If you launch Vitest browser with more than 127 files, Vitest prints error somewhere in the middle:
SyntaxError: Failed to construct 'WebSocket': The URL 'ws://localhost:undefined/' is invalid.
❯ setupWebSocket ../../../../@vite/client:509:20
❯ fallback ../../../../@vite/client:488:22
❯ WebSocket.<anonymous> ../../../../@vite/client:524:13
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Unhandled Rejection ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
SyntaxError: Failed to construct 'WebSocket': The URL 'ws://localhost:undefined/' is invalid.
❯ setupWebSocket ../../../../@vite/client:509:20
❯ fallback ../../../../@vite/client:488:22
❯ WebSocket.<anonymous> ../../../../@vite/client:524:13
If file parallelization disabled situation becomes even worse. You don't see any error for some time, Vitests stucks in the end and after a while you see error:
Error: Cannot connect to the server in 60 seconds
❯ ../../../../__vitest_browser__/rpc-slP7oy1q.js:551:16