4xx Client Error

HTTP 412 Precondition Failed

One or more conditions given in the request headers (like If-Match or If-Unmodified-Since) evaluated to false on the server. The conditional request cannot be fulfilled.

Common Causes

  • If-Match ETag doesn't match current resource version
  • If-Unmodified-Since date is older than last modification
  • Optimistic concurrency check failed
  • Conditional PUT/PATCH with stale data

How to Fix

  1. 1Fetch the latest resource version and update your ETag/date
  2. 2Re-read the resource before attempting the conditional update
  3. 3Remove precondition headers if strict versioning isn't needed

Example

HTTP 412 Precondition Failed
PUT /api/doc/1 HTTP/1.1
If-Match: "old-etag"

→ 412 Precondition Failed
→ Resource was modified by another client

Related Client Error Codes

400

Bad Request

401

Unauthorized

402

Payment Required

403

Forbidden

404

Not Found

405

Method Not Allowed

406

Not Acceptable

407

Proxy Authentication Required

408

Request Timeout

409

Conflict

410

Gone

411

Length Required

413

Content Too Large

414

URI Too Long

415

Unsupported Media Type

416

Range Not Satisfiable

417

Expectation Failed

418

I'm a Teapot

421

Misdirected Request

422

Unprocessable Content

423

Locked

424

Failed Dependency

425

Too Early

426

Upgrade Required

428

Precondition Required

429

Too Many Requests

431

Request Header Fields Too Large

451

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