DriftGuard is a tool that monitors third-party APIs for silent schema changes, the kind that return 200 OK while quietly breaking your integration. This blog covers the building of DriftGuard in public: technical decisions, lessons learned, and the occasional post-mortem on API changes that have broken real production systems. If you've ever spent a Friday night debugging why your data looks wrong only to discover an upstream API quietly renamed a field, this blog is for you.