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Architecture

How MCP Router is built

Overview

MCP Router is a macOS menu bar app with a Rust core.

  • Swift/SwiftUI — The UI, HTTP server, process management
  • Rust — MCP protocol handling via FFI
┌───────────────────────────────────────────┐
│             MCP Router App                │
├───────────────────────────────────────────┤
│  SwiftUI         │  HTTP Server           │
│  (Menu Bar UI)   │  (localhost:19104)     │
├───────────────────────────────────────────┤
│            Process Manager                │
│      (Spawns and manages servers)         │
├───────────────────────────────────────────┤
│            Rust Core (FFI)                │
│   (MCP protocol, JSON-RPC, stdio)         │
└───────────────────────────────────────────┘
              │
              ▼
    ┌─────────┬─────────┐
    │ Server  │ Server  │  (stdio or http)
    └─────────┴─────────┘

HTTP Server

Listens on localhost:19104. Receives JSON-RPC requests from MCP clients (Claude Code, Codex, etc.).

When a request comes in:

  1. Look at the X-Workspace-Token header
  2. Find the matching workspace
  3. Route to the right servers
  4. Return the response

The 4 meta tools (list_servers, list_tools, describe, call) are handled by the router itself. Tool calls get forwarded to backend servers.

Process Manager

Spawns MCP server processes and keeps track of them.

For stdio servers, it runs the command and talks to the process via stdin/stdout. For http servers, it just sends HTTP requests to the configured URL.

Rust Core

The MCP protocol has some fiddly parts — JSON-RPC message framing, request-response correlation, proper error handling over stdio.

Rust handles this. Swift calls into Rust via C FFI for the protocol work.

Data Storage

Server configs and workspace settings are stored locally. The menu bar app manages everything — no external database.

File locations:

  • ~/.vimo/mcp-router/ — Settings, workspaces, servers config

Source Layout

mcp-router/
├── core/                 # Rust core (FFI)
│   └── src/
├── mcp-router/           # Swift app
│   ├── Models/           # Data models
│   ├── Views/            # SwiftUI views
│   └── Services/         # HTTP server, process manager, etc.
└── scripts/              # Build scripts

Want to dig deeper? Check the GitHub repo.