mcp-middleware

Middleware for MCP servers. mcp-middleware sits between your client and a working MCP server so you can intercept the calls you care about — everything else passes straight through.

Why

You've got a working MCP server you don't want to (or can't) modify, and you need to:

• tweak a request before it hits the server,
• rewrite or enrich a response on its way back,
• post-process a tool's output,
• short-circuit a specific call with your own logic,
• log, meter, or redact traffic.

mcp-middleware lets you do any of the above as a thin layer in front, without forking or patching the underlying server.

Install

npm install mcp-middleware

Usage

import { mcpMiddleware } from 'mcp-middleware';
import { StdioClientTransport } from '@modelcontextprotocol/sdk/client/stdio.js';
import { CallToolRequestSchema } from '@modelcontextprotocol/sdk/types.js';
import { z } from 'zod';

const { connect } = await mcpMiddleware({
  makeTransport: () => new StdioClientTransport({ command: 'some-mcp-server' }),
  requestHandler: async (req, extra, client) => {
    const callTool = CallToolRequestSchema.safeParse(req);
    if (callTool.success && callTool.data.params.name === 'greet') {
      return { content: [{ type: 'text', text: 'intercepted' }] };
    }
    return client.request(req, z.any(), extra);
  },
});

await connect();

Recipe: modify a request or response

Mutate the request before it's forwarded, the response before it's returned, or both.

import { mcpMiddleware } from 'mcp-middleware';
import { StdioClientTransport } from '@modelcontextprotocol/sdk/client/stdio.js';
import { CallToolRequestSchema } from '@modelcontextprotocol/sdk/types.js';
import { z } from 'zod';

const { connect } = await mcpMiddleware({
  makeTransport: () => new StdioClientTransport({ command: 'some-mcp-server' }),
  requestHandler: async (req, extra, client) => {
    const call = CallToolRequestSchema.safeParse(req);

    // Request side: force `read_only: true` on every tool call.
    if (call.success) {
      req.params.arguments = { ...req.params.arguments, read_only: true };
    }

    const res = await client.request(req, z.any(), extra);

    // Response side: redact email addresses in any text content.
    if (call.success && Array.isArray(res.content)) {
      res.content = res.content.map((c) =>
        c.type === 'text'
          ? { ...c, text: c.text.replace(/[\w.-]+@[\w.-]+/g, '[redacted]') }
          : c,
      );
    }

    return res;
  },
});

await connect();

Recipe: hide internal tools

Strip specific tools out of tools/list; everything else is forwarded untouched.

import { mcpMiddleware } from 'mcp-middleware';
import { StdioClientTransport } from '@modelcontextprotocol/sdk/client/stdio.js';
import { ListToolsRequestSchema } from '@modelcontextprotocol/sdk/types.js';
import { z } from 'zod';

const HIDDEN = new Set(['internal-debug', 'dangerous-reset']);

const { connect } = await mcpMiddleware({
  makeTransport: () => new StdioClientTransport({ command: 'some-mcp-server' }),
  requestHandler: async (req, extra, client) => {
    if (ListToolsRequestSchema.safeParse(req).success) {
      const res = await client.request(req, z.any(), extra);
      return { ...res, tools: res.tools.filter((t) => !HIDDEN.has(t.name)) };
    }
    return client.request(req, z.any(), extra);
  },
});

await connect();