Client Error (4xx)

A 4xx client error is any HTTP status code from 400 to 499. It tells the client the request itself was faulty or not permitted, for example malformed syntax, missing authentication, a forbidden resource or a URL that does not exist.

Why it matters

The class says whose move it is. Repeating a 404 will not help, but fixing the link will. For site owners, a rising 4xx rate often points to broken internal links, deleted pages, expired sessions or an aggressive firewall rule. Search engines drop pages that return 404 or 410 over time, so unplanned 4xx responses on important URLs cost traffic.

Example

The common ones:

400 Bad Request          malformed request
401 Unauthorized         missing or invalid credentials
403 Forbidden            authenticated but not allowed
404 Not Found            no such resource
410 Gone                 deliberately removed
429 Too Many Requests    rate limited

If your monitor sees 403 from a page that works in a browser, a WAF or bot rule may be blocking the monitor's user agent or IP rather than the page being down.

Common mistakes

• Treating a 4xx as an outage. Many are expected, like a 404 on a mistyped URL.
• Returning 403 or 404 when you mean 401, confusing clients about whether logging in would help.
• Ignoring 4xx spikes after a deploy or migration.

Related

• Status code class
• Server error 5xx
• 404 page
• 410 Gone

This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.