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Adapters

Express and Fastify parity is a shipped goal, not an assumption. The same @AiStream handler streams identically on both adapters, verified by twin samples and an end-to-end suite per adapter.

How Parity Works​

@AiStream writes to the underlying Node ServerResponse, which both adapters expose:

  • Express returns the Node ServerResponse directly from getResponse().
  • Fastify returns a FastifyReply whose .raw property is the underlying ServerResponse.

The package normalizes both: it resolves the raw Node response from whichever shape the adapter hands it. That is why the cookbook's Express-only recipe (which imports express's Response type) becomes adapter-agnostic under @AiStream.

Fastify reply.hijack()​

On Fastify, @AiStream calls reply.hijack() before streaming. This tells Fastify to relinquish control of the response lifecycle so the AI SDK can own the raw socket. Without it, Fastify would still try to send its own reply after the stream — which throws ERR_HTTP_HEADERS_SENT when a client disconnects mid-stream.

Express has no equivalent and omits hijack, so the package calls it only when present. This is the one adapter-specific behavior in the package, and it is internal — your handler code is identical on both.

Switching Adapters​

Nothing in your controller changes. Only the bootstrap differs:

// Express
const app = await NestFactory.create(AppModule);
// Fastify
const app = await NestFactory.create<NestFastifyApplication>(
  AppModule,
  new FastifyAdapter(),
);

Verified By​

  • sample/01-fastify-parity mounts one controller on both an Express app and a Fastify app and asserts the streamed payloads match byte-for-byte.
  • The package end-to-end suite mirrors the Express tests on Fastify (pre-stream guard rejection, text and UI message streams, pre-stream filter).
  • sample/02-abort-signal verifies disconnect cancellation on both adapters.