The request-response model that defined back-end engineering for two decades is being stretched into something different. Back-ends still serve requests, but increasingly they also set goals, call tools on their own initiative, and run for a few minutes or hours before returning anything. That changes a lot of what back-end engineers have to think about: orchestration, async pipelines, tool governance, state management, and a different shape of failure.Here’s what is actually shifting, and what stays the same.