SDK makes HTTPS fetches through an HTTP proxy

We have a corporate proxy, running on a localhost port, for third-party applications to be able to talk to internal services over HTTP. We can't complete connections over this proxy with the Typescript SDK (via the inspector) because, as far as I can tell, Node's fetch implementation ends up trying to run an HTTPS connection through the proxy and fails as the initial CONNECT to set up a tunnel gets early-terminated in a situation where the SDK is expecting the result of a plain HTTP call. In particular, here:

https://github.com/modelcontextprotocol/typescript-sdk/blob/68a0ca350576a9f7908010f1e4fef2f414cd0f9a/src/client/streamableHttp.ts#L455-L458

we get Unexpected content type: null, which our logs show is in response to that CONNECT request. (at the JSON-RPC layer, this is from the initialize request)

Ideally, when we have an HTTP_PROXY env var and no HTTPS_PROXY, we wouldn't be trying to use HTTPS at all.

As a reproduction of the CONNECT request being made, I had Claude write me a little fake version of an HTTP proxy:

st-ajgray1:mcp-repro ajgray$ cat proxy-server.js 
const http = require('http');
const net = require('net');

const server = http.createServer((req, res) => {
  console.log(`Regular HTTP request: ${req.method} ${req.url}`);
  
  // Handle normal HTTP requests
  res.writeHead(200);
  res.end('HTTP request handled');
});

server.on('connect', (req, clientSocket, head) => {
  console.log(`CONNECT request detected for: ${req.url}`);
  
  // Log but reject the CONNECT request
  clientSocket.write('HTTP/1.1 400 Bad Request\r\n\r\n');
  clientSocket.end();
});

server.listen(8888, () => {
  console.log('Proxy server running on port 8888');
});

Running the inspector while pointing at that proxy:

st-ajgray1:inspector ajgray$ HTTP_PROXY=http://127.0.0.1:8888 npm run start

And then trying to connect to a Streamable HTTP server fails:

because this CONNECT request is detected:

st-ajgray1:mcp-repro ajgray$ node proxy-server.js 
Proxy server running on port 8888
CONNECT request detected for: fake-mcp-server.com:80

AIUI, this is consistent with the underlying fetch library, undici, trying to make this an HTTPS request. Due to limitations of our proxy, only HTTP would play nice with our systems; we would prefer that the SDK here make undici stay in HTTP-land here.

Start in src/client/streamableHttp.ts around lines 455-458 and reproduce through the inspector with HTTP_PROXY and the provided proxy-server.js. Trace how the SDK and Node fetch choose the proxy for the initialize request; done means an HTTP-only proxy does not trigger CONNECT and Streamable HTTP initialization succeeds.