HTTP 408 vs 504 — Request Timeout vs Gateway Timeout Comparison

Quick Cheat Sheet

Aspect	408 Request Timeout	504 Gateway Timeout
Class	4xx — Client Error	5xx — Server Error
Who timed out?	Client (didn't finish sending)	Upstream server (didn't respond in time)
Issued by	Origin server	Reverse proxy / gateway / CDN
Client should retry?	Yes, often safe	Yes, with backoff

What Each Code Means

408 Request Timeout (RFC 9110 § 15.5.9) is sent when the server was waiting for the client to finish sending the request and gave up. The client took too long to produce headers or upload a body. The server is essentially saying: "I'm closing this connection because you went silent."

504 Gateway Timeout (RFC 9110 § 15.6.5) is fundamentally different: it means the responding server is itself a proxy/gateway, and the upstream server it depends on didn't respond in time. The client did everything right; the failure is downstream of the proxy.

How They Show Up in Production

You will almost never see a real 408 in modern web apps — browsers don't typically stall mid-request, and most servers just close the connection silently. 408 is most often seen with slow file uploads or flaky mobile networks.

504 is everywhere in microservice architectures:

• Nginx returns 504 when an upstream block exceeds proxy_read_timeout (default 60s)
• AWS Application Load Balancer returns 504 when the target group doesn't respond within the idle timeout (default 60s)
• Cloudflare returns 504 when your origin takes longer than 100s (520-series for shorter failures)
• API Gateway returns 504 when the integration (Lambda, HTTP backend) exceeds the integration timeout (max 29s for REST APIs)

Debugging Each

• For 408: check client-side network logs, body size, and any expect: 100-continue flow. On the server, look for client_body_timeout and client_header_timeout in Nginx.
• For 504: the failure is upstream — check application logs of the origin, not the proxy. Common causes: a downstream DB query, a third-party API call, or a Lambda cold start exceeding the timeout.

A Subtle Rule

If you operate a reverse proxy and your own backend takes too long, return 504 (you are the gateway). Only return 408 from the actual origin server. Many devs incorrectly send 408 from a proxy.