Next.js App Router chat route: streamText to UI message stream wiring

The App Router chat endpoint has one correct shape in AI SDK 7. Get any piece wrong and the client either hangs, never aborts, or throws a parse error.

app/api/chat/route.ts:

import { convertToModelMessages, createUIMessageStreamResponse, streamText, toUIMessageStream } from 'ai';

export const maxDuration = 30;

export async function POST(req: Request) {
  const { messages } = await req.json();

  const result = streamText({
    model: 'your-model-id',
    instructions: 'You are a helpful assistant.',
    messages: await convertToModelMessages(messages),
    abortSignal: req.signal,
  });

  return createUIMessageStreamResponse({
    stream: toUIMessageStream({ stream: result.stream }),
  });
}

Checklist:
1. messages from useChat are UIMessage objects, not model messages. Convert them with convertToModelMessages, and await it (it is async since v6).
2. Pass abortSignal: req.signal so a client stop() actually cancels the provider request instead of leaving a zombie generation running.
3. In AI SDK 7 the prompt option for system-level text is instructions, not system. The v7 codemod is npx @ai-sdk/codemod v7/rename-system-to-instructions.
4. Return createUIMessageStreamResponse, not a raw Response. The UI message stream protocol is what useChat's DefaultChatTransport parses.
5. Set export const maxDuration = 30 on the route. Vercel serverless functions time out otherwise, and long generations die mid-stream.
6. Never call streamText in a client component or inside the page that renders useChat. Route handlers are server-only; the 'use client' component talks to them over HTTP.