# HTTP Status Codes

> A status code reference written for debugging, not for the spec: what each code means, why your side of the stack is seeing it, and what to do about it.

- URL: https://bytescope.dev/en/http-status/
- Locale: en · Also: [ja](https://bytescope.dev/ja/http-status.md) · [zh-Hant](https://bytescope.dev/zh-Hant/http-status.md)
- Runs: Everything runs locally in your browser — nothing is uploaded.
- Requires: Any modern browser (Chrome, Edge, Firefox, Safari), desktop and mobile.
- Updated: 2026-09-08

## What it does

### Written for the moment somebody hands you a number

Almost nobody looks up a status code out of curiosity. They look it up because a deploy went out and half the requests turned red, because a client is reporting an error nobody can reproduce, or because a monitoring alert quoted three digits and nothing else. So every page here leads with what the code tells you about *where* the failure is, and only then explains what the specification says. The definition is the short part; deciding whether to look at your router, your proxy or your upstream is the part that takes the afternoon.

### Three readers, three different answers

A 404 seen by a front-end developer is usually a route that works on click and breaks on refresh. The same 404 seen by a back-end developer is usually a proxy prefix that was added or stripped before the application saw the path. Seen by a visitor it is a link that has moved. Those are three problems with three fixes, and a page that merges them into one paragraph helps none of the three — so every code here answers them separately, and says plainly when the honest answer for one of them is that there is nothing to do.

### Standard, vendor-specific, or just habit

RFC 9110 defines the status codes every conforming client and server agrees on, and a few widely used ones live in their own documents — 429 in RFC 6585, 308 in RFC 7538. Plenty of codes you will meet in production are in neither: 499 comes from nginx's source, and the whole 520-526 range is Cloudflare's, invented so that a failure between the edge and your origin is not reported as a generic 502. Each page says which of the three it is, because the difference decides whose documentation is the authority — and this site runs on Cloudflare, so the 52x pages are written from the same side of that connection you are on.

## Limits

Does not test a live URL; it is a reference of HTTP status codes with meaning, causes and fixes.

## FAQ

### What do the HTTP status code classes mean?

The first digit is the whole classification. 1xx is informational and rarely surfaces in application code; 2xx means the request succeeded; 3xx means further action is needed, usually following a redirect; 4xx means the request itself was faulty; and 5xx means the server failed while apparently handling a valid request. RFC 9110 also says a client that meets an unrecognised code must treat it as the `x00` of its class, which is why an unknown 599 is safe to handle as a 500.

### How do I see which status code a page actually returned?

In a browser, open the developer tools network panel and read the Status column on the request itself rather than on the assets it pulled in. From a terminal, `curl -sS -o /dev/null -w '%{http_code}\n' https://example.com/page` prints just the number, and adding `-D -` prints the response headers with it. The terminal answer is the one to trust when the two disagree: an extension, a service worker or a cached response can change what the browser shows you.

### Is a 4xx my fault and a 5xx the server's?

That is the intent of the split, but it is a rule of thumb rather than a guarantee. Plenty of 4xx responses are produced by a server that is misconfigured — a 404 from a proxy that mangled the path, or a 403 from a rule nobody meant to apply — and plenty of 5xx responses are triggered by a request that asked for too much. Use the class to decide which side to look at first, not to decide who is to blame.

### Which of these codes are not actual standards?

418 is a joke from an April Fools' RFC that servers still return. 499 is not registered with IANA at all: it comes from nginx and means the client closed the connection before the server answered, so it shows up in your access log rather than in a browser. The 520 to 526 range is Cloudflare's own, and only Cloudflare's documentation defines it. 429, despite feeling like an addition, is a real standard — RFC 6585 — as is 308, from RFC 7538.

### Do status codes affect SEO?

Some of them decisively. A 301 passes ranking signals to the new URL while a 302 tells a crawler the move is temporary and to keep the old one, so using 302 for a permanent move quietly costs you. A 404 invites re-crawling for a while, whereas a 410 tells an indexer the resource is finished. The worst case is a soft 404 — an error page served with a 200 — because a search engine then indexes the apology as content.