[vite-plugin] Vitest in Browser Mode errors on startup with Cloudflare Vite plugin

[jeremy-code]

What versions & operating system are you using?

System:
  OS: macOS 26.5.2
  CPU: (8) arm64 Apple M1
  Memory: 142.52 MB / 8.00 GB
  Shell: 5.9 - /bin/zsh
Binaries:
  Node: 24.18.0 - /Users/jeremy/.nodenv/versions/24.18.0/bin/node
  Yarn: 1.22.22 - /opt/homebrew/bin/yarn
  npm: 11.16.0 - /Users/jeremy/.nodenv/versions/24.18.0/bin/npm
  pnpm: 11.17.0 - /opt/homebrew/bin/pnpm
  Deno: 2.9.3 - /opt/homebrew/bin/deno
npmPackages:
  @cloudflare/vite-plugin: ^1.47.0 => 1.47.0 
  wrangler: ^4.114.0 => 4.114.0

Please provide a link to a minimal reproduction

https://github.com/jeremy-code/cloudflare-vite-vitest

Describe the Bug

Steps to reproduce

1. Create Vite config with @cloudflare/vite-plugin
2. Create Vitest config extending Vite config using Vitest browser mode
3. Attempt to run vitest

This only occurs in browser mode. In the minimal reproduction, the error logged is "module is not defined".

FWIW, though I could not reproduce it, on a different project, I receive the error: TypeError: Cannot read properties of undefined (reading 'wrapDynamicImport') with the same error stack. See also: withastro/astro#16275, vitest-dev/vitest#10319, sveltejs/kit#14149

Please provide any relevant error logs

$ vitest --run

⎯⎯⎯⎯⎯⎯⎯ Startup Error ⎯⎯⎯⎯⎯⎯⎯⎯
ReferenceError: module is not defined
    at runInRunnerObject (workers/runner-worker/index.js:107:3)
    at getWorkerEntryExportTypes (workers/runner-worker/index.js:246:24)
    at null.<anonymous> (workers/runner-worker/index.js:350:37)
    at maybeCaptureError (workers/runner-worker/index.js:51:10) {
  [cause]: undefined
}



[ELIFECYCLE] Test failed. See above for more details.

[workers-devprod]

Note

This is an automated, advisory triage report generated by workers-devprod. It is not an official maintainer response — a maintainer will follow up.

🤖 Automated triage report

Issue Triage: #14869

Summary

Running Vitest in browser mode with a config that extends a Vite config using @cloudflare/vite-plugin fails at startup with ReferenceError: module is not defined (or Cannot read properties of undefined (reading 'wrapDynamicImport')).

Classification

• Type: Bug
• Component: packages/vite-plugin-cloudflare (with likely involvement of packages/vitest-pool-workers)
• Severity: Medium
• Has Reproduction: Yes (with link)
• Quality: Complete

Findings

• Created: 2026-07-26
• Author: jeremy-code
• Version: @cloudflare/vite-plugin 1.47.0, wrangler 4.114.0
• Labels: none
• Comments: 0

Analysis

• The report is complete: it includes exact package versions, OS (macOS 26.5.2 / Node 24.18.0), clear reproduction steps, a minimal reproduction repo (https://github.com/jeremy-code/cloudflare-vite-vitest), and the full error stack. This is enough to investigate directly.
• The error only occurs in Vitest browser mode. The stack trace points into workers/runner-worker/index.js (the vitest-pool-workers runner), suggesting a conflict between the Cloudflare Vite plugin's environment/module handling and Vitest's browser-mode runner — module is not defined implies CommonJS-style module referencing in an ESM context.
• The reporter links to parallel reports in other framework projects ([Bug]: Vitest Astro fails at running Storybook tests withastro/astro#16275, browser: dynamicImportPlugin leaks into SSR environment, breaks SvelteKit dev middleware vitest-dev/vitest#10319, SSR Error: "Cannot read properties of undefined (reading 'wrapDynamicImport')" with Vite 7 sveltejs/kit#14149), indicating the underlying cause may be shared and could partly originate in Vitest's browser-mode handling. This should be verified against workers-sdk code before assuming ownership.
• No known duplicate within workers-sdk, not stale (just filed), not spam, not user error, and not a runtime/workerd concern — it is a tooling integration bug in the Vite plugin.

Recommendation

Status: NEEDS VERIFICATION

Reasoning: The issue is well-formed with a clean reproduction and clearly affects the @cloudflare/vite-plugin + Vitest browser-mode integration. A maintainer needs to run the reproduction to confirm the fault lies in workers-sdk (vite-plugin / vitest-pool-workers) versus upstream Vitest before deciding on a fix path.

Action: A maintainer should clone the reproduction and run vitest --run in browser mode to confirm the module is not defined failure, then determine whether the root cause is in packages/vite-plugin-cloudflare, packages/vitest-pool-workers, or upstream Vitest. Given the cross-project references, coordinate with the linked Vitest issue if the cause is upstream.

Suggested Labels: none

Suggested Comment

Thanks for the detailed report and the minimal reproduction — that's really helpful. We'll take a look at running Vitest in browser mode alongside @cloudflare/vite-plugin and try to pin down whether the module is not defined error originates in our plugin, in vitest-pool-workers, or upstream in Vitest itself (given the related reports you linked). We'll follow up here with what we find.

Structured summary (JSON)

{
  "issueNumber": 14869,
  "title": "[vite-plugin] Vitest in Browser Mode errors on startup",
  "githubUrl": "https://github.com/cloudflare/workers-sdk/issues/14869",
  "recommendation": "NEEDS VERIFICATION",
  "difficulty": "medium",
  "reasoning": "Well-formed bug with a minimal reproduction affecting the @cloudflare/vite-plugin plus Vitest browser-mode integration, failing at startup with 'module is not defined'. Needs a maintainer to reproduce and confirm whether the cause is in workers-sdk or upstream Vitest.",
  "suggestedAction": "Clone the reproduction, run vitest in browser mode to confirm the error, then determine whether the root cause is in vite-plugin-cloudflare, vitest-pool-workers, or upstream Vitest (coordinating with the linked Vitest issue if upstream).",
  "hasSuggestedComment": true,
  "suggestedLabels": []
}