412 Precondition Failed

412 Precondition Failed means one of the conditional headers the client sent, If-Match, If-None-Match, If-Modified-Since, or If-Unmodified-Since, evaluated to false against the resource's current state, so the server declined to perform the requested method. Conditional headers let a client state an assumption about the resource, such as only doing this if it still matches this ETag, and have the server enforce that assumption atomically, rather than the client checking and acting in two separate, race-prone steps.

The most common real use is protecting against lost updates: a client fetches a resource along with its ETag, later sends a PUT or DELETE with If-Match set to that ETag, and if another process has modified the resource in the meantime, the ETag no longer matches and the server returns 412 instead of silently applying the write over someone else's change. If-Unmodified-Since works the same way using a timestamp rather than an opaque tag.

412 is easy to confuse with 304 Not Modified, but the two apply to genuinely different situations: 304 is a successful response to a conditional GET using If-None-Match, telling the client its cached copy is still perfectly fine to use, while 412 is a failure response to a conditional unsafe method such as PUT, DELETE, or PATCH, telling the client that its stated assumption about the resource's current state turned out to be wrong.