HTTP 499 or client closed connection — request cancelled

Server logs show HTTP 499 (nginx) or 'client closed connection' errors. The response was being generated but the client disconnected before receiving it.

(1) Slow responses: if your backend takes >30s, clients or load balancers timeout. Optimize the endpoint or increase timeouts. (2) User navigation: user clicked away, closing the pending request. Handle AbortSignal server-side to stop work early. (3) Duplicate requests: frontend retrying without cancelling the first request. Use AbortController. (4) Load balancer timeout: AWS ALB defaults to 60s, nginx proxy_read_timeout defaults to 60s — increase if needed. (5) For long operations: use async job pattern — accept request, return job ID, client polls for result.

HTTP 499 is nginx-specific (not in the HTTP standard) meaning the client closed the socket before the server could send a response. This wastes server resources computing unused responses.