An AI SDK doesn't make the model smarter — it deletes the streaming parser, the tool-calling loop, the JSON-validation prayer, and the per-provider glue you'd otherwise hand-roll against a raw HTTP endpoint. Here's exactly what you get over `fetch`, using the Vercel AI SDK, and where the abstraction leaks.
Why streaming an 8-second answer makes it feel fast even though it isn't, how to pick between long-poll, SSE, and WebSockets, and how to build a NestJS SSE endpoint that proxies a streaming LLM call — including the proxy-buffering, cancellation, and mid-stream-error gotchas that bite everyone the first time.