@cloudflare/vitest-pool-workers initialization failure with vitest 4.1.x + miniflare 4.202604xx

[ahwlsqja]

Summary

@cloudflare/vitest-pool-workers fails during initialization in our test setup before any tests run.

We originally planned to run a workerd-based integration test for a Cloudflare Worker app, but had to temporarily fall back to a plain Node pool + app.fetch(req, env) approach because the worker pool failed to initialize.

Versions

Observed with combinations around:

• vitest 4.1.0 / 4.1.x
• @cloudflare/vitest-pool-workers 0.14.4 through 0.14.7
• miniflare 4.20260410.0 through 4.20260415.0
• wrangler 4.61.0

We confirmed the relevant package repository is cloudflare/workers-sdk (packages/vitest-pool-workers).

Symptoms

The failure happens before the actual test body executes.

The errors we observed referenced missing internal Vitest exports such as:

• @vitest/utils/helpers.filterOutComments
• @vitest/expect.ChaiStyleAssertions

This suggests a compatibility issue between the worker-pool/miniflare layer and the installed Vitest internals, rather than a bug in our worker application code.

Impact

Because of this, we could not keep the intended workerd-runtime integration test path for our Worker app.

We temporarily replaced it with:

• Node pool
• direct app.fetch(new Request(...), env) calls
• real PostgreSQL test DB
• mock KV binding

That still lets us validate our route logic and state invariants, but it does not validate:

• actual workerd runtime behavior
• binding rendering behavior
• Hyperdrive runtime behavior inside the worker environment

Question

Is this a known compatibility issue for the vitest 4.1.x + @cloudflare/vitest-pool-workers 0.14.x + miniflare 4.202604xx combination?

If yes, is there an existing tracking issue or recommended workaround?

If helpful, I can provide a minimized reproduction next.

[emily-shen]

Hi, could you please provide a link to a minimal reproduction?

[ahwlsqja]

I put together a minimal public reproduction here:

• https://github.com/ahwlsqja/workers-sdk-13581-workspace-repro

Repro shape:

• pnpm workspace
• packages/legacy pins vitest@4.0.18
• packages/repro uses vitest@4.1.0 + @cloudflare/vitest-pool-workers@0.14.7
• tiny Worker + tiny cloudflareTest() config + one test

Steps:

pnpm install
cd packages/repro
pnpm test

With the version-skewed workspace layout, I can reproduce initialization failure before the test body runs. In my local run the error was:

SyntaxError: The requested module '@vitest/expect' does not provide an export named 'ChaiStyleAssertions'

In the original app/workspace I also observed a related failure using:

SyntaxError: The requested module '@vitest/utils/helpers' does not provide an export named 'filterOutComments'

If needed I can also add a second branch / second repro variant for the filterOutComments path specifically, but this workspace repro seems to capture the same class of failure: pool-workers startup breaking under Vitest internal package version skew.

[emily-shen]

I've pulled down your repro and I'm not seeing any errors when running the test - it passes on my machine 😅.

Just to clarify, you're seeing issues when running vitest 4.1.0 and vitest pool workers 0.14.x? But vitest 4.0.x + vitest pool workers 0.14.x is fine?

We do expect vitest pool workers 0.13+ and vitest 4.1.0 to be compatible, and we've not received any other reports of this not working :(

[edmundhung]

You shouldn't need to install miniflare yourself as vitest-pool-workers already listed it as its own dependencies.

Can you please try the followings:

• Remove miniflare from your devDependencies
• Update wrangler, @cloudflare/vitest-pool-workers and all vitest related packages to the latest version.