If-Unmodified-Since
RequestActiveIf-Unmodified-Since makes a request conditional – the server only processes it if the resource has NOT been modified since the given date. Used with PUT and PATCH to implement optimistic concurrency: reject an update if the resource changed since the client last fetched it.
If-Unmodified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT
Description
If-Unmodified-Since is the complement of If-Modified-Since. Where If-Modified-Since says 'only give me this if it changed', If-Unmodified-Since says 'only process this if it has NOT changed'. It is used for safe concurrent writes – a client fetches a resource, modifies it locally, then PUTs back with If-Unmodified-Since set to the original Last-Modified date. If the resource was updated by another client in between, the server returns 412 Precondition Failed.
Examples
Conditional PUT
http
PUT /api/config HTTP/1.1
If-Unmodified-Since: Thu, 01 Jan 2026 12:00:00 GMT
Content-Type: application/json