LABSCO SUMMARY

Solves the "I need auth on someone else's server" problem.

Most MCP servers have no notion of who is calling; forking one to add a check means maintaining the fork. A proxy layer that rejects before forwarding avoids that entirely, and the same seam lets you relabel tool descriptions or collapse several calls into one. The chain is plain Python, so what you can express is not limited by a config schema.

What it is

A composable middleware framework for MCP, modelled on Ruby Rack. Each layer looks like a standard MCP server to the client while forwarding to the next one downstream, so you can add cross-cutting behaviour to a server you did not write and cannot change.

What you get

A transparent proxy pattern — mcp_chain() builds a chain, .then() adds a layer, serve() runs it
Two middleware positions per layer: one that transforms metadata (the tool list a client sees) and one that transforms requests and responses
Requests rejected before they reach the downstream server — the auth example returns {"error": "Authentication required", "code": 401} when no token is present
Tool descriptions rewritten in flight, so the same downstream server presents differently to different clients
An existing MCP server wrapped by name — ExternalMCPServer("postgres", "postgres-mcp")
Shell commands exposed as tools without writing a server — CLIMCPServer takes a name, a list of commands and a description for each
Middleware that can orchestrate several MCP calls behind one tool, turning a granular API into a single higher-level operation

Requirements

Python and uvx — no setup beyond that. Write a small Python file that builds the chain, then run uvx mcp-chain your_server.py and point the client's config at that command. The package is mcp-chain 0.1.9 with the entry point mcp-chain. Chains are ordinary Python functions taking (next_server, request_dict), so logging, rate limiting and header injection are a few lines each.

Setup effort

One command — uvx mcp-chain cli_server.py

See installation options

USE CASES

01

Putting a gate in front of a server that has none

Add an auth layer that checks for a token and returns an error before the request ever reaches the downstream server.

02

Seeing what an agent is actually calling

Insert a logging layer that records the method on the way in and the outcome on the way out.

03

Making a generic server speak your organisation's language

Rewrite tool descriptions in the metadata layer so the model sees context specific to your setup.

04

Exposing a few CLI commands as tools

Declare the commands with a description each and serve them, instead of writing an MCP server from scratch.

BEFORE YOU INSTALL

What to know first

Requires

uvx on PATH

Where it runs

The documented client runs the server on your machine.

INSTALLATION

Connect your client

Server setup

TERMINAL

uvx mcp-chain cli_server.py

MCP Clients

Any MCP client

ANY MCP CLIENT · JSON

{
  "mcpServers": {
    "dev-tools": {
      "command": "uvx",
      "args": [
        "mcp-chain",
        "cli_server.py"
      ]
    }
  }
}

MCP Chain