4xx Client Error

HTTP 428 Precondition Required

The server requires the request to be conditional. This is intended to prevent the 'lost update' problem, where a client GETs a resource, modifies it, and PUTs it back while a third party has modified it.

Common Causes

  • Server requires If-Match or If-Unmodified-Since header
  • Optimistic concurrency control is enforced
  • API mandates conditional requests for write operations

How to Fix

  1. 1Add an If-Match header with the current ETag
  2. 2Fetch the resource first to get its ETag, then send conditional request
  3. 3Include If-Unmodified-Since with the last known modification date

Example

HTTP 428 Precondition Required
PUT /api/doc/1 HTTP/1.1
(no If-Match header)

→ 428 Precondition Required
→ "Include If-Match header for updates"

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

412

Precondition Failed

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

429

Too Many Requests

431

Request Header Fields Too Large

451

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