HTTP headers / request header
If-Match
If-Match: "33a64df551425fcc"
What it does
Conditional write: "apply my PUT/PATCH/DELETE only if the resource still has this ETag". The backbone of optimistic locking; mismatch → 412.
The gotcha
APIs answering 428 are demanding exactly this header — fetch, take the ETag, retry.
Debugging headers on a phone
The fastest way to see what If-Match is actually doing is to send the request and read both sides raw — ReqPad shows auto-generated and custom headers for every request, on all six protocols, with history. Related references: status codes · Authorization · Accept · Accept-Encoding · Accept-Language · If-None-Match · If-Modified-Since
Inspect If-Match on a live request.
Build the request, send it, read raw headers and timing — from your iPhone. Free to start.