The CORS Header Was Right There and the Browser Blocked It Anyway

The browser console showed exactly what CORS errors always show — a request blocked for violating the same-origin policy — except the response headers, visible in the network tab, clearly included Access-Control-Allow-Origin: * . The header the browser wanted was right there. The browser rejected the request anyway. The detail that's easy to miss in the network tab Chrome's network inspector, by default, coalesces duplicate header names into a single display line — so Access-Control-Allow-Origin: * shown once in the UI can actually mean the header was sent twice by the server, and the browser is showing you a merged, deduplicated view rather than the literal wire response. curl -s -D - https://api.example.com/data -o /dev/null | grep -i access-control Access-Control-Allow-Origin: * Access-Control-Allow-Origin: https://app.example.com Two separate headers, both valid individually, sent by two different layers that each thought they were the one responsible for CORS: our nginx reverse proxy had a blanket add_header Access-Control-Allow-Origin *; for general API access, and the application server behind it independently set a specific origin for authenticated routes. Neither config was wrong on its own. Together, they produced a response with the header appearing twice — and per the Fetch spec, a response with multiple Access-Control-Allow-Origin values is treated as invalid, so the browser blocks the request rather than guessing which one you meant. Why this is worse than a missing header A missing CORS header fails immediately, obviously, the same way every time. A duplicate header fails in a way that looks, from the response body alone, like the header is present and correct — because it is present, twice, which is precisely the state that trips the spec's validation. Every piece of evidence you'd normally check says "this should work," and it still doesn't. The fix Removed the blanket nginx header and let the application server be the single source of truth for COR