# 412 Precondition Failed

The HTTP **`412 Precondition Failed`** [client error response](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status#client_error_responses) status code indicates that access to the target resource was denied.
This happens with [conditional requests](https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Conditional_requests) on methods other than [GET](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/GET) or [HEAD](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/HEAD) when the condition defined by the [If-Unmodified-Since](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Unmodified-Since) or [If-Match](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-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

```http
412 Precondition Failed
```

## Examples

### Precondition failed

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

### Avoiding mid-air collisions

With the help of the `ETag` and the [If-Match](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Match) headers, you can prevent conflicts or mid-air collisions.
For example, when editing some wiki pages, content is hashed and put into an `ETag` in successful responses:

```http
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
```

When saving changes to a wiki page (posting data), the [POST](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST) request will contain the [If-Match](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Match) header containing the `ETag` values that the client stored from the last edit to check freshness of the resource on the server:

```http
If-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"
```

If the hashes don't match, the document has been edited in-between and a `412 Precondition Failed` error is thrown.

## Specifications

## See also

- [HTTP response status codes](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status)
- [Conditional requests](https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Conditional_requests)
- [304](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/304)
- [If-Unmodified-Since](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Unmodified-Since), [If-Match](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Match)
- [428](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/428)