412 Precondition Failed

What 412 Precondition Failed means {#what}

Used in conditional requests where the client sets preconditions via headers like If-Match or If-Unmodified-Since. If those conditions aren’t met (e.g., the resource was modified by someone else), the server rejects the request with 412.

When you’ll see a 412 {#when}

• Optimistic concurrency control: you fetch a resource, get its ETag, then try to update it — if it changed in the meantime, you get 412
• If-Match header that no longer matches

When to use 412 {#use}

• Implement ETags for safe concurrent updates
• Prevents the “lost update” problem in collaborative APIs