[v2] AWS Lambda + API Gateway Example

[microadam]

Attempting to build a fresh MCP server, v2 is obviously the future, so trying to get it running using that. Is there a simple example that could be shown on how to get this running with streaming on AWS with API gateway and lambda?

I had it working without any streaming, but trying to use streamifyResponse and then everything just stops working. A simple working example would be very handy.

Many thanks

Code example:

import type { APIGatewayProxyEvent, StreamifyHandler } from "aws-lambda";
import { Readable } from "node:stream";
import { pipeline } from "node:stream/promises";

function buildQueryString(
  params: APIGatewayProxyEvent["queryStringParameters"]
): string {
  if (!params) {
    return "";
  }

  const search = new URLSearchParams();
  for (const [key, value] of Object.entries(params)) {
    if (value !== undefined) {
      search.set(key, value);
    }
  }

  const query = search.toString();
  return query ? `?${query}` : "";
}

function toFetchRequest(event: APIGatewayProxyEvent): Request {
  const domain = event.requestContext.domainName;
  const query = buildQueryString(event.queryStringParameters);
  const url = `https://${domain}${event.path}${query}`;

  const headers = new Headers();
  if (event.headers) {
    for (const [key, value] of Object.entries(event.headers)) {
      if (value !== undefined) {
        headers.set(key, value);
      }
    }
  }

  let body: string | undefined;
  if (event.body) {
    body = event.isBase64Encoded
      ? Buffer.from(event.body, "base64").toString("utf8")
      : event.body;
  }

  return new Request(url, {
    method: event.httpMethod,
    headers,
    body: body && event.httpMethod !== "GET" ? body : undefined
  });
}


export const handler: StreamifyHandler<APIGatewayProxyEvent, void> =
  awslambda.streamifyResponse(async (event, responseStream) => {
    const request = toFetchRequest(event);
    const response = await mcpHandler.fetch(request);

    const httpResponseStream = awslambda.HttpResponseStream.from(
      responseStream,
      {
        statusCode: response.status,
        headers: Object.fromEntries(response.headers)
      }
    );

    if (response.body) {
      await pipeline(Readable.fromWeb(response.body), httpResponseStream);
    } else {
      httpResponseStream.end();
    }
  });

NOTE: mcpHandler is omitted here. Its just the basic response of createMcpHandler from the server package

In terms of API Gateway and the lambda config, the "responseTransferMode" is set to "STREAM" on ApiGateway and the lambda is being invoked via its streaming method

[microadam]

turns out the code is fine and that responseTransferMode" is set to "STREAM wasn't actually being set correctly. Will close, but perhaps the code is useful to someone if they stumble across this :)