Streaming LLM responses in TypeScript: SSE, ReadableStream, and the React 19 useChat hook.

The first time I wired an LLM response that streamed token by token instead of arriving as one lump after 4 seconds, I shipped it to production the same afternoon. The difference in perceived speed is that obvious to anyone who has used ChatGPT and then tried a non-streaming competitor. Users will wait 8 seconds if they can see the cursor moving. They will not wait 4 seconds for a blank screen.

This tutorial walks the full stack from scratch. By the end you have a working Next.js API route that streams from an LLM over Server-Sent Events, a frontend that parses the stream manually with ReadableStream, and then the same UI rebuilt with the Vercel AI SDK's useChat hook so you can see what the abstraction actually buys you.

TL;DR

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Streaming LLM responses in TypeScript: SSE, ReadableStream, and the React 19 useChat hook.

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