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Vite dev server returns 431 Request Header Fields Too Large

431 Request Header Fields Too Large

The Vite dev server can return 431 errors when request headers become too large, often because of oversized cookies or local browser state.

431 Request Header Fields Too Largedevelopment servers

Observed message

431 Request Header Fields Too Large

Why it happens

  • The browser is sending oversized cookies to the dev server.
  • Local development state attached to the request headers has grown too large.
  • Repeated local testing on the same host accumulated header-heavy browser state.

How to fix it

  1. Clear cookies and related browser storage for the local development host.
  2. Retry the dev server request in a clean browser profile or private window.
  3. Check whether local tooling is adding large custom headers during development.

Official reference

Vite: Troubleshooting

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Vite 431 Request Header Fields Too Large: causes and fixes | Error Atlas