Bug: `mount(...) is not available on the server` error when running tests after a fresh install

[evert1989]

Summary

Running npm test after a fresh install using npx sv create my-app will have a failing test.

FAIL   client  src/routes/page.svelte.test.ts > /+page.svelte > should render h1
Svelte error: lifecycle_function_unavailable
`mount(...)` is not available on the server
https://svelte.dev/e/lifecycle_function_unavailable
 ❯ Module.lifecycle_function_unavailable node_modules/svelte/src/internal/server/errors.js:9:16
      7|  */
      8| export function lifecycle_function_unavailable(name) {
      9|  const error = new Error(`lifecycle_function_unavailable\n\`${name}(...)\` is not available on the server\nhtt…
       |                ^
     10|
     11|  error.name = 'Svelte error';
 ❯ Module.mount node_modules/svelte/src/index-server.js:25:4
 ❯ mount node_modules/@testing-library/svelte/src/core/modern.svelte.js:27:28
 ❯ render node_modules/@testing-library/svelte/src/pure.js:75:26
 ❯ src/routes/page.svelte.test.ts:8:3

Here are the hardware/software specs:

• Apple M3 Pro
• Node: v23.11.1
• NPM: 10.9.2

Reproduction steps

• npx sv create my-app
  • add vitest and playwright to the project
• cd my-app
• npm test

Relevant files

• vite.config.ts

import { svelteTesting } from '@testing-library/svelte/vite';
import { sveltekit } from '@sveltejs/kit/vite';
import { defineConfig } from 'vite';

export default defineConfig({
  plugins: [sveltekit()],
  test: {
    workspace: [
      {
        extends: './vite.config.ts',
        plugins: [svelteTesting()],
        test: {
          name: 'client',
          environment: 'jsdom',
          clearMocks: true,
          include: ['src/**/*.svelte.{test,spec}.{js,ts}'],
          exclude: ['src/lib/server/**'],
          setupFiles: ['./vitest-setup-client.ts']
        }
      },
      {
        extends: './vite.config.ts',
        test: {
          name: 'server',
          environment: 'node',
          include: ['src/**/*.{test,spec}.{js,ts}'],
          exclude: ['src/**/*.svelte.{test,spec}.{js,ts}']
        }
      }
    ]
  },
});

Expected behaviour

All tests succeed when doing a clean install

Actual behaviour

The unit test for +page.svelte fails

Current way to fix it manually

Adding the following to the vite.config.ts fixes the issue.

resolve: process.env.VITEST ? { conditions: ['browser'] } : undefined

Extra suggestion

There's a deprecation warning when running tests

 DEPRECATED  The `workspace` option is deprecated and will be removed in the next major. To hide this warning, rename `workspace` option to `projects`.

This can be resolved by chaning workspace to projects