428 Precondition Required

428 Precondition Required (RFC 6585) indicates that the origin server requires the request to be conditional. This is used to prevent the "lost update" problem: without conditional requests, a client might unknowingly overwrite changes made by another user since the resource was last fetched.

The server is essentially saying: "I require you to prove you have the latest version before I allow this modification."

• REST API enforcing optimistic concurrency: requires If-Match on PUT/PATCH/DELETE
• CMS or document editor requiring version checks before saving
• Shared resource management preventing silent overwrites

• Include the appropriate conditional header: If-Match with the current ETag
• Fetch the resource first to get the ETag before modifying
• Implement optimistic locking in your API client
• Document that conditional headers are required in API documentation