[api-extractor] Add support for new TS declaration format when using module resolution 'bundler' or 'nodenext'

apps/api-extractor/src/api/ExtractorConfig.ts

@@ -258,8 +258,11 @@ export class ExtractorConfig {
     path.join(__dirname, '../schemas/api-extractor-defaults.json')
   );
 
-  /** Match all three flavors for type declaration files (.d.ts, .d.mts, .d.cts) */
-  private static readonly _declarationFileExtensionRegExp: RegExp = /\.d\.(c|m)?ts$/i;
+  /**
+   * Match all three flavors for type declaration files (.d.ts, .d.mts, .d.cts)
+   * including the new TS 5 bundle resolutions (.d.\{extension\}.ts, .d.\{extension\}.mts, .d.\{extension\}.cts)
+   **/
+  private static readonly _declarationFileExtensionRegExp: RegExp = /\.d(\.[^./\\]+)?\.(c|m)?ts$/i;
 
   /** {@inheritDoc IConfigFile.projectFolder} */
   public readonly projectFolder: string;

apps/api-extractor/src/api/IConfigFile.ts

@@ -450,7 +450,7 @@ export interface IConfigFile {
    *
    * @remarks
    *
-   * The file extension must be ".d.ts" and not ".ts".
+   * The file extension must be a declaration file (e.g. ".d.ts", ".d.mts", ".d.\{extension\}.ts"), not ".ts".
    * The path is resolved relative to the "projectFolder" location.
    */
   mainEntryPointFilePath: string;

apps/api-extractor/src/api/test/ExtractorConfig.test.ts

@@ -0,0 +1,31 @@
+// Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT license.
+// See LICENSE in the project root for license information.
+
+import { ExtractorConfig } from '../ExtractorConfig';
+
+describe('ExtractorConfig', () => {
+  describe('hasDtsFileExtension', () => {
+    it.each([
+      ['test.ts', false],
+      ['test.cts', false],
+      ['test.mts', false],
+      ['test.d.ts', true],
+      ['test.d.mts', true],
+      ['test.d.cts', true],
+      ['test.css', false],
+      ['test.css.ts', false],
+      ['test.css.d.ts', true],
+      ['test.d.css.ts', true],
+      ['test.json', false],
+      ['test.json.ts', false],
+      ['test.json.d.ts', true],
+      ['test.d.json.ts', true],
+      ['video.d.mp4.ts', true],
+      ['font.d.woff2.ts', true],
+      ['model.d.3mf.ts', true]
+    ])('file "%s" has dts file extension equals "%s"', (file, expected) => {
+      const result = ExtractorConfig.hasDtsFileExtension(file);
+      expect(result).toEqual(expected);
+    });
+  });
+});

apps/api-extractor/src/schemas/api-extractor-template.json

@@ -38,7 +38,7 @@
    * (REQUIRED) Specifies the .d.ts file to be used as the starting point for analysis.  API Extractor
    * analyzes the symbols exported by this module.
    *
-   * The file extension must be ".d.ts" and not ".ts".
+   * The file extension must be a declaration file (e.g. ".d.ts", ".d.mts", ".d.{extension}.ts"), not ".ts".
    *
    * The path is resolved relative to the folder of the config file that contains the setting; to change this,
    * prepend a folder token such as "<projectFolder>".

build-tests-samples/heft-web-rig-library-tutorial/config/api-extractor.json

@@ -40,7 +40,7 @@
    * (REQUIRED) Specifies the .d.ts file to be used as the starting point for analysis.  API Extractor
    * analyzes the symbols exported by this module.
    *
-   * The file extension must be ".d.ts" and not ".ts".
+   * The file extension must be a declaration file (e.g. ".d.ts", ".d.mts", ".d.{extension}.ts"), not ".ts".
    *
    * The path is resolved relative to the folder of the config file that contains the setting; to change this,
    * prepend a folder token such as "<projectFolder>".

common/changes/@microsoft/api-extractor/bartvandenende-wm-4899_2026-06-08-04-32.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@microsoft/api-extractor",
+      "comment": "Add support for new d.ts extension format when using TS moduleResolution 'bundler' or 'nodenext'.",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@microsoft/api-extractor"
+}