What header satisfies a 428?

Usually If-Match with the ETag you last read. Some APIs also accept If-Unmodified-Since.

428 for a missing precondition, 412 for a precondition that was present but no longer true.

HTTP 428 Precondition Required

The server requires this request to be conditional — to carry If-Match or similar — so that a write cannot overwrite a change it has not seen.

Defined in RFC 6585 §3 — 428 Precondition Required · MDN reference

Where you meet HTTP 428 in production

APIs that enforce optimistic concurrency on every write: configuration stores, CMS records, collaborative documents — anywhere a lost update is expensive.

Why you would test it

Send a PUT with no If-Match and expect 428; send it with a stale ETag and expect 412. Testing both proves the server enforces optimistic concurrency rather than merely supporting it.

What your client should do about a 428

Read the resource first, take its ETag, and send it back in If-Match on the write. Do not invent an ETag or send If-Match: * just to get past the check — that disables the protection it exists for.

428 versus the codes it gets confused with

428 vs 412 Precondition Failed — 412 means you sent a condition and it failed. 428 means you did not send one at all.
428 vs 409 Conflict — 409 is a conflict the server detected by itself; 428 requires you to declare the version you expect.

Endpoints that return 428

1 endpoint in this playground answers with 428. Every one is free, needs no signup, and can be called from the browser or with curl.

GET https://funapi.dev/api/faults/v1/status/{code} — Returned on request (Fault Injection API) Open & run this endpoint

Questions about HTTP 428

What header satisfies a 428?: Usually If-Match with the ETag you last read. Some APIs also accept If-Unmodified-Since.
428 or 412?: 428 for a missing precondition, 412 for a precondition that was present but no longer true.

Other client error codes

All HTTP status codes · All 39 mock REST APIs · Getting started guide

Last updated 2 October 2026