Nginx 431 - Request Header Fields Too Large - in reverse proxy

I am using auth_request module in nginx to act as an authentication module.

location ~ ^/apigw/cws/(.*)$ {
        log_subrequest on;
        auth_request  /_sessionvalidate;
        auth_request_set $token $upstream_http_authorization;
        proxy_set_header Authorization $token;
        proxy_pass http://cws/$1$is_args$args;
}



location = /_sessionvalidate {
        internal;
        proxy_method POST;
        proxy_busy_buffers_size   512k;
        proxy_buffers   4 512k;
        proxy_buffer_size   256k;
        proxy_pass_request_body off;
        proxy_set_header Content-Length "";
        proxy_set_header X-Original-URI $request_uri;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_pass http://apigw/sessionValidate/;
}

The problem here is when i try to pass the token passed by the sessionvalidate to the cws upstream server

 proxy_set_header Authorization $token;

I get 431 error. I tried setting up the below in server/http directive

        large_client_header_buffers 4 64k;
        client_header_buffer_size 16k;

but I am still facing the error. I am not sure how make sure increase the header size of the upstream request. Please help.