428 Precondition Required

The server requires the request to be conditional (If-Match etc.) to prevent lost updates.

428 at a glance

Code: 428
Name: Precondition Required
Class: 4xx Client error
Status: IANA-registered standard code
Defined in: RFC 6585, Section 3
Cacheable by default: No by default. Caches store it only when the response carries explicit freshness (Cache-Control max-age, s-maxage or Expires).

What 428 means

428 Precondition Required (RFC 6585) is returned when a client tries to modify a resource without sending a precondition such as If-Match. The server forces clients to prove they have seen the latest version, so two clients editing at once cannot silently overwrite each other.

What typically causes a 428

APIs that enforce optimistic concurrency, e.g. some REST APIs for configuration or documents.
Firmware and device-management APIs protecting shared state.

Fix it on the server

Explain in the body which header is required, and return ETags on GET so clients can supply them.

Fix it as a client or visitor

GET the resource, read its ETag, and resend the update with If-Match: "<etag>".

Headers that go with 428

Header	Role
`If-Match`	The precondition the client must send.
`ETag`	Value to use in If-Match.

Example response

HTTP/1.1 428 Precondition Required
Content-Type: application/json

{"error": "Send If-Match with the current ETag"}