diff --git a/.vscode/settings.json b/.vscode/settings.json
new file mode 100644
index 0000000..0c8d273
--- /dev/null
+++ b/.vscode/settings.json
@@ -0,0 +1,6 @@
+{
+	"eslint.workingDirectories": [
+		{"directory": "./packages/core", "changeProcessCWD": true},
+		{"directory": "./packages/react", "changeProcessCWD": true}
+	]
+}
diff --git a/config/eslint-config/index.js b/config/eslint-config/index.js
index 631dfb3..457ab1c 100644
--- a/config/eslint-config/index.js
+++ b/config/eslint-config/index.js
@@ -4,12 +4,14 @@ module.exports = {
 		ecmaVersion: 2019,
 		sourceType: 'module',
 	},
-	plugins: ['@typescript-eslint'],
+	plugins: ['@typescript-eslint', 'require-extensions'],
+
 	extends: [
 		'eslint:recommended',
 		'plugin:@typescript-eslint/recommended-type-checked',
 		'plugin:react/recommended',
 		'plugin:react-hooks/recommended',
+		'plugin:require-extensions/recommended',
 		'prettier',
 	],
 	rules: {
diff --git a/config/eslint-config/package.json b/config/eslint-config/package.json
index f60d7bd..f751cfe 100644
--- a/config/eslint-config/package.json
+++ b/config/eslint-config/package.json
@@ -5,5 +5,8 @@
 	"main": "index.js",
 	"files": [
 		"index.js"
-	]
+	],
+	"devDependencies": {
+		"eslint-plugin-require-extensions": "^0.1.3"
+	}
 }
diff --git a/packages/core/.eslintrc.cjs b/packages/core/.eslintrc.cjs
index 37b6c39..267f462 100644
--- a/packages/core/.eslintrc.cjs
+++ b/packages/core/.eslintrc.cjs
@@ -2,7 +2,7 @@ module.exports = {
 	root: true,
 	extends: ['@formulier/eslint-config'],
 	parserOptions: {
-		project: true,
+		project: './tsconfig.lint.json',
 		tsConfigRootDir: __dirname,
 	},
 }
diff --git a/packages/core/tests/array-utils.test.ts b/packages/core/tests/array-utils.test.ts
index acd0252..14e17f8 100644
--- a/packages/core/tests/array-utils.test.ts
+++ b/packages/core/tests/array-utils.test.ts
@@ -1,6 +1,6 @@
 import {expect, it} from 'vitest'
 
-import {insert, move, push, remove, swap} from '../src/array-utils'
+import {insert, move, push, remove, swap} from '../src/array-utils.js'
 
 const [A, B, C, D] = ['a', 'b', 'c', 'd']
 
diff --git a/packages/core/tests/form.test.ts b/packages/core/tests/form.test.ts
index 3b8987d..05f8121 100644
--- a/packages/core/tests/form.test.ts
+++ b/packages/core/tests/form.test.ts
@@ -1,7 +1,7 @@
 import {expect, it, vi} from 'vitest'
 
-import {Formulier} from '../src/form'
-import {getPath} from '../src/state-utils'
+import {Formulier} from '../src/form.js'
+import {getPath} from '../src/state-utils.js'
 
 const INITIAL_VALUES = {a: {b: {c: 'c', d: 'd'}}}
 
diff --git a/packages/core/tests/state-utils.test.ts b/packages/core/tests/state-utils.test.ts
index 0e5b639..fbea130 100644
--- a/packages/core/tests/state-utils.test.ts
+++ b/packages/core/tests/state-utils.test.ts
@@ -1,6 +1,6 @@
 import {describe, expect, it} from 'vitest'
 
-import {getPath, isInteger, isObject, removeKey, setKey, setPath, toPath} from '../src/state-utils'
+import {getPath, isInteger, isObject, removeKey, setKey, setPath, toPath} from '../src/state-utils.js'
 
 const SOURCE = {
 	a: {b: {c: 'c'}},
diff --git a/packages/core/tsconfig.json b/packages/core/tsconfig.json
index cbde135..08dfec6 100644
--- a/packages/core/tsconfig.json
+++ b/packages/core/tsconfig.json
@@ -1,6 +1,6 @@
 {
 	"extends": "@formulier/tsconfig/base",
-	"include": ["src/index.ts"],
+	"files": ["src/index.ts"],
 	"compilerOptions": {
 		"outDir": "dist",
 		"types": ["vitest/importMeta"]
diff --git a/packages/core/tsconfig.lint.json b/packages/core/tsconfig.lint.json
new file mode 100644
index 0000000..cfa64ac
--- /dev/null
+++ b/packages/core/tsconfig.lint.json
@@ -0,0 +1,4 @@
+{
+	"extends": "./tsconfig.json",
+	"include": ["src/**/*", "tests/**/*"]
+}
diff --git a/packages/react/.eslintrc.cjs b/packages/react/.eslintrc.cjs
index 37b6c39..267f462 100644
--- a/packages/react/.eslintrc.cjs
+++ b/packages/react/.eslintrc.cjs
@@ -2,7 +2,7 @@ module.exports = {
 	root: true,
 	extends: ['@formulier/eslint-config'],
 	parserOptions: {
-		project: true,
+		project: './tsconfig.lint.json',
 		tsConfigRootDir: __dirname,
 	},
 }
diff --git a/packages/react/cypress/component/form-tests.cy.tsx b/packages/react/cypress/component/form-tests.cy.tsx
index fd3dcd7..50ded28 100644
--- a/packages/react/cypress/component/form-tests.cy.tsx
+++ b/packages/react/cypress/component/form-tests.cy.tsx
@@ -1,6 +1,6 @@
 import * as React from 'react'
 
-import {FormProvider, useCreateForm, useFormField, useFormInstance} from '../../src'
+import {FormProvider, useCreateForm, useFormField, useFormInstance} from '../../src/index.js'
 
 function Form({children}: {children: React.ReactNode}) {
 	const form = useCreateForm({
diff --git a/packages/react/cypress/support/component.tsx b/packages/react/cypress/support/component.tsx
index 8e8f5ac..4e0fa36 100644
--- a/packages/react/cypress/support/component.tsx
+++ b/packages/react/cypress/support/component.tsx
@@ -1,7 +1,7 @@
 import * as React from 'react'
 import {mount} from 'cypress/react18'
 
-import './commands'
+import './commands.js'
 
 declare global {
 	// eslint-disable-next-line @typescript-eslint/no-namespace
diff --git a/packages/react/tests/use-create-form.test.ts b/packages/react/tests/use-create-form.test.ts
index c3ed365..e13e30c 100644
--- a/packages/react/tests/use-create-form.test.ts
+++ b/packages/react/tests/use-create-form.test.ts
@@ -1,8 +1,8 @@
 import {expect, it} from 'vitest'
 import {renderHook} from '@testing-library/react'
 
-import {useCreateForm} from '../src/form'
-import {useFormField} from '../src/field'
+import {useCreateForm} from '../src/form.js'
+import {useFormField} from '../src/field.js'
 
 interface FormState {
 	a: {b: {c: string; d: string}}
diff --git a/packages/react/tests/use-form-field.test.ts b/packages/react/tests/use-form-field.test.ts
index 7911387..1406f9b 100644
--- a/packages/react/tests/use-form-field.test.ts
+++ b/packages/react/tests/use-form-field.test.ts
@@ -1,10 +1,10 @@
 import {expect, it} from 'vitest'
 import {renderHook} from '@testing-library/react'
 
-import {useCreateForm} from '../src/form'
-import {useFormField} from '../src/field'
+import {useCreateForm} from '../src/form.js'
+import {useFormField} from '../src/field.js'
 
-import type {FieldOptions} from '../src/types'
+import type {FieldOptions} from '../src/types.js'
 
 interface FormState {
 	a: {b: {c: string; d: string}}
diff --git a/packages/react/tsconfig.json b/packages/react/tsconfig.json
index 9571fbe..a673caf 100644
--- a/packages/react/tsconfig.json
+++ b/packages/react/tsconfig.json
@@ -3,7 +3,7 @@
 	"files": ["src/index.ts"],
 	"compilerOptions": {
 		"outDir": "dist",
-		"jsx": "react-jsx",
-		"types": ["vitest/importMeta"]
+		"types": ["vitest/importMeta"],
+		"jsx": "react-jsx"
 	}
 }
diff --git a/packages/react/tsconfig.lint.json b/packages/react/tsconfig.lint.json
new file mode 100644
index 0000000..9fb6d41
--- /dev/null
+++ b/packages/react/tsconfig.lint.json
@@ -0,0 +1,4 @@
+{
+	"extends": "./tsconfig.json",
+	"include": ["src/**/*", "tests/**/*", "cypress/**/*"]
+}
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index fe84026..6d35e5d 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -45,7 +45,11 @@ importers:
         specifier: ^5.3.3
         version: 5.3.3
 
-  config/eslint-config: {}
+  config/eslint-config:
+    devDependencies:
+      eslint-plugin-require-extensions:
+        specifier: ^0.1.3
+        version: 0.1.3(eslint@8.56.0)
 
   config/tsconfig: {}
 
@@ -3013,6 +3017,15 @@ packages:
       string.prototype.matchall: 4.0.10
     dev: true
 
+  /eslint-plugin-require-extensions@0.1.3(eslint@8.56.0):
+    resolution: {integrity: sha512-T3c1PZ9PIdI3hjV8LdunfYI8gj017UQjzAnCrxuo3wAjneDbTPHdE3oNWInOjMA+z/aBkUtlW5vC0YepYMZIug==}
+    engines: {node: '>=16'}
+    peerDependencies:
+      eslint: '*'
+    dependencies:
+      eslint: 8.56.0
+    dev: true
+
   /eslint-scope@7.2.2:
     resolution: {integrity: sha512-dOt21O7lTMhDM+X9mB4GX+DZrZtCUJPL/wlcTqxyrx5IvO0IYtILdtrQGQp+8n5S0gwSVmOf9NQrjMOgfQZlIg==}
     engines: {node: ^12.22.0 || ^14.17.0 || >=16.0.0}