fasthttp defaults to a 4 KB read buffer per connection. Any request
whose header line exceeds that returns a flat HTTP 431 from Fiber
before the request reaches a handler — affecting clients carrying
chunked NextAuth cookies, mTLS client-cert headers, or large bearer
tokens.
16 KB matches the cluster ingress-nginx large_client_header_buffers
allowance. Tested 4–8 KB header payloads through shell-api.gosec.internal
— all return normal app responses instead of 431.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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