Microservices were designed around a simple contract: a known caller sends a predictable request, expects a typed response, and moves on. That contract held for years. Then AI agents arrived.An agent doesn't call your service once. It might call it three times in a single reasoning loop, fan out to five services simultaneously, retry on ambiguous output, or decide mid-flight that a different endpoint is more appropriate. The distributed systems assumptions baked into your architecture rate limiting, idempotency, circuit breakers, auth flows were never built for a non-deterministic, autonomous caller. This article walks through the five core assumption breaks and how to address each one practically.