412 Precondition Failed

Client error: the problem lies with the request. Repeating it unchanged will normally fail again.

When a server should send it. When a condition in the request headers (If-Match, If-Unmodified-Since, If-None-Match on a write) is false. It is the server half of optimistic locking: the client's copy is out of date.

Common causes. Optimistic locking: the resource changed since you last fetched it.

What to do. Fetch the current version and its ETag, then retry.

Browsers and HTTP clients. API clients re-fetch the resource to get the current ETag, then retry. Browsers send conditional headers only for their own cache checks, which give 304 rather than 412.

Google Search. Like every 4xx except 429, it tells Google the content does not exist: an indexed URL is removed from the index, and a new one is not processed. Crawl frequency for the URL gradually drops. Source: Google Search Central, How HTTP status codes, and network and DNS errors affect Google Search.

Caching. Not cacheable by default (RFC 9110 section 15.1 does not list it). A cache stores it only when the response says so with Cache-Control or Expires.

HTTP/1.1 412 Precondition Failed
Content-Type: text/plain; charset=utf-8

Precondition Failed

location /example {
    return 412;
}

RewriteEngine On
RewriteRule ^example$ - [R=412,L]

http_response_code(412);
echo 'Precondition Failed';
exit;

res.writeHead(412, { 'Content-Type': 'text/plain' });
res.end('Precondition Failed');

# in a view function
return 'Precondition Failed', 412