typescript-eslint · kirkwaiblinger · Jun 30, 2025 · Jun 30, 2025 · Jun 30, 2025 · Jun 30, 2025
diff --git a/packages/typescript-eslint/src/config-helper.ts b/packages/typescript-eslint/src/config-helper.ts
@@ -174,6 +174,13 @@ function configImpl(...configs: unknown[]): ConfigArray {
               ` 'basePath' in 'extends' is not allowed.`,
           );
         }
+
+        if ('extends' in extension) {
+          throw new TypeError(
+            `tseslint.config(): Config at index ${configIndex}${nameErrorPhrase} has an 'extends' array that contains a config with an 'extends' property at index ${extensionIndex}.` +
+              ` Nested 'extends' is not allowed.`,
+          );
+        }
       }
       if (nonObjectExtensions.length > 0) {
         const extensionIndices = nonObjectExtensions.join(', ');

diff --git a/packages/typescript-eslint/tests/config-helper.test.ts b/packages/typescript-eslint/tests/config-helper.test.ts
@@ -416,4 +416,19 @@ describe('config helper', () => {
       "tseslint.config(): Config at index 0 (anonymous) has an 'extends' array that contains a config with a 'basePath' property at index 0. 'basePath' in 'extends' is not allowed.",
     );
   });
+
+  it('should error when trying to use nested extends', () => {
+    expect(() => {
+      tseslint.config({
+        extends: [
+          {
+            extends: [
+              { rules: { rule1: 'error' } },
+              { rules: { rule2: 'error' } },
+            ],
+          },
+        ],
+      });
+    }).toThrow();
+  });
 });