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 GET
or HEAD
when the condition defined by the If-Unmodified-Since
or 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 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 POST
request will contain the 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 412
Precondition Failed
error is thrown.
Specifications
Specification | Title |
---|---|
RFC 7232, section 4.2: 412 Precondition Failed | Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests |
Browser compatibility
The information below has been pulled from MDN's GitHub (https://github.com/mdn/browser-compat-data).
The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
412 | Chrome Full support Yes | Edge Full support 12 | Firefox Full support Yes | IE Full support Yes | Opera Full support Yes | Safari Full support Yes | WebView Android Full support Yes | Chrome Android Full support Yes | Firefox Android Full support Yes | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support Yes |
Legend
- Full support
- Full support