CORS Configuration Toolkit

CORS is the #1 source of cryptic browser errors in web development. This skill eliminates the guesswork with a complete diagnostic decision tree, production-ready configurations for 8 frameworks (Express, FastAPI, Django, Next.js, Nginx, Caddy, Go, and more), a security audit checklist, preflight debugging guide, and testing scripts.

Includes: • Diagnostic tree that maps browser errors to exact fixes • Copy-paste configs for Express.js, FastAPI, Django, Next.js, Nginx, Caddy, Go • Security audit checklist covering origin reflection attacks, wildcard misuse, regex bypasses • Preflight deep-dive explaining when and why OPTIONS requests fire • curl-based testing commands to verify CORS outside the browser • Common pitfalls: wildcard + credentials, Vary: Origin, duplicate middleware, localhost vs 127.0.0.1

Worth $2 because every developer hits CORS issues repeatedly, and the answers on Stack Overflow are usually wrong or outdated. This gives your agent the complete mental model.