Nuxt `multiApp` breaks nuxt/test-utils

[RaphaelDDL]

Environment

Nuxt project info (That's stackblitz's terminal info):

Operating system	Linux 5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/141.0.0.0 Safari/537.36
CPU	Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz (4 cores)
Node.js version	v20.19.1
nuxt/cli version	3.31.3
Package manager	npm@10.8.2
Nuxt version	4.2.2
Nitro version	2.12.9
Builder	vite@7.3.0
Config	compatibilityDate, devtools, future, modules
Modules	@nuxt/test-utils/module@3.21.0

Reproduction

https://stackblitz.com/edit/github-sqshpc8h?file=nuxt.config.ts&view=editor

(using stackblitz's template for github/nuxt/test-utils/tree/main/examples/app-vitest, but I removed all non-necessary tests)

Describe the bug

We are moving a project from Vue to Nuxt and we are using future: { multiApp: true } as part of this project's solution. When moving tests, we noticed they don't work and are throwing errors that other projects weren't. Upon further investigation, we pinpointed to the multiApp flag.

Running tests (npm run test) with future: { multiApp: false } passes tests. When changing to future: { multiApp: true }, we got the following errors:

During vitest running the file:

stderr | test/app.nuxt.spec.ts
[nuxt] error caught during app initialization TypeError: Cannot read properties of undefined (reading 'app')
    at setup (/home/projects/jvmikwmyxv.github/node_modules/nuxt/dist/app/plugins/router.js:48:139)
    at eval (/home/projects/jvmikwmyxv.github/node_modules/nuxt/dist/app/nuxt.js:156:60)
    at fn (/home/projects/jvmikwmyxv.github/node_modules/nuxt/dist/app/nuxt.js:238:44)
    at Object.runWithContext (/home/projects/jvmikwmyxv.github/node_modules/@vue/runtime-core/dist/runtime-core.cjs.js:4218:18)
    at callWithNuxt (/home/projects/jvmikwmyxv.github/node_modules/nuxt/dist/app/nuxt.js:244:24)
    at eval (/home/projects/jvmikwmyxv.github/node_modules/nuxt/dist/app/nuxt.js:55:41)
    at EffectScope.run (/home/projects/jvmikwmyxv.github/node_modules/@vue/reactivity/dist/reactivity.cjs.js:85:16)
    at Object.runWithContext (/home/projects/jvmikwmyxv.github/node_modules/nuxt/dist/app/nuxt.js:55:31)
    at applyPlugin (/home/projects/jvmikwmyxv.github/node_modules/nuxt/dist/app/nuxt.js:156:39)
    at executePlugin (/home/projects/jvmikwmyxv.github/node_modules/nuxt/dist/app/nuxt.js:175:23)

which is referencing to NuxtPlugin nuxt:router's setup where useRuntimeConfig is not returning an app in useRuntimeConfig().app.baseURL

and Vitest report after finished running shows the following:

 FAIL   nuxt  test/app.nuxt.spec.ts [ test/app.nuxt.spec.ts ]
TypeError: Cannot read properties of undefined (reading 'afterEach')
 ❯ setupNuxt node_modules/@nuxt/test-utils/dist/runtime/shared/nuxt.mjs:6:14
      4|   const nuxtApp = useNuxtApp();
      5|   await nuxtApp.callHook("page:finish");
      6|   useRouter().afterEach(() => nuxtApp.callHook("page:finish"));
       |              ^
      7| }
      8| 
 ❯ eval node_modules/@nuxt/test-utils/dist/runtime/entry.mjs:4:3

There seems to be some kind of issue in Router when multiApp is preset. We do have appId and app.rootId set in nuxt.config, but this made no change to the tests so I did not include in the stackblitz.