Issue Review: cloudflare/workers-sdk#11082

Summary

vitest-pool-workers intermittently fails with "Fallback service failed to fetch module; Connection refused" when running tests multiple times in succession with large bundles and multiple test files.

Findings

• Created: 2025-10-24
• Updated: 2025-11-26
• Version: @cloudflare/vitest-pool-workers 0.8.71 -> 0.12.6
• Component: vitest-pool-workers
• Labels: bug, vitest
• Comments: 2

Key Evidence

• Maintainer @petebacondarwin confirmed reproducibility on 2025-10-27: "Thanks for the great reproduction! I can see this happening locally too. Adding to the backlog to be looked into."
• This issue reopens #8853, which was closed due to lack of reproduction
• Related issue #7569 was closed with reference to this new issue
• A workaround exists: poolOptions.workers.singleWorker = true flag
• No PRs have been merged explicitly fixing this issue
• No changelog entries mention fixing this connection refused/fallback service issue
• Reproduction repository provided: https://github.com/dmaretskyi/vpw-repro

Root Cause Analysis

The error appears to be related to resource exhaustion when running vitest-pool-workers repeatedly. Key factors:

1. Running tests multiple times in quick succession
2. Having multiple separate test-suite files
3. Having workers with large bundle sizes (large dependencies like effect)

The error message connect(): Can't assign requested address and Connection refused suggest port exhaustion or timing issues with workerd's fallback service module resolution.

Recommendation

Status: KEEP OPEN

Reasoning: This is a confirmed, reproducible bug that a maintainer has added to the backlog. No fix has been merged yet, and the reproduction is clear. The issue is actively tracked.

Action: No action needed - wait for fix to be developed. Consider subscribing for updates.

Suggested Comment

N/A - Issue is confirmed and on backlog, no further action needed at this time.