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412
Client Error

HTTP status code 412 Precondition Failed

412 Precondition Failed (RFC 9110, 15.5.13): an If-Match or If-Unmodified-Since condition was false, so nothing changed — usually a stale ETag.

Definition (RFC 9110)

HTTP status code 412 Precondition Failed is defined in RFC 9110 (HTTP Semantics), Section 15.5.13 — the current authoritative specification for HTTP semantics, published June 2022, which obsoletes the older RFC 7231 and RFC 2616.

Description

412 Precondition Failed occurs when conditional request headers (If-Match, If-None-Match, If-Modified-Since, If-Unmodified-Since) evaluate to false. This is used to prevent mid-air collisions: if a resource has been modified since you last read it, your update is rejected.

Essential for optimistic concurrency control in REST APIs.

Common Causes

  • Resource was modified by another client between read and write (lost update)
  • Stale ETag value in If-Match header
  • Conditional GET where the resource hasn't changed
  • Cache validation failure

How to Fix

  • Re-fetch the resource to get the latest ETag before updating
  • Implement merge strategies or conflict resolution UI
  • Use optimistic locking: include version number or ETag in update requests
  • Inform the user that the resource was updated by someone else

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Related Status Codes

400
400 Bad Request
The server cannot process the request due to malformed syntax.
 401
401 Unauthorized
Authentication is required and has failed or not been provided.
 402
402 Payment Required
402 Payment Required — code reserved for future use, but currently used by some APIs to signal "payment needed".
 403
403 Forbidden
403 Forbidden (RFC 9110, 15.5.4): the server understood the request but refuses it. Re-sending the same credentials will not help.
 404
404 Not Found
HTTP 404 Not Found is the status code a server returns when it is reachable but has nothing at the requested URL, without saying whether that is temporary or permanent. Search engines drop URLs that keep returning it.
 405
405 Method Not Allowed
The HTTP method used is not allowed for this resource.

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