Allow for custom fetch to be used from the MultiServerMCPClient so that requests can be signed for authentication when calling remote MCP server over HTTP

[waltmayf]

Checked

• I searched existing ideas and did not find a similar one
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• I've clearly described the feature request and motivation for it

Feature request

I want to host my MCP server in an AWS Lambda function, and use AWS IAM authentication between my MCP client and the MCP server. I can use the "AWS_IAM" authentication type with the AWS Lambda function url, and that requires the requests to be signed using my AWS credentials. With the current implementation of MultiServerMCPClient, I can't sign requests before sending them, and so I can't authenticate against my AWS Lambda function. This leaves my only options being to 1/ use OAuth to authenticate with my AWS Lambda function (Which means using AWS IAM for everything except this function), or 2/ Using static headers, which is less secure.

Motivation

Adding the ability to sign requests before they are sent to the MCP server would make the MCP client - server connections more secure, and would facilitate wider adoption of the langgraph.js platform because it would be possible to meet more customer's security requirements.

Proposal (If applicable)

The easiest way to do this is probably to let the user supply their own fetch which will sign the requests before sending them.

Here's what the end result could look like:

import { MultiServerMCPClient } from "@langchain/mcp-adapters";
const mcpClient = new MultiServerMCPClient({
      useStandardContentBlocks: true,
      mcpServers: {
          math: {
              url: process.env.A4E_MCP_SERVER_URL!,
              fetch: myCustomFetchWhichSignsRequests
          }
      }
  })