HTTP headers / request header
Cookie
Cookie: session=abc123; theme=dark
What it does
Sends the stored cookies back to the server, semicolon-separated in a single header.
The gotcha
Cookie bloat is the top cause of 431 errors; API clients with cookie jars may send session state you forgot you had — check what is actually in the jar.
Debugging headers on a phone
The fastest way to see what Cookie 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-Match
Inspect Cookie on a live request.
Build the request, send it, read raw headers and timing — from your iPhone. Free to start.