MCP Proxy Hub

An MCP proxy server that aggregates and serves multiple MCP resource servers through a single interface. This server acts as a central hub that can:

• Connect to and manage multiple MCP resource servers
• Expose their combined capabilities through a unified interface
• Handle routing of requests to appropriate backend servers
• Aggregate responses from multiple sources

Features

Resource Management

• Discover and connect to multiple MCP resource servers
• Aggregate resources from all connected servers
• Maintain consistent URI schemes across servers
• Handle resource routing and resolution

Tool Aggregation

• Expose tools from all connected servers with server name prefixes
• Apply tool filtering based on configuration (exposedTools/hiddenTools)
• Support tool name remapping via configuration
• Route tool calls to appropriate backend servers

Custom Tool Support

• Define compound tools that combine functionality from multiple servers
• Execute subtools using server and tool name specifications
• Provide detailed documentation through tool descriptions
• Specify execution with a standardized format:

{
    "server": "server_name",
    "tool": "tool_name",
    "args": {
      // Tool-specific arguments
    }
  }

Environment Variable Support

• Automatically expand environment variables in tool arguments
• Automatically replace sensitive values with variable references in responses
• Configure which variables should be expanded/unexpanded via configuration
• Support for both global (all servers) and server-specific environment variables
• Server-specific variables take precedence over global variables with the same name
• Each variable can be independently configured for expansion and unexpansion
• Environment variables are only expanded when using the `${VARIABLE_NAME}` syntax (e.g., `${API_KEY}`). The `$VARIABLE_NAME` syntax is not supported.
• Secure handling of sensitive information like API keys

Prompt Handling

• Aggregate prompts from all connected servers
• Route prompt requests to appropriate backends
• Handle multi-server prompt responses

Configuration

The server requires a JSON configuration file that specifies the MCP servers to connect to. Copy the example config(config.example.json) and modify it for your needs:

cp config.example.json config.json

Configuration Options

MCP Server Configuration

• Stdio-type Server:

• SSE-type Server:

• Streamable HTTP-type Server:
  • `type`: "streamable-http" (required)
  • `url`: URL of the Streamable HTTP server (required)
  • `headers`: Object of HTTP headers to send with requests (optional)
  • `exposedTools`: Array of tools to expose (optional)
  • `hiddenTools`: Array of tools to hide (optional)
  • `envVars`: Environment variable configuration for tool arguments and responses (optional)
  • `timeout`: Request timeout in seconds for downstream tool calls (optional, overrides the top-level `timeout`; `0` disables the timeout)
  • `enable`: Whether to enable the server (optional, default: true)

Tool Filtering Configuration

• exposedTools:

• hiddenTools:
  • Hides specified tools
  • Array of tool name strings to hide

Environment Variables Configuration

• Server-specific envVars:

"envVars": [
      { "name": "API_KEY", "value": "my-api-key", "expand": true, "unexpand": true },
      { "name": "USER_ID", "value": "user123", "expand": true, "unexpand": false }
    ]

• Global envVars:

"envVars": [
      { "name": "GLOBAL_API_KEY", "value": "global-api-key", "expand": true, "unexpand": true },
      { "name": "GLOBAL_ENV", "value": "production", "expand": true, "unexpand": false }
    ]

Timeout Configuration

Controls how long the proxy hub waits for a downstream server to respond to a tool call (`tools/call`) or tool listing (`tools/list`) before aborting.

• Top-level `timeout`: Global default in seconds applied to all servers.
• Per-server `timeout` (inside each `mcpServers[name]`): Overrides the global value for that server.
• `0`: Disables the timeout for that scope (no upper bound).
• Unset: Falls back to the MCP SDK default (60 seconds).

Values whose millisecond conversion (`timeout * 1000`) exceeds `2147483647` ms (the maximum safe `setTimeout` delay) are capped to that ceiling. Negative, `NaN`, or non-number values are ignored and fall through to the next level with a warning.

Example:

{
  "timeout": 30,
  "mcpServers": {
    "slow-server": { "command": "...", "timeout": 0 },
    "fast-server": { "command": "...", "timeout": 5 }
  }
}

Server Transport Configuration

Configure how the proxy hub itself is served via the `serverTransport` section:

"serverTransport": {
  "type": "streamable-http",
  "port": 3006,
  "host": "0.0.0.0",
  "path": "/mcp",
  "auth": {
    "type": "bearer",
    "token": "your-secret-token"
  }
}

• `type`: Transport type ("stdio", "sse", or "streamable-http")
• `port`: Port number for HTTP-based transports (default: 3006)
• `host`: Host to bind to (default: "0.0.0.0")
• `path`: URL path for Streamable HTTP endpoint (default: "/mcp")
• `auth`: Authentication configuration (optional)
  • `type`: "bearer" (currently the only supported type)
  • `token`: The bearer token required for authentication

Authentication can also be configured via the `MCP_PROXY_AUTH_TOKEN` environment variable.

Custom Tool Configuration

• tools:
  • Object with custom tool names as keys
  • Each tool has `description` and `subtools`
  • `subtools` is keyed by server name and contains each server's tool list

Environment Variables

• `MCP_PROXY_CONFIG_PATH`: Path to the configuration file
• `MCP_PROXY_LOG_DIRECTORY_PATH`: Path to the log directory
• `MCP_PROXY_LOG_LEVEL`: Log level ("debug" or "info")
• `MCP_PROXY_AUTH_TOKEN`: Bearer token for authenticating incoming requests to the proxy server
• `MCP_PROXY_PATH`: URL path for Streamable HTTP endpoint (default: "/mcp")
• `KEEP_SERVER_OPEN`: Whether to keep the server open after client disconnection in SSE mode (set to "1" to enable)
• `PORT`: Port for the SSE/Streamable HTTP server (default: 3006)
• `HOST`: Host to bind for the HTTP server (default: "0.0.0.0")

Development

Install dependencies:

npm install

Build the server:

npm run build

For development with auto-rebuild:

npm run watch

For development with continuous run:

# Stdio
npm run dev
# SSE
npm run dev:sse
# Streamable HTTP
npm run dev:http

CLI

The CLI provides two modes of operation for interacting with the MCP Proxy Hub.

Direct Execution Mode

You can execute commands directly from your terminal. This is useful for scripting and automation.

• List available tools:

mcp-proxy-hub-cli list

• Call a tool:

mcp-proxy-hub-cli call  [args...]

Example:

mcp-proxy-hub-cli call my_tool param1=value1 -o output

Interactive Mode

If you run the CLI without any arguments, it will start in interactive mode. This provides a shell-like interface for running commands.

mcp-proxy-hub-cli

Once in interactive mode, you can use the following commands:

• `list`: List available tools.
• `call [args...]`: Call a tool with arguments.
• `exit`: Exit the interactive session.

Installation

To use with Claude Desktop, add the server config:

On MacOS: `~/Library/Application Support/Claude/claude_desktop_config.json`

On Windows: `%APPDATA%/Claude/claude_desktop_config.json`

{
  "mcpServers": {
    "mcp-proxy-hub": {
      "command": "/path/to/mcp-proxy-hub/build/index.js",
      "env": {
        "MCP_PROXY_CONFIG_PATH": "/absolute/path/to/your/config.json",
        "KEEP_SERVER_OPEN": "1"
      }
    }
  }
}

`KEEP_SERVER_OPEN` will keep the SSE running even if a client disconnects. This is useful when multiple clients connect to the MCP proxy.

Debugging

Since MCP servers communicate over stdio, debugging can be challenging. We recommend using the MCP Inspector, which is available as a package script:

npm run inspector

The Inspector will provide a URL to access debugging tools in your browser.