499 Token Required or Client Closed Request
HTTP response status code 499 is an unofficial client error that is specific to both ArcGIS with 499 Token Required and nginx with 499 Client Closed Request.
Usage specific to ArcGIS
When the 499 Token Required status code is received, the server is expecting a token to accompany the HTTP request but it was not submitted. To solve this error, the HTTP request needs to be resubmitted with a valid token.
Usage specific to nginx
When the 499 Client Closed Request status code is generated, it means that the client has closed the HTTP Connection in advance of the server completing the task. As such, the final response cannot be sent and this status code is only available in the nginx logs.
Note
Search engines like Google will not index a URL with 499 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 499 Token Required / Client Closed Request status code is specific to ArcGIS, where the HTTP request required a token that was not included, and nginx, where the client closed the HTTP request prematurely.