If a server component reads the raw Cookie request header itself (headers().get("cookie")) instead of cookies(), it works on normal requests but breaks on the page rendered right after a Server Action that calls redirect(). For that render, Next.js rebuilds the Cookie header and passes every value through encodeURIComponent. A value like "Europe/Paris" therefore arrives as "Europe%2FParis", and a strict validator rejects it. The page silently falls back to a default.

A Server Action reports refusals by redirecting to ?erreur=<encodeURIComponent(message)>. A bulk action (e.g. 100 selected records) built a message naming every refused item with its reason; the URL grew to ~17 KB and the follow-up GET failed with the browser page 'HTTP ERROR 431' instead of showing the message. Nothing was written, but the user got no explanation.