What it means
The server requires the request to be conditional (If-Match etc.) to prevent lost updates.
Common causes
- PUT or PATCH without If-Match on an API that enforces optimistic locking
How to fix it
As a client or visitor
GET the resource, take its ETag, and send it in If-Match with your update.
As the site or API owner
Explain in the body which header is required.
Example response
HTTP/1.1
HTTP/1.1 428 Precondition Required
Date: Tue, 07 Oct 2025 09:30:00 GMT
Server: nginx
Content-Type: application/problem+json
{
"type": "about:blank",
"title": "Precondition Required",
"status": 428,
"detail": "Send If-Match with the ETag you last read."
}Specification
FAQ
› Why would an API require If-Match?
To prevent two clients from overwriting each other's changes without noticing.
› 428 vs 412?
428: no precondition was sent. 412: one was sent but failed.