5xx Server error
503 Service Unavailable
The server is temporarily unable to handle the request.
Overload, maintenance or no healthy instances behind a load balancer. It may include Retry-After.
Common causes and fixes
- Retry later with backoff; respect
Retry-Afterif present. - On your own servers: check health checks, instance counts, connection pools and whether a maintenance mode is on.
- LLM APIs return 503 (or 529) when overloaded — retry with backoff or fall back to another model.
Example response
HTTP/1.1 503 Service Unavailable
Retry-After: 120
Date: Thu, 22 Oct 2026 08:00:00 GMT Check a URL’s status with curl -I https://example.com, which prints the status line and headers.