All HTTP status codes

HTTP 412 Precondition Failed

Client Error

A conditional header (If-Match, If-Unmodified-Since…) evaluated to false — the resource changed since the client last saw it.

Defined in RFC 9110

Common causes

  • Optimistic locking: ETag in If-Match is stale
  • Conditional uploads guarding against overwrites

How to fix it

  1. GET the resource again, take the fresh ETag, reapply changes

  2. Surface a 'document changed elsewhere' flow to end users

Related status codes

409 Conflict

The request conflicts with the resource's current state — classic cases are duplicate creation and stale optimistic-locking versions.

304 Not Modified

The cached copy the client holds is still valid — the server sends no body. The result of conditional requests with If-None-Match (ETag) or If-Modified-Since.

428 Precondition Required

The server requires the request to be conditional — send If-Match/If-Unmodified-Since to avoid lost-update races.

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