4xx Client error
412 Precondition Failed
A condition in the request headers (such as If-Match) was not met.
Used for optimistic concurrency: the client says “update only if the version is still X”, and the server refuses because it changed.
Common causes and fixes
- Fetch the latest version (and its ETag), merge the changes and retry.
Example response
HTTP/1.1 412 Precondition Failed
Date: Thu, 22 Oct 2026 08:00:00 GMT Check a URL’s status with curl -I https://example.com, which prints the status line and headers.