# Fastify (/docs/adapters/fastify)

## Installation

`xmcp` can work on top of your existing Fastify project. To get started, run the following command in your project directory:

```bash
npx init-xmcp@latest
```

After setting up the project, your build command should look like this:

```json
{
  "scripts": {
    "build": "xmcp build && tsc"
  }
}
```

`xmcp build` bundles your tools into `.xmcp/adapter`.

## Usage

Install Fastify and register the MCP handler on your server:

```bash
npm install fastify
```

```typescript
import Fastify from "fastify";
import { xmcpHandler } from "@xmcp/adapter";

const app = Fastify({ logger: false });

app.post("/mcp", xmcpHandler);
app.get("/mcp", xmcpHandler); // required for SSE / streaming clients

await app.listen({ port: 3000 });
```

<Callout variant="info">
  For browser-origin clients, add CORS and preflight handling at the Fastify app
  level (e.g. `@fastify/cors`). The `xmcpHandler` handles the MCP endpoint only
  — it does not register an OPTIONS route.
</Callout>

## AWS Lambda

Use `@fastify/aws-lambda` as the Lambda bridge:

```bash
npm install fastify @fastify/aws-lambda
```

```typescript
import Fastify from "fastify";
import awsLambdaFastify from "@fastify/aws-lambda";
import { xmcpHandler } from "@xmcp/adapter";

const app = Fastify({ logger: false });

app.post("/mcp", xmcpHandler);

export const handler = awsLambdaFastify(app);
```

The default buffered form (`awsLambdaFastify(app)`) works for all JSON-RPC POST requests (`initialize`, `tools/call`, etc.). GET/SSE is not supported in buffered mode — see the streaming form below.

### SSE support on Lambda

To support SSE via `GET /mcp`, use a Lambda Function URL with `InvokeMode: RESPONSE_STREAM` and switch to the streaming form:

```typescript
import Fastify from "fastify";
import awsLambdaFastify from "@fastify/aws-lambda";
import { xmcpHandler } from "@xmcp/adapter";
import { promisify } from "node:util";
import stream from "node:stream";

const pipeline = promisify(stream.pipeline);
const app = Fastify({ logger: false });

app.post("/mcp", xmcpHandler);
app.get("/mcp", xmcpHandler);

const proxy = awsLambdaFastify(app, { payloadAsStream: true });

export const handler = awslambda.streamifyResponse(
  async (event, responseStream, context) => {
    const { meta, stream: bodyStream } = await proxy(event, context);
    responseStream = awslambda.HttpResponseStream.from(responseStream, meta);
    await pipeline(bodyStream, responseStream);
  }
);
```

<Callout variant="info">
  `middleware.ts` is not supported in adapter mode.
</Callout>

## xmcp.config.ts

```typescript
import type { XmcpConfig } from "xmcp";

const config: XmcpConfig = {
  http: true,
  experimental: {
    adapter: "fastify",
  },
};

export default config;
```