494 Request header too large
HTTP response status code 494 Request header too large is an unofficial client error specific to nginx and is returned by the server to indicate that the client either sent an HTTP request that is too large, or one or more of the HTTP headers submitted is too long.
Usage
When the 494 Request header too large status code is received, it indicates that the server is unwilling to process the HTTP request due to the HTTP request being too large, or that the content from one or more of the HTTP headers sent is too long.
Note
494 Request header too large is similar to the official 431 Request Header Fields Too Large status code.
Note
In the nginx documentation, it states that if the client HTTP request headers are too large for the buffers then a 414 URI Too Long) or 400 Bad Request will be returned to the client.
Note
Search engines like Google will not index a URL with 494 Request header too large response status, and consequently, URLs that have been indexed in the past but are now returning this HTTP status code will be removed from the search results.
Takeaway
The 494 Request header too large status code is a client error that is sent by the server when the client presents an HTTP request that is too large, or at least one of the HTTP headers sent is too long.