error?

Import the plugin and register it under plugins, then reference its rules.