mcp-relay

A lightweight MCP (Model Context Protocol) stdio-to-HTTP relay written in Rust.

Why?

I wanted to use O'Reilly's MCP server in Claude Desktop. The problem: Claude Desktop only supports stdio-based MCP servers, but O'Reilly's server uses Streamable HTTP with Bearer token authentication.

The existing Node.js solution (mcp-remote) works, but consumes ~70MB of RAM just to sit there and proxy JSON. That felt wrong for what is essentially "read stdin, POST it, write response to stdout."

So I built mcp-relay in Rust. It does the same job in ~2.6MB of RAM with a ~1.9MB binary.

Features

Bridges MCP stdio transport to remote Streamable HTTP servers
Custom HTTP headers via -H flag (for Bearer tokens, API keys, etc.)
Async handling of JSON-RPC notifications (fire-and-forget) and requests (wait for response)
Tiny footprint: ~1.9MB binary, ~2.6MB RAM at runtime

Install

cargo install mcp-relay

Usage

mcp-relay [OPTIONS] <URL>

Options

| Flag | Description | |------|-------------| | -H <KEY> <VALUE> | Add custom HTTP header (can be repeated) | | -h, --help | Print help |

Example

mcp-relay \
  -H Authorization "Bearer your-token-here" \
  https://api.example.com/mcp/

Claude Desktop Configuration

Add to your claude_desktop_config.json:

{
  "mcpServers": {
    "my-server": {
      "command": "mcp-relay",
      "args": [
        "-H", "Authorization", "Bearer your-token-here",
        "https://api.example.com/mcp/"
      ]
    }
  }
}

If cargo install puts the binary in a path Claude Desktop can't find, use the full path (e.g., /Users/you/.cargo/bin/mcp-relay).

Comparison

| | mcp-relay (Rust) | mcp-remote (Node.js) | |---|---|---| | Binary size | ~1.9 MB | ~70 MB (node_modules) | | RAM usage | ~2.6 MB | ~70 MB | | Dependencies | 4 crates | npm ecosystem | | Startup | Instant | ~2s (npx download) |

License

MIT

MCP Servers