ESLint "couldn't find an eslint.config.* file" (ESLint 10)

This error means ESLint looked for a config file in the new format and found none. Usually there is a config: an .eslintrc.json, .eslintrc.js or .eslintrc.yml that worked for years. ESLint 9 made eslint.config.* the default, and ESLint 10 removed support for .eslintrc completely, so that file is now simply ignored.

I reproduced it with a project that had only an .eslintrc.json, on both versions:

ESLint 10 also exits with code 2, so CI fails on this step. There’s no switch that brings the old format back. The config has to move.

The fix

npx @eslint/migrate-config .eslintrc.json

npm install -D @eslint/js @eslint/eslintrc

import js from "@eslint/js";
import globals from "globals";
import { defineConfig } from "eslint/config";

export default defineConfig([
  js.configs.recommended,
  { languageOptions: { globals: globals.node } },
]);

“‘console’ is not defined” after migrating

The next surprise is a flood of no-undef errors for things that obviously exist:

2:1  error  'console' is not defined  no-undef

.eslintrc had env: { node: true } or env: { browser: true } to declare those globals. Flat config has no env.

ESLint 10 with React and Next.js plugins

This is the part behind most “eslint 10” searches. Several of the most-used plugins don’t declare support for ESLint 10 yet. Their peerDependencies on October 10, 2026:

How that showed up depended on how the plugin got into the project. Installing eslint-plugin-react directly next to ESLint 10 failed outright, and nothing was installed:

npm error code ERESOLVE
npm error Found: eslint@10.12.0
npm error Could not resolve dependency:
npm error peer eslint@"^3 || ^4 || ^5 || ^6 || ^7 || ^8 || ^9.7" from eslint-plugin-react@7.37.5

Through eslint-config-next, npm only warned (npm warn ERESOLVE overriding peer dependency, three times) and installed everything.

In both cases the plugins worked on ESLint 10. With eslint-config-next’s core-web-vitals config, a test file got errors and warnings from react/jsx-key, react-hooks/rules-of-hooks, jsx-a11y/alt-text and @next/next/no-img-element, so all of those plugins loaded and ran. The block is the declared version range, not the code.

{
  "overrides": {
    "eslint-plugin-react": { "eslint": "$eslint" }
  }
}

“‘React’ must be in scope when using JSX”

If you set up eslint-plugin-react by hand with its recommended config, every JSX file gets this:

2:10  error  'React' must be in scope when using JSX  react/react-in-jsx-scope

The recommended config still assumes the old JSX transform from before React 17.

import react from "eslint-plugin-react";
import { defineConfig } from "eslint/config";

export default defineConfig([
  {
    files: ["**/*.jsx"],
    ...react.configs.flat.recommended,
    ...react.configs.flat["jsx-runtime"],
    rules: { ...react.configs.flat.recommended.rules, ...react.configs.flat["jsx-runtime"].rules },
    settings: { react: { version: "19.0" } },
  },
]);

One more thing if you’re on Next.js: create-next-app 16.4 with its defaults no longer adds ESLint at all, so there’s no lint script or config to migrate. You add both yourself.

What didn’t work

How this was tested

Fresh npm projects on Windows 11 with Node 24.14.0 and npm 11.9.0:

• an .eslintrc.json-only project on ESLint 10.12.0 and 9.39.5, with and without ESLINT_USE_FLAT_CONFIG=false
• @eslint/migrate-config on a config without env and one with env: { node: true }, run before and after installing the packages it asked for, and a hand-written flat config
• eslint 10.12.0 with eslint-config-next 16.4.0 and next 16.4.0, linting a JSX file written to break rules from each plugin
• eslint 10.12.0 with eslint-plugin-react 7.37.5 installed directly, then with --legacy-peer-deps, then with an overrides entry

I only tested the rules those configs enable. A plugin that hasn’t declared ESLint 10 support could still have a rule that breaks on it.

The code is public, so you can run it yourself: eslint10 in nk-repro.