428 Precondition Required

What’s actually happening

A GET works but a PUT, PATCH, or DELETE comes back 428. The API is refusing the write on purpose: it won't let you overwrite a resource blind. It wants you to send the ETag (or last-modified date) you last saw, so it can confirm nobody changed the record between your read and your write. This is the API designer deliberately closing the lost-update hole, where two clients both GET, both edit, and the second save silently clobbers the first.

Common causes

• Your write request omits the If-Match header (or If-Unmodified-Since) that the endpoint requires.
• Your HTTP client strips the ETag from the GET response, so you never have a validator to send back.
• You're hand-testing in curl/Postman and simply forgot to copy the ETag from the read into the write.
• An API gateway policy enforces conditional requests on a whole route, and a new endpoint slipped under it without the client being updated.
• You cached the ETag but send it on the wrong header (If-None-Match instead of If-Match), which the server treats as missing.

How to fix it

1. GET the resource and capture its ETagRead the record first and grab the ETag response header, e.g. ETag: "a1b2c3". This is the validator the server is asking for. Store it alongside the data in your client state.
2. Send it back on If-Match for the writeOn the PUT/PATCH/DELETE, add If-Match: "a1b2c3". If the API uses dates instead, send If-Unmodified-Since with the Last-Modified value from the GET. The 428 turns into a normal 200/204.
3. Stop your HTTP layer from discarding the ETagSome wrappers normalize away response headers. Log the raw GET response and confirm the ETag survives to your code. If your framework auto-handles ETags (e.g. some ORMs / API clients), enable that feature instead of doing it by hand.
4. Read the error body for the exact header nameA spec-compliant 428 names the precondition it wants. If it asks for If-Match and you've been sending If-None-Match, that's why it still reads as missing, they're different operators.