v5 browser mode no longer serves /mockServiceWorker.js (undocumented)

Describe the bug

Vitest 4.1 browser mode served MSW's worker at /mockServiceWorker.js via the vitest:browser:resolve-virtual plugin. setupWorker().start() worked with no public/ file.

That plugin is gone in 5.0.0. Same start() now 404s:

[MSW] Failed to register a Service Worker for scope ('http://localhost:…/') with script ('http://localhost:…/mockServiceWorker.js'): Service Worker script does not exist at the given path.

Did you forget to run "npx msw init <PUBLIC_DIR>"?

The v5 migration guide does not mention this. I only found it by reading the plugin source after the tests failed.

This looks like leftover fallout from #7556 / #7576 (stop bundling MSW, mock modules with page.route). Fair change. The resolve was still in 4.1.10 though, so anyone who relied on it hits a wall on 5.0 with no note in the guide.

4.1.10 still has the virtual resolve:

https://github.com/vitest-dev/vitest/blob/v4.1.10/packages/browser/src/node/plugin.ts#L313-L320

{
  name: 'vitest:browser:resolve-virtual',
  async resolveId(rawId) {
    if (rawId === '/mockServiceWorker.js') {
      return this.resolve('msw/mockServiceWorker.js', distRoot, {
        skipSelf: true,
      })
    }
  },
},

5.0.0's plugin list does not.

I am not asking to bring MSW back as a Vitest dependency. Either restore a tiny resolve that maps /mockServiceWorker.js to the project's msw package, or say in the migration guide that browser-mode setupWorker() now needs msw init public.

Reproduction

1. Vitest 4.1 browser project with setupWorker() from msw/browser in a setup file. No public/mockServiceWorker.js.
2. worker.start() succeeds.
3. Upgrade to 5.0.0. Do not add a public worker file.
4. worker.start() throws the MSW "script does not exist" error.

Workaround that unblocked us: msw init public --save and commit the generated file.

System Info

vitest: 5.0.0 (broke)
vitest: 4.1.10 (worked)
@vitest/browser-playwright: 5.0.0
msw: 2.15.0 (unchanged across the upgrade)

Used Package Manager

pnpm

Start by comparing the browser plugin in packages/browser/src/node/plugin.ts at v4.1.10 with the v5.0.0 plugin list, then read the v5 migration guide entry for browser mode. Determine whether the missing /mockServiceWorker.js behavior should be restored or documented, and verify that either the worker loads without a public file or the migration guidance clearly explains the required msw init workaround.