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412
Client Error

412 Precondition Failed

One or more conditions given in the request headers evaluated to false on the server.

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
Payment is required to access this resource.
 403
403 Forbidden
The server understood the request but refuses to authorize it.
 404
404 Not Found
The server cannot find the requested resource.
 405
405 Method Not Allowed
The HTTP method used is not allowed for this resource.

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