Most HTTP bugs can be pinned down with one tool you already have. This guide shows the curl options we use daily: verbose output, header-only requests, --resolve and --connect-to for testing servers before DNS changes, timing breakdowns, protocol versions and TLS checks.
A low but stubborn rate of 502s at the load balancer and socket hang up errors between services turned out to be one race: Node closed idle keep-alive connections after 5 seconds while its callers kept reusing them.
A practical guide to cookie attributes for engineers who keep getting logged out of embedded widgets or failing security reviews: what SameSite, Secure, HttpOnly, __Host- and Partitioned really do, with tested defaults for common setups.
Moving our SPA from cookie auth to bearer tokens broke every API call with a CORS error. The Authorization header triggered an OPTIONS preflight, our nginx auth check rejected it, and error responses carried no CORS headers.