diff --git a/packages/rtk-query-codegen-openapi/src/index.ts b/packages/rtk-query-codegen-openapi/src/index.ts
index 41a251d63d..7b68762719 100644
--- a/packages/rtk-query-codegen-openapi/src/index.ts
+++ b/packages/rtk-query-codegen-openapi/src/index.ts
@@ -8,6 +8,14 @@ export type { ConfigFile } from './types';
 
 const require = createRequire(__filename);
 
+export async function generateEndpoints(
+  options: Omit<GenerationOptions, 'outputFile'> & { outputFile?: never }
+): Promise<string>;
+
+export async function generateEndpoints(
+  options: Omit<GenerationOptions, 'outputFile'> & Required<Pick<GenerationOptions, 'outputFile'>>
+): Promise<void>;
+
 export async function generateEndpoints(options: GenerationOptions): Promise<string | void> {
   const schemaLocation = options.schemaFile;
 
diff --git a/packages/rtk-query-codegen-openapi/test/generateEndpoints.test-d.ts b/packages/rtk-query-codegen-openapi/test/generateEndpoints.test-d.ts
new file mode 100644
index 0000000000..2d4ba28bc4
--- /dev/null
+++ b/packages/rtk-query-codegen-openapi/test/generateEndpoints.test-d.ts
@@ -0,0 +1,31 @@
+import { generateEndpoints } from '@rtk-query/codegen-openapi';
+import * as path from 'node:path';
+
+const withoutOutputFile = {
+  apiFile: './fixtures/emptyApi.ts',
+  schemaFile: path.join(__dirname, 'fixtures', 'petstore.json'),
+};
+
+const withOutputFile = {
+  apiFile: './fixtures/emptyApi.ts',
+  outputFile: './test/tmp/out.ts',
+  schemaFile: path.join(__dirname, 'fixtures', 'petstore.json'),
+};
+
+describe('generateEndpoints return type narrowing', () => {
+  test('narrows to Promise<string> when outputFile is omitted', () => {
+    expectTypeOf(generateEndpoints(withoutOutputFile)).toEqualTypeOf<Promise<string>>();
+
+    expectTypeOf(generateEndpoints(withoutOutputFile)).resolves.toBeString();
+
+    expectTypeOf(generateEndpoints).toBeCallableWith(withoutOutputFile).returns.resolves.toBeString();
+  });
+
+  test('narrows to Promise<void> when outputFile is provided', () => {
+    expectTypeOf(generateEndpoints(withOutputFile)).toEqualTypeOf<Promise<void>>();
+
+    expectTypeOf(generateEndpoints(withOutputFile)).resolves.toBeVoid();
+
+    expectTypeOf(generateEndpoints).toBeCallableWith(withOutputFile).returns.resolves.toBeVoid();
+  });
+});
diff --git a/packages/rtk-query-codegen-openapi/test/generateEndpoints.test.ts b/packages/rtk-query-codegen-openapi/test/generateEndpoints.test.ts
index 20c895219d..c452a4253b 100644
--- a/packages/rtk-query-codegen-openapi/test/generateEndpoints.test.ts
+++ b/packages/rtk-query-codegen-openapi/test/generateEndpoints.test.ts
@@ -933,3 +933,26 @@ describe('esmExtensions option', () => {
     expect(content).toContain("import { api } from '../../fixtures/emptyApi'");
   });
 });
+
+describe('generateEndpoints return type narrowing', () => {
+  const schemaFile = resolve(__dirname, 'fixtures', 'petstore.json');
+
+  test('returns a string when outputFile is omitted', async () => {
+    const result = await generateEndpoints({
+      apiFile: './fixtures/emptyApi.ts',
+      schemaFile,
+    });
+
+    expect(result).toBeTypeOf('string');
+  });
+
+  test('returns void when outputFile is provided', async () => {
+    const result = await generateEndpoints({
+      apiFile: './fixtures/emptyApi.ts',
+      outputFile: './test/tmp/out.ts',
+      schemaFile,
+    });
+
+    expect(result).toBeUndefined();
+  });
+});
diff --git a/packages/rtk-query-codegen-openapi/vitest.config.mts b/packages/rtk-query-codegen-openapi/vitest.config.mts
index de73ff27d2..d411d934b9 100644
--- a/packages/rtk-query-codegen-openapi/vitest.config.mts
+++ b/packages/rtk-query-codegen-openapi/vitest.config.mts
@@ -1,23 +1,42 @@
-import path from 'node:path';
-import { fileURLToPath } from 'node:url';
+import * as path from 'node:path';
 import tsconfigPaths from 'vite-tsconfig-paths';
 import { defineConfig } from 'vitest/config';
-
-// No __dirname under Node ESM
-const __filename = fileURLToPath(import.meta.url);
-const __dirname = path.dirname(__filename);
+import packageJson from './package.json' with { type: 'json' };
 
 export default defineConfig({
-  plugins: [tsconfigPaths({ projects: ['./tsconfig.json'] })],
+  plugins: [
+    tsconfigPaths({
+      configNames: ['tsconfig.json'],
+      projects: ['./tsconfig.json'],
+      root: import.meta.dirname,
+    }),
+  ],
+
+  root: import.meta.dirname,
+
   test: {
+    name: {
+      label: packageJson.name,
+    },
+
     alias: process.env.TEST_DIST
       ? {
-          '@rtk-query/codegen-openapi': path.join(__dirname, '../..', 'node_modules/@rtk-query/codegen-openapi'),
+          '@rtk-query/codegen-openapi': path.join(import.meta.dirname, '..', '..', 'node_modules', packageJson.name),
         }
       : undefined,
+
+    dir: path.join(import.meta.dirname, 'test'),
+    root: import.meta.dirname,
     testTimeout: 10_000,
+
+    typecheck: {
+      enabled: true,
+      tsconfig: path.join(import.meta.dirname, 'tsconfig.json'),
+    },
+
     pool: 'forks',
     globals: true,
     setupFiles: ['./test/vitest.setup.ts'],
+    watch: false,
   },
 });
diff --git a/yarn.lock b/yarn.lock
index 98e3fb4c30..65d31d73da 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -10230,27 +10230,27 @@ __metadata:
   languageName: node
   linkType: hard
 
-"@vitest/expect@npm:4.0.4":
-  version: 4.0.4
-  resolution: "@vitest/expect@npm:4.0.4"
+"@vitest/expect@npm:4.0.18":
+  version: 4.0.18
+  resolution: "@vitest/expect@npm:4.0.18"
   dependencies:
     "@standard-schema/spec": "npm:^1.0.0"
     "@types/chai": "npm:^5.2.2"
-    "@vitest/spy": "npm:4.0.4"
-    "@vitest/utils": "npm:4.0.4"
-    chai: "npm:^6.0.1"
+    "@vitest/spy": "npm:4.0.18"
+    "@vitest/utils": "npm:4.0.18"
+    chai: "npm:^6.2.1"
     tinyrainbow: "npm:^3.0.3"
-  checksum: 10/7d6f4e501c5b84c63fd99259a2b0000b81b075fd62c1a4ff479c50136d46d6cf90bcdd514f6304bf972fbba148949fda23ebd2926fa26f0fd428f1010f1cff63
+  checksum: 10/2115bff1bbcad460ce72032022e4dbcf8572c4b0fe07ca60f5644a8d96dd0dfa112986b5a1a5c5705f4548119b3b829c45d1de0838879211e0d6bb276b4ece73
   languageName: node
   linkType: hard
 
-"@vitest/mocker@npm:4.0.4":
-  version: 4.0.4
-  resolution: "@vitest/mocker@npm:4.0.4"
+"@vitest/mocker@npm:4.0.18":
+  version: 4.0.18
+  resolution: "@vitest/mocker@npm:4.0.18"
   dependencies:
-    "@vitest/spy": "npm:4.0.4"
+    "@vitest/spy": "npm:4.0.18"
     estree-walker: "npm:^3.0.3"
-    magic-string: "npm:^0.30.19"
+    magic-string: "npm:^0.30.21"
   peerDependencies:
     msw: ^2.4.9
     vite: ^6.0.0 || ^7.0.0-0
@@ -10259,54 +10259,54 @@ __metadata:
       optional: true
     vite:
       optional: true
-  checksum: 10/070ddbd7f9907482aa1812e01d13ff66cea540479435a857072fc7528f4b583d62f0600eed112b5780e9698812c944b679fc959e764670de23621eba9be9e177
+  checksum: 10/46f584a4c1180dfb513137bc8db6e2e3b53e141adfe964307297e98321652d86a3f2a52d80cda1f810205bd5fdcab789bb8b52a532e68f175ef1e20be398218d
   languageName: node
   linkType: hard
 
-"@vitest/pretty-format@npm:4.0.4":
-  version: 4.0.4
-  resolution: "@vitest/pretty-format@npm:4.0.4"
+"@vitest/pretty-format@npm:4.0.18":
+  version: 4.0.18
+  resolution: "@vitest/pretty-format@npm:4.0.18"
   dependencies:
     tinyrainbow: "npm:^3.0.3"
-  checksum: 10/64b0430012f8c34748a39dd1ea061f51d17053086d961830e93a4083b7cda50d822d6db8154398d7f800e15e1efc45c6460d7c1c5f06e21f09a282663824b9c9
+  checksum: 10/4cafc7c9853097345bd94e8761bf47c2c04e00d366ac56d79928182787ff83c512c96f1dc2ce9b6aeed4d3a8c23ce12254da203783108d3c096bc398eed2a62d
   languageName: node
   linkType: hard
 
-"@vitest/runner@npm:4.0.4":
-  version: 4.0.4
-  resolution: "@vitest/runner@npm:4.0.4"
+"@vitest/runner@npm:4.0.18":
+  version: 4.0.18
+  resolution: "@vitest/runner@npm:4.0.18"
   dependencies:
-    "@vitest/utils": "npm:4.0.4"
+    "@vitest/utils": "npm:4.0.18"
     pathe: "npm:^2.0.3"
-  checksum: 10/3bd7e8944d8770ae7dd7457fb83751def5cb4834766cb86b7231cc0fd77d4e889b5ec6da70ae82e06902b19c216f114a5a670e4422ba2db8384dee09dc24b832
+  checksum: 10/d7deebf086d7e084f449733ecea6c9c81737a18aafece318cbe7500e45debea00fa9dbf9315fd38aa88550dd5240a791b885ac71665f89b154d71a6c63da5836
   languageName: node
   linkType: hard
 
-"@vitest/snapshot@npm:4.0.4":
-  version: 4.0.4
-  resolution: "@vitest/snapshot@npm:4.0.4"
+"@vitest/snapshot@npm:4.0.18":
+  version: 4.0.18
+  resolution: "@vitest/snapshot@npm:4.0.18"
   dependencies:
-    "@vitest/pretty-format": "npm:4.0.4"
-    magic-string: "npm:^0.30.19"
+    "@vitest/pretty-format": "npm:4.0.18"
+    magic-string: "npm:^0.30.21"
     pathe: "npm:^2.0.3"
-  checksum: 10/dba29abb40e8a4e8931a9522076f0884a579280c440d1e7591c32c4f8d75366b18e3f78920eed13337909384767d0403d15f384275f22d7c0bc5c36a0c4f5482
+  checksum: 10/50aa5fb7fca45c499c145cc2f20e53b8afb0990b53ff4a4e6447dd6f147437edc5316f22e2d82119e154c3cf7c59d44898e7b2faf7ba614ac1051cbe4d662a77
   languageName: node
   linkType: hard
 
-"@vitest/spy@npm:4.0.4":
-  version: 4.0.4
-  resolution: "@vitest/spy@npm:4.0.4"
-  checksum: 10/f3bca9c53840c6f271d676a5b7a06535eb4a80e4b2e52e179ff56cb53f7f725ba1c683cb13d2ef1283057c09bf260634b9b9b318bd2a1755853c067bc9a0dd6a
+"@vitest/spy@npm:4.0.18":
+  version: 4.0.18
+  resolution: "@vitest/spy@npm:4.0.18"
+  checksum: 10/f7b1618ae13790105771dd2a8c973c63c018366fcc69b50f15ce5d12f9ac552efd3c1e6e5ae4ebdb6023d0b8d8f31fef2a0b1b77334284928db45c80c63de456
   languageName: node
   linkType: hard
 
-"@vitest/utils@npm:4.0.4":
-  version: 4.0.4
-  resolution: "@vitest/utils@npm:4.0.4"
+"@vitest/utils@npm:4.0.18":
+  version: 4.0.18
+  resolution: "@vitest/utils@npm:4.0.18"
   dependencies:
-    "@vitest/pretty-format": "npm:4.0.4"
+    "@vitest/pretty-format": "npm:4.0.18"
     tinyrainbow: "npm:^3.0.3"
-  checksum: 10/032aa6fad679efe270015bd0ceec55915b3ff4597f5e0b4ee9ae2ac54d5e8a2f903ef64533cc73f05264d8959c01104a32283c2b0298f4e903a003c9bf427514
+  checksum: 10/e8b2ad7bc35b2bc5590f9dc1d1a67644755da416b47ab7099a6f26792903fa0aacb81e6ba99f0f03858d9d3a1d76eeba65150a1a0849690a40817424e749c367
   languageName: node
   linkType: hard
 
@@ -12271,10 +12271,10 @@ __metadata:
   languageName: node
   linkType: hard
 
-"chai@npm:^6.0.1":
-  version: 6.2.0
-  resolution: "chai@npm:6.2.0"
-  checksum: 10/199422854e253d8711ea3f220365c6a850c450abf68b31131d2a0f703cbfc5cb48e6c81567e0adbe80e83cdcae6dba82d069a41a77c16bdf6703329c5c3447ef
+"chai@npm:^6.2.1":
+  version: 6.2.2
+  resolution: "chai@npm:6.2.2"
+  checksum: 10/13cda42cc40aa46da04a41cf7e5c61df6b6ae0b4e8a8c8b40e04d6947e4d7951377ea8c14f9fa7fe5aaa9e8bd9ba414f11288dc958d4cee6f5221b9436f2778f
   languageName: node
   linkType: hard
 
@@ -14127,18 +14127,6 @@ __metadata:
   languageName: node
   linkType: hard
 
-"debug@npm:^4.4.3":
-  version: 4.4.3
-  resolution: "debug@npm:4.4.3"
-  dependencies:
-    ms: "npm:^2.1.3"
-  peerDependenciesMeta:
-    supports-color:
-      optional: true
-  checksum: 10/9ada3434ea2993800bd9a1e320bd4aa7af69659fb51cca685d390949434bc0a8873c21ed7c9b852af6f2455a55c6d050aa3937d52b3c69f796dab666f762acad
-  languageName: node
-  linkType: hard
-
 "decamelize@npm:^1.2.0":
   version: 1.2.0
   resolution: "decamelize@npm:1.2.0"
@@ -16179,9 +16167,9 @@ __metadata:
   linkType: hard
 
 "expect-type@npm:^1.2.2":
-  version: 1.2.2
-  resolution: "expect-type@npm:1.2.2"
-  checksum: 10/1703e6e47b575f79d801d87f24c639f4d0af71b327a822e6922d0ccb7eb3f6559abb240b8bd43bab6a477903de4cc322908e194d05132c18f52a217115e8e870
+  version: 1.3.0
+  resolution: "expect-type@npm:1.3.0"
+  checksum: 10/a5fada3d0c621649261f886e7d93e6bf80ce26d8a86e5d517e38301b8baec8450ab2cb94ba6e7a0a6bf2fc9ee55f54e1b06938ef1efa52ddcfeffbfa01acbbcc
   languageName: node
   linkType: hard
 
@@ -21440,7 +21428,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"magic-string@npm:^0.30.19":
+"magic-string@npm:^0.30.21":
   version: 0.30.21
   resolution: "magic-string@npm:0.30.21"
   dependencies:
@@ -23874,6 +23862,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"obug@npm:^2.1.1":
+  version: 2.1.1
+  resolution: "obug@npm:2.1.1"
+  checksum: 10/bdcf9213361786688019345f3452b95a1dc73710e4b403c82a1994b98bad6abc31b26cb72a482128c5fd53ea9daf6fbb7d0e0e7b2b7e9c8be6d779deeccee07f
+  languageName: node
+  linkType: hard
+
 "on-finished@npm:2.4.1":
   version: 2.4.1
   resolution: "on-finished@npm:2.4.1"
@@ -29894,6 +29889,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"std-env@npm:^3.10.0":
+  version: 3.10.0
+  resolution: "std-env@npm:3.10.0"
+  checksum: 10/19c9cda4f370b1ffae2b8b08c72167d8c3e5cfa972aaf5c6873f85d0ed2faa729407f5abb194dc33380708c00315002febb6f1e1b484736bfcf9361ad366013a
+  languageName: node
+  linkType: hard
+
 "std-env@npm:^3.7.0":
   version: 3.9.0
   resolution: "std-env@npm:3.9.0"
@@ -29901,13 +29903,6 @@ __metadata:
   languageName: node
   linkType: hard
 
-"std-env@npm:^3.9.0":
-  version: 3.10.0
-  resolution: "std-env@npm:3.10.0"
-  checksum: 10/19c9cda4f370b1ffae2b8b08c72167d8c3e5cfa972aaf5c6873f85d0ed2faa729407f5abb194dc33380708c00315002febb6f1e1b484736bfcf9361ad366013a
-  languageName: node
-  linkType: hard
-
 "stop-iteration-iterator@npm:^1.0.0":
   version: 1.1.0
   resolution: "stop-iteration-iterator@npm:1.1.0"
@@ -30834,6 +30829,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"tinyexec@npm:^1.0.2":
+  version: 1.0.2
+  resolution: "tinyexec@npm:1.0.2"
+  checksum: 10/cb709ed4240e873d3816e67f851d445f5676e0ae3a52931a60ff571d93d388da09108c8057b62351766133ee05ff3159dd56c3a0fbd39a5933c6639ce8771405
+  languageName: node
+  linkType: hard
+
 "tinyglobby@npm:^0.2.11, tinyglobby@npm:^0.2.12":
   version: 0.2.13
   resolution: "tinyglobby@npm:0.2.13"
@@ -32272,43 +32274,43 @@ __metadata:
   linkType: hard
 
 "vitest@npm:^4":
-  version: 4.0.4
-  resolution: "vitest@npm:4.0.4"
-  dependencies:
-    "@vitest/expect": "npm:4.0.4"
-    "@vitest/mocker": "npm:4.0.4"
-    "@vitest/pretty-format": "npm:4.0.4"
-    "@vitest/runner": "npm:4.0.4"
-    "@vitest/snapshot": "npm:4.0.4"
-    "@vitest/spy": "npm:4.0.4"
-    "@vitest/utils": "npm:4.0.4"
-    debug: "npm:^4.4.3"
+  version: 4.0.18
+  resolution: "vitest@npm:4.0.18"
+  dependencies:
+    "@vitest/expect": "npm:4.0.18"
+    "@vitest/mocker": "npm:4.0.18"
+    "@vitest/pretty-format": "npm:4.0.18"
+    "@vitest/runner": "npm:4.0.18"
+    "@vitest/snapshot": "npm:4.0.18"
+    "@vitest/spy": "npm:4.0.18"
+    "@vitest/utils": "npm:4.0.18"
     es-module-lexer: "npm:^1.7.0"
     expect-type: "npm:^1.2.2"
-    magic-string: "npm:^0.30.19"
+    magic-string: "npm:^0.30.21"
+    obug: "npm:^2.1.1"
     pathe: "npm:^2.0.3"
     picomatch: "npm:^4.0.3"
-    std-env: "npm:^3.9.0"
+    std-env: "npm:^3.10.0"
     tinybench: "npm:^2.9.0"
-    tinyexec: "npm:^0.3.2"
+    tinyexec: "npm:^1.0.2"
     tinyglobby: "npm:^0.2.15"
     tinyrainbow: "npm:^3.0.3"
     vite: "npm:^6.0.0 || ^7.0.0"
     why-is-node-running: "npm:^2.3.0"
   peerDependencies:
     "@edge-runtime/vm": "*"
-    "@types/debug": ^4.1.12
+    "@opentelemetry/api": ^1.9.0
     "@types/node": ^20.0.0 || ^22.0.0 || >=24.0.0
-    "@vitest/browser-playwright": 4.0.4
-    "@vitest/browser-preview": 4.0.4
-    "@vitest/browser-webdriverio": 4.0.4
-    "@vitest/ui": 4.0.4
+    "@vitest/browser-playwright": 4.0.18
+    "@vitest/browser-preview": 4.0.18
+    "@vitest/browser-webdriverio": 4.0.18
+    "@vitest/ui": 4.0.18
     happy-dom: "*"
     jsdom: "*"
   peerDependenciesMeta:
     "@edge-runtime/vm":
       optional: true
-    "@types/debug":
+    "@opentelemetry/api":
       optional: true
     "@types/node":
       optional: true
@@ -32326,7 +32328,7 @@ __metadata:
       optional: true
   bin:
     vitest: vitest.mjs
-  checksum: 10/5a7cd573e1a1daeaa7d874d9a676945aab67c1b5167119e903d50be33f3490f919ea575119efdc473a42d8c42b176bbbc31c12f3c7f1fdd2eb817cc8e162c10a
+  checksum: 10/6c6464ebcf3af83546862896fd1b5f10cb6607261bffce39df60033a288b8c1687ae1dd20002b6e4997a7a05303376d1eb58ce20afe63be052529a4378a8c165
   languageName: node
   linkType: hard