# Express

You can use the AI SDK in an [Express](https://expressjs.com/) server to generate and stream text and objects to the client.

## Examples

The examples start a simple HTTP server that listens on port 8080. You can e.g. test it using `curl`:

```bash
curl -X POST http://localhost:8080
```

<Note>
  The examples use the OpenAI `gpt-4o` model. Ensure that the OpenAI API key is
  set in the `OPENAI_API_KEY` environment variable.
</Note>

**Full example**: [github.com/vercel/ai/examples/express](https://github.com/vercel/ai/tree/main/examples/express)

### Data Stream

You can use the `pipeDataStreamToResponse` method to pipe the stream data to the server response.

```ts filename='index.ts'
import { openai } from '@ai-sdk/openai';
import { streamText } from 'ai';
import express, { Request, Response } from 'express';

const app = express();

app.post('/', async (req: Request, res: Response) => {
  const result = streamText({
    model: openai('gpt-4o'),
    prompt: 'Invent a new holiday and describe its traditions.',
  });

  result.pipeDataStreamToResponse(res);
});

app.listen(8080, () => {
  console.log(`Example app listening on port ${8080}`);
});
```

### Sending Custom Data

`pipeDataStreamToResponse` can be used to send custom data to the client.

```ts filename='index.ts' highlight="8-11,18"
import { openai } from '@ai-sdk/openai';
import { pipeDataStreamToResponse, streamText } from 'ai';
import express, { Request, Response } from 'express';

const app = express();

app.post('/stream-data', async (req: Request, res: Response) => {
  // immediately start streaming the response
  pipeDataStreamToResponse(res, {
    execute: async dataStreamWriter => {
      dataStreamWriter.writeData('initialized call');

      const result = streamText({
        model: openai('gpt-4o'),
        prompt: 'Invent a new holiday and describe its traditions.',
      });

      result.mergeIntoDataStream(dataStreamWriter);
    },
    onError: error => {
      // Error messages are masked by default for security reasons.
      // If you want to expose the error message to the client, you can do so here:
      return error instanceof Error ? error.message : String(error);
    },
  });
});

app.listen(8080, () => {
  console.log(`Example app listening on port ${8080}`);
});
```

### Text Stream

You can send a text stream to the client using `pipeTextStreamToResponse`.

```ts filename='index.ts' highlight="13"
import { openai } from '@ai-sdk/openai';
import { streamText } from 'ai';
import express, { Request, Response } from 'express';

const app = express();

app.post('/', async (req: Request, res: Response) => {
  const result = streamText({
    model: openai('gpt-4o'),
    prompt: 'Invent a new holiday and describe its traditions.',
  });

  result.pipeTextStreamToResponse(res);
});

app.listen(8080, () => {
  console.log(`Example app listening on port ${8080}`);
});
```

## Troubleshooting

- Streaming not working when [proxied](/docs/troubleshooting/streaming-not-working-when-proxied)


## Navigation

- [Node.js HTTP Server](/v4/cookbook/api-servers/node-http-server)
- [Express](/v4/cookbook/api-servers/express)
- [Hono](/v4/cookbook/api-servers/hono)
- [Fastify](/v4/cookbook/api-servers/fastify)
- [Nest.js](/v4/cookbook/api-servers/nest)


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