Redundant call to start.

[todor-a]

Hey. I'm getting:

[MSW] Found a redundant "worker.start()" call. Note that starting the worker while mocking is already enabled will have no effect. Consider removing this "worker.start()" call.

and while I understand this is just a warning, I still wonder why it happens. My MSW instance live here:

import { type SetupWorker } from 'msw/browser';
import { test as testBase } from 'vitest';

import { worker } from '@network-mocks';

export const browserTest = testBase.extend<{ worker: SetupWorker }>({
    worker: [
        async ({}, use) => {
            await worker.start({ quiet: true });

            await use(worker);

            worker.resetHandlers();
        },
        {
            auto: true,
        },
    ],
});

Here is a proof that no one else is referencing it:

Here is my system info (note vitest beta):

  System:
    OS: macOS 15.7.1
    CPU: (10) arm64 Apple M1 Pro
    Memory: 234.66 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.19.5 - /usr/local/bin/node
    Yarn: 1.22.22 - /opt/homebrew/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 10.12.2 - /Users/todor/Library/pnpm/pnpm
    bun: 1.2.20 - /Users/todor/.bun/bin/bun
    Deno: 2.4.2 - /Users/todor/.deno/bin/deno
    Watchman: 2025.09.15.00 - /opt/homebrew/bin/watchman
  Browsers:
    Brave Browser: 141.1.83.112
    Chrome: 141.0.7390.76
    Firefox: 142.0.1
    Safari: 26.0
  npmPackages:
    @vitejs/plugin-react: 4.7.0 => 4.7.0 
    @vitest/browser-playwright: ^4.0.0-beta.15 => 4.0.0-beta.15 
    msw: 2.11.2 => 2.11.2 
    playwright: 1.52.0 => 1.52.0 
    vite: 7.1.5 => 7.1.5 
    vitest: 4.0.0-beta.17 => 4.0.0-beta.17 
    vitest-browser-react: ^2.0.0-beta.4 => 2.0.0-beta.4 
    vitest-canvas-mock: ^0.3.3 => 0.3.3 
    vitest-localstorage-mock: ^0.1.2 => 0.1.2 
    vitest-mock-extended: ^3.1.0 => 3.1.0 

[kettanaito]

Hi, @todor-a. That warning is printed because your fixture calls worker.start() but never calls worker.stop(). Since the worker reference becomes a share state across tests through your fixture, you're ending up with the 2nd test attempting to start the worker that's already been started by the first.

I believe you should call worker.stop() in the cleanup phase of your fixture (like so). Let me know if that helps.

[todor-a]

That seems to have fixed it. Do you want me to update this example?

[kettanaito]

Yes, please! A pull request would be most welcome.