index.md

---

title: 412 Precondition Failed
slug: Web/HTTP/Status/412
browser-compat: http.status.412

---

{{HTTPSidebar}}

The HyperText Transfer Protocol (HTTP) 412 Precondition Failed client error response code indicates that access to the target resource has been denied. This happens with conditional requests on methods other than {{HTTPMethod("GET")}} or {{HTTPMethod("HEAD")}} when the condition defined by the {{HTTPHeader("If-Unmodified-Since")}} or {{HTTPHeader("If-None-Match")}} headers is not fulfilled. In that case, the request, usually an upload or a modification of a resource, cannot be made and this error response is sent back.

Status

412 Precondition Failed

Examples

ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
ETag: W/"0815"

Avoiding mid-air collisions

With the help of the ETag and the {{HTTPHeader("If-Match")}} headers, you can detect mid-air edit collisions.

For example, when editing MDN, the current wiki content is hashed and put into an Etag in the response:

ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"

When saving changes to a wiki page (posting data), the {{HTTPMethod("POST")}} request will contain the {{HTTPHeader("If-Match")}} header containing the ETag values to check freshness against.

If-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"

If the hashes don't match, it means that the document has been edited in-between and a {{HTTPStatus("412")}} Precondition Failed error is thrown.

Specifications

{{Specifications}}

Browser compatibility

The information below has been pulled from MDN's GitHub (https://github.com/mdn/browser-compat-data).

{{Compat}}

See also

• {{HTTPStatus("304")}}

• {{HTTPHeader("If-Unmodified-Since")}}

• {{HTTPHeader("If-None-Match")}}

• {{HTTPStatus("428")}}