An If-Match, If-None-Match or If-Unmodified-Since condition that evaluated false - almost always because the resource changed since the client read it. It is a signal to re-read and retry, not a client bug.

412 when the failure was specifically a precondition the client supplied, which is a clearer signal for automated retry. 409 for conflicts detected some other way, such as a uniqueness violation.

How does this prevent lost updates?

Without a precondition, two clients that both read version 1 and both write will have the second silently overwrite the first. With If-Match, the second write fails with 412, and the client re-reads and reapplies its change.

412 Precondition Failed

A conditional request's precondition was not met, so the request was not applied.

Class 4xx · Client error
Retry Retry with care

What 412 means

412 is the failure counterpart of 304. The client attached a condition - If-Match with an ETag, If-Unmodified-Since with a date - and the condition was false, so the server refused to proceed. Nothing was changed.

This is the mechanism for optimistic concurrency control on writes. The client reads a resource and its ETag, sends the ETag back with If-Match on the update, and a 412 tells it that someone else changed the resource in the meantime. The client re-reads and retries, and no update is silently lost.

It is worth preferring over 409 for this specific case, because it says precisely what failed. A 409 tells the client there is a conflict; a 412 tells it the conflict was detected by its own precondition, which is a clearer signal for automated retry.

The related 428 Precondition Required lets a server insist on conditional requests, refusing unconditional writes so that clients cannot accidentally overwrite.

Headers this status expects

ETag - the current validator, so the client can retry against the new state.

Should a client retry?

Re-read the resource to get its current ETag, then retry the update with that value. Do not retry with the stale ETag.

FAQ

What causes a 412?: An If-Match, If-None-Match or If-Unmodified-Since condition that evaluated false - almost always because the resource changed since the client read it. It is a signal to re-read and retry, not a client bug.
412 or 409?: 412 when the failure was specifically a precondition the client supplied, which is a clearer signal for automated retry. 409 for conflicts detected some other way, such as a uniqueness violation.
How does this prevent lost updates?: Without a precondition, two clients that both read version 1 and both write will have the second silently overwrite the first. With If-Match, the second write fails with 412, and the client re-reads and reapplies its change.

Often confused with

409 Conflict The request conflicts with the current state of the resource.
304 Not Modified The cached copy is still current, so the server sent no body.