Glossary · Networking

Server Error (5xx)

A 5xx response means the server failed to complete a valid request, so the fault lies with the server, a proxy or an upstream dependency, not the client.

Updated November 4, 2026

A 5xx server error is an HTTP status code from 500 to 599. It signals that the request looked valid but the server, or something behind it, could not complete it.

Why it matters

5xx responses are the clearest signal that users are being hurt right now, which is why error-rate alerts usually count them. They also separate causes: 500 is a generic application failure, 502 and 504 point to a bad or slow upstream behind a proxy, and 503 means the service is unavailable or overloaded and is often temporary. Search engines slow their crawling when they see sustained 5xx.

Example

A quick triage order:

500  check application error log and recent deploys
502  is the upstream process running and listening?
503  overload, maintenance or capacity limit
504  upstream too slow; check timeouts and slow queries

For a planned outage, return 503 with a Retry-After header so crawlers treat it as temporary.

Common mistakes

  • Debugging the browser when the response came from a proxy or CDN in front of the origin.
  • Showing a friendly page with status 200, which hides the failure from monitors.
  • Retrying instantly in a loop, which makes an overloaded server worse.

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