diff --git a/apps/api-extractor/src/api/ExtractorConfig.ts b/apps/api-extractor/src/api/ExtractorConfig.ts
index f3e33f4682..080902d37f 100644
--- a/apps/api-extractor/src/api/ExtractorConfig.ts
+++ b/apps/api-extractor/src/api/ExtractorConfig.ts
@@ -33,6 +33,7 @@ import { PackageMetadataManager } from '../analyzer/PackageMetadataManager';
 import { MessageRouter } from '../collector/MessageRouter';
 import type { IApiModelGenerationOptions } from '../generators/ApiModelGenerator';
 import apiExtractorSchema from '../schemas/api-extractor.schema.json';
+import apiExtractorDefaults from '../schemas/api-extractor-defaults.json';
 
 /**
  * Tokens used during variable expansion of path fields from api-extractor.json.
@@ -228,9 +229,9 @@ interface IExtractorConfigParameters {
   enumMemberOrder: EnumMemberOrder;
 }
 
-const _defaultConfig: Partial<IConfigFile> = JsonFile.load(
-  path.join(__dirname, '../schemas/api-extractor-defaults.json')
-);
+// The defaults file deliberately omits fields that are required once a section is used
+// (for example "apiReport.enabled"), so it does not structurally match IConfigFile.
+const _defaultConfig: Partial<IConfigFile> = apiExtractorDefaults as unknown as Partial<IConfigFile>;
 
 /**
  * Match all three flavors for type declaration files (.d.ts, .d.mts, .d.cts)
diff --git a/apps/api-extractor/src/schemas/api-extractor-defaults.json b/apps/api-extractor/src/schemas/api-extractor-defaults.json
index bb6b6157aa..a5401209c4 100644
--- a/apps/api-extractor/src/schemas/api-extractor-defaults.json
+++ b/apps/api-extractor/src/schemas/api-extractor-defaults.json
@@ -1,8 +1,6 @@
 {
   "projectFolder": "<lookup>",
 
-  // ("mainEntryPointFilePath" is required)
-
   "bundledPackages": [],
 
   "newlineKind": "crlf",
@@ -15,7 +13,6 @@
   },
 
   "apiReport": {
-    // ("enabled" is required)
     "reportFileName": "<unscopedPackageName>.api.md",
     "reportFolder": "<projectFolder>/etc/",
     "reportTempFolder": "<projectFolder>/temp/",
@@ -23,13 +20,11 @@
   },
 
   "docModel": {
-    // ("enabled" is required)
     "apiJsonFilePath": "<projectFolder>/temp/<unscopedPackageName>.api.json",
     "includeForgottenExports": false
   },
 
   "dtsRollup": {
-    // ("enabled" is required)
     "untrimmedFilePath": "<projectFolder>/dist/<unscopedPackageName>.d.ts",
     "alphaTrimmedFilePath": "",
     "betaTrimmedFilePath": "",
diff --git a/common/changes/@microsoft/api-extractor/fix-api-extractor-esm-bundling_2026-08-17-04-39-03.json b/common/changes/@microsoft/api-extractor/fix-api-extractor-esm-bundling_2026-08-17-04-39-03.json
new file mode 100644
index 0000000000..98066ac42c
--- /dev/null
+++ b/common/changes/@microsoft/api-extractor/fix-api-extractor-esm-bundling_2026-08-17-04-39-03.json
@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "comment": "Fix an error that occurred when `@microsoft/api-extractor` was bundled into ESM output, caused by loading the default config from a `__dirname`-relative path at module initialization.",
+      "type": "patch",
+      "packageName": "@microsoft/api-extractor"
+    }
+  ],
+  "packageName": "@microsoft/api-extractor"
+}