HTTP 499 in Nginx means that the client closed the connection before the server answered the request. In my experience is usually caused by client side timeout. As I know it’s an Nginx specific error code.

If the client closes the connection because the server is taking too long to process, in addition to optimizing the server handler, nginx can also add a configuration directive

proxy_ignore_client_abort on;

to ignore client disconnections. But this only ignores the error, and does not fundamentally solve the problem. The fundamental solution to the problem is to optimize the configuration and processing speed of the program. For example, whether php-fpm has enough processes, etc.

Some other 499 possible factors (reference).

It can mean that you’re sending requests too quickly – from memory you have a limit of 100 http-requests per 100 seconds, but this may have changed

It can mean that the simulator is too busy or cannot allocate a request for technical reasons

SSL failure

If there is a space inurl, thehttp_responsestatus code will be 499.

Requests must fully complete after 60 seconds, or else the response will be thrown away and thehttp_responsestatus code will be 499.

Requesting a client connection with a timeout set too short, such as 1s, may also result in a 499 error.