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HTTP_STATUS_HEADER_FIELDS_TOO_LARGE
The response can identify the offending header, but it may also be returned for the combined header section. Large cookies, long authorization tokens, forwarding chains, and custom tracing headers are frequent causes.
Capture the request headers and their encoded sizes. Remove stale cookies or reduce oversized headers; changing only the body will not affect a 431 response.
Servers can reject one exceptionally large field or the combined header section. Cookies are a frequent source because they are automatically sent on many requests. Inspect actual wire headers rather than only application input, then reduce unnecessary cookies or metadata instead of simply increasing limits without understanding the growth.
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