Running Unit Tests on a New Project give an " transport was disconnected, cannot call "fetchModule" error

[Reboog711]

Describe the bug

I've created a brand new Svelte Project. Before I make any changes, I run npm run test and am getting this error:

12:55:39 PM [vite] (ssr) Error when evaluating SSR module /@fs/C:/Projects/blog/svelte-test-2/node_modules/@sveltejs/kit/src/runtime/server/index.js: transport was disconnected, cannot call "fetchModule"
at reviveInvokeError (file:///C:/Projects/blog/svelte-test-2/node_modules/vite/dist/node/module-runner.js:475:14)
at Object.invoke (file:///C:/Projects/blog/svelte-test-2/node_modules/vite/dist/node/module-runner.js:549:11)
at async SSRCompatModuleRunner.getModuleInformation (file:///C:/Projects/blog/svelte-test-2/node_modules/vite/dist/node/module-runner.js:1059:7)
at async request (file:///C:/Projects/blog/svelte-test-2/node_modules/vite/dist/node/module-runner.js:1076:83)
at async eval (C:\Projects\blog\svelte-test-2\node_modules@sveltejs\kit\src\runtime\server\page\actions.js:11:1)
at async ESModulesEvaluator.runInlinedModule (file:///C:/Projects/blog/svelte-test-2/node_modules/vite/dist/node/module-runner.js:910:3)
at async SSRCompatModuleRunner.directRequest (file:///C:/Projects/blog/svelte-test-2/node_modules/vite/dist/node/module-runner.js:1119:59)
at async SSRCompatModuleRunner.directRequest (file:///C:/Projects/blog/svelte-test-2/node_modules/vite/dist/node/chunks/dep-Bj7gA1-0.js:18770:22)
at async SSRCompatModuleRunner.cachedRequest (file:///C:/Projects/blog/svelte-test-2/node_modules/vite/dist/node/module-runner.js:1037:73)
at async eval (C:\Projects\blog\svelte-test-2\node_modules@sveltejs\kit\src\runtime\server\page\index.js:7:1)

Reproduction

1. Run npx sv create svelte-test-3 at your console, per instructions
2. Select SvelteKit Minimal
3. Select "Yes, using TypeScript Syntax"
4. Check Vitest
5. Select Unit Testing (But I get the same error if I choose Component Testing)
6. Select npm

7. Run cd svelte-test-3 to change to the project directory
8. run npm run test

C:\Projects\blog\svelte-test-3>npm run test

svelte-test-3@0.0.1 test
npm run test:unit -- --run

svelte-test-3@0.0.1 test:unit
vitest --run

The following Vite config options will be overridden by SvelteKit:

• root
  The following Vite config options will be overridden by SvelteKit:
• root

RUN v3.2.4 C:/Projects/blog/svelte-test-3

The following Vite config options will be overridden by SvelteKit:

• root
  The following Vite config options will be overridden by SvelteKit:
• base
  ✓ server src/demo.spec.ts (1 test) 4ms
  ✓ client (chromium) src/routes/page.svelte.spec.ts (1 test) 9ms

Test Files 2 passed (2)
Tests 2 passed (2)
Start at 15:02:01
Duration 4.21s (transform 61ms, setup 14ms, collect 327ms, tests 13ms, environment 0ms, prepare 5.38s)

3:02:05 PM [vite] (ssr) Error when evaluating SSR module /@fs/C:/Projects/blog/svelte-test-3/node_modules/@sveltejs/kit/src/runtime/server/index.js: transport was disconnected, cannot call "fetchModule"
at reviveInvokeError (file:///C:/Projects/blog/svelte-test-3/node_modules/vite/dist/node/module-runner.js:475:14)
at Object.invoke (file:///C:/Projects/blog/svelte-test-3/node_modules/vite/dist/node/module-runner.js:549:11)
at async SSRCompatModuleRunner.getModuleInformation (file:///C:/Projects/blog/svelte-test-3/node_modules/vite/dist/node/module-runner.js:1059:7)
at async request (file:///C:/Projects/blog/svelte-test-3/node_modules/vite/dist/node/module-runner.js:1076:83)
at async eval (C:\Projects\blog\svelte-test-3\node_modules@sveltejs\kit\src\runtime\server\page\render.js:2:1)
at async ESModulesEvaluator.runInlinedModule (file:///C:/Projects/blog/svelte-test-3/node_modules/vite/dist/node/module-runner.js:910:3)
at async SSRCompatModuleRunner.directRequest (file:///C:/Projects/blog/svelte-test-3/node_modules/vite/dist/node/module-runner.js:1119:59)
at async SSRCompatModuleRunner.directRequest (file:///C:/Projects/blog/svelte-test-3/node_modules/vite/dist/node/chunks/dep-Bj7gA1-0.js:18770:22)
at async SSRCompatModuleRunner.cachedRequest (file:///C:/Projects/blog/svelte-test-3/node_modules/vite/dist/node/module-runner.js:1037:73)
at async eval (C:\Projects\blog\svelte-test-3\node_modules@sveltejs\kit\src\runtime\server\page\index.js:14:1)

Or screenshot

I have changed no code in the generated sample, but if you need me to upload a repo I'm happy to do so.

Logs

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (8) x64 Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz
    Memory: 14.12 GB / 31.93 GB
  Binaries:
    Node: 24.6.0 - C:\nvm4w\nodejs\node.EXE
    npm: 10.2.4 - ~\AppData\Roaming\npm\npm.CMD
  Browsers:
    Chrome: 139.0.7258.139
    Edge: Chromium (139.0.3405.102)
    Internet Explorer: 11.0.19041.5794
  npmPackages:
    svelte: ^5.0.0 => 5.38.3

---
If it is relevant, I'm also using nvm for Windows, version 1.2.2

Severity

blocking all usage of svelte

[imbrem]

I'm getting the same bug. As a temporary fix, going into package.json and changing

"test": "npm run test:unit -- --run && npm run test:e2e",

to

"test": "npm run test:unit && npm run test:e2e",

fixes things; I noticed this since npm run test:unit works fine. I have no idea why, though!

[Reboog711]

If you follow the instructions I set up above, no e2e tests were included.

So, this is the test command in my generated project

"test": "npm run test:unit -- --run"

The test:unit command is this:

"test:unit": "vitest",

Running the test:unit command seems to work without the error, so thank you for that insight. However, it puts the test run in a "watch" mode, waiting for files to change. It does not quit the tests after a single run.

The problem appears to only occur when running the tests once and quitting. If I change the test:unit command to this:

"test:unit": "vitest run",

Then I see the same error as the original test command.

Do you see a browser window pop up for you, or are you running in a headless mode? By default, I'm seeing a browser window pop up.

[Reboog711]

I was able to fix this issue by specifying headless in the vite.config.ts:

instances: [{ 
  browser: 'chromium', 
  headless: true  // <-- this is the new property 
}]

[imbrem]

Yes, but I kind of like to be able to visualize the tests. As a workaround I just do the watch

[gurjeet]

I was able to fix this issue by specifying headless in the vite.config.ts:

instances: [{ 
  browser: 'chromium', 
  headless: true  // <-- this is the new property 
}]

This worked for me to resolve the error. Since I was running tests over SSH, the "headful" browser leads to these error messages.

[Reboog711]

Yes, but I kind of like to be able to visualize the tests. As a workaround I just do the watch

That's fine for local dev. Less fine if you're running them in Jenkins or as part of some other automated build process.

[imbrem]

I meant more that this issue should really be fixed haha, since it is annoying that things are in watch mode!