response is sent back.

##  Status

http

    
    
    412 Precondition Failed
    

##  Examples

http

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

###  Avoiding mid-air collisions

With the help of the ` ETag ` and the [ ` If-Match ` ](/en-
US/docs/Web/HTTP/Headers/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:

http

    
    
    ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
    

When saving changes to a wiki page (posting data), the [ ` POST ` ](/en-
US/docs/Web/HTTP/Methods/POST) request will contain the [ ` If-Match ` ](/en-
US/docs/Web/HTTP/Headers/If-Match) header containing the ` ETag ` values to
check freshness against.

http

    
    
    If-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"
    

If the hashes don't match, it means that the document has been edited in-
between and a [ ` 412 ` ](/en-US/docs/Web/HTTP/Status/412) ` Precondition
Failed ` error is thrown.

##  Specifications

Specification  
---  
[ HTTP Semantics  
#  status.412  ](https://www.rfc-editor.org/rfc/rfc9110#status.412)  
  
##  Browser compatibility

BCD tables only load in the browser  with JavaScript enabled. Enable
JavaScript to view data.

##  See also

  * [ ` 304 ` ](/en-US/docs/Web/HTTP/Status/304)
  * [ ` If-Unmodified-Since ` ](/en-US/docs/Web/HTTP/Headers/If-Unmodified-Since)
  * [ ` If-None-Match ` ](/en-US/docs/Web/HTTP/Headers/If-None-Match)
  * [ ` 428 ` ](/en-US/docs/Web/HTTP/Status/428)

##  Help improve MDN

Was this page helpful to you?

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[ Learn how to contribute
](https://github.com/mdn/content/blob/main/CONTRIBUTING.md "This will take you
to our contribution guidelines on GitHub.") .

This page was last modified on  Apr 15, 2024  by  [ MDN contributors ](/en-
US/docs/Web/HTTP/Status/412/contributors.txt) .

[ View this page on GitHub
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