MCP Proxy Server

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✨ Key Features Highlight

• 🌐 Web UI Management: Easily manage all connected MCP servers through an intuitive web interface (optional, requires enabling).

• 🔧 Granular Tool Control: Enable or disable individual tools, and override names/descriptions via the Web UI.

• 🛡️ Flexible Endpoint Authentication: Secure your HTTP-based endpoints (/sse, /mcp) with flexible authentication options (Authorization: Bearer <token> or X-API-Key: <key>).

• 🔄 Robust Session Handling & Concurrency:

  • Improved SSE session handling for client reconnections (relying on server-sent endpoint events) and support for concurrent connections.

  • Streamable HTTP endpoint (/mcp) also supports concurrent client interactions.

• 🚀 Versatile MCP Operations (Server & Proxy):

  • Acts as a Proxy: Connects to and aggregates multiple backend MCP servers of various types (Stdio, SSE, Streamable HTTP).

  • Acts as a Server: Exposes these aggregated capabilities through its own Streamable HTTP (/mcp) and SSE (/sse) endpoints. Can also run in a pure Stdio mode.

• ✨ Real-time Install Output: Monitor Stdio server installation progress (stdout/stderr) directly in the Web UI.

• ✨ Web Terminal: Access a command-line terminal within the Admin UI for direct server interaction (optional, use with caution due to security risks).

This server acts as a central hub for Model Context Protocol (MCP) resource servers. It can:

• Connect to and manage multiple backend MCP servers (Stdio, SSE, and Streamable HTTP types).

• Expose their combined capabilities (tools, resources) through a single, unified SSE interface, a Streamable HTTP interface, or act as a single Stdio-based MCP server itself.

• Handle routing of requests to the appropriate backend servers.

• Aggregate responses if needed (though primarily acts as a proxy).

• Support multiple simultaneous SSE client connections with optional API key authentication.

Related MCP server: MCP Proxy Server

Features

Resource & Tool Management via Proxy

• Discovers and connects to multiple MCP resource servers defined in config/mcp_server.json.

• Aggregates tools and resources from all connected active servers.

• Routes tool calls and resource access requests to the correct backend server.

• Maintains consistent URI schemes.

✨ Optional Web Admin UI (ENABLE_ADMIN_UI=true)

Provides a browser-based interface for managing the proxy server configuration and connected tools. Features include:

• Server Configuration: View, add, edit, and delete server entries (mcp_server.json). Supports Stdio, SSE, and HTTP server types with relevant options (type, command, args, env, url, apiKey, bearerToken, install config).

• Tool Configuration: View all tools discovered from active backend servers. Enable or disable specific tools. Override the display name and description for each tool (tool_config.json).

• Live Reload: Apply server and tool configuration changes by triggering a configuration reload without needing to restart the entire proxy server process.

• Stdio Server Installation: For Stdio servers, you can define installation commands in the configuration. The Admin UI allows you to:

  • Trigger the execution of these installation commands.

  • Monitor installation progress in real-time with live stdout and stderr output streamed directly to the UI.

• Web Terminal: Access an integrated web-based terminal that provides shell access to the environment where the proxy server is running.

  • Security Warning: This feature grants significant access and should be used with extreme caution, especially if the admin interface is exposed.

Configuration

Configuration is primarily done via environment variables and JSON files located in the ./config directory.

1. Server Connections (config/mcp_server.json)

This file defines the backend MCP servers the proxy should connect to.

Example config/mcp_server.json:

{
  "mcpServers": {
    "unique-server-key1": {
      "type": "stdio",
      "name": "My Stdio Server",
      "active": true,
      "command": "/path/to/server/executable",
      "args": ["--port", "1234"],
      "env": {
        "API_KEY": "server_specific_key"
      },
      "installDirectory": "/custom_install_path/unique-server-key1",
      "installCommands": [
        "git clone https://github.com/some/repo unique-server-key1",
        "cd unique-server-key1 && npm install && npm run build"
      ]
    },
    "another-sse-server": {
      "type": "sse",
      "name": "My SSE Server",
      "active": true,
      "url": "http://localhost:8080/sse",
      "apiKey": "sse_server_api_key"
    },
    "http-mcp-server": {
      "type": "http",
      "name": "My Streamable HTTP Server",
      "active": true,
      "url": "http://localhost:8081/mcp",
      "bearerToken": "some_secure_token_for_http_server"
    },
    "stdio-default-install": {
        "type": "stdio",
        "name": "Stdio Server with Default Install Path",
        "active": true,
        "command": "my_other_server",
        "installCommands": ["echo 'Installing to default location...'"]
    }
  }
}

Fields:

• mcpServers: (Required) An object where each key is a unique identifier for a backend server.

• name: (Optional) A user-friendly display name for the server (used in Admin UI).

• active: (Optional, default: true) Set to false to prevent the proxy from connecting to this server.

• type: (Required) Specifies the transport type. Must be one of "stdio", "sse", or "http".

• command: (Required if type is "stdio") The command to execute the server process.

• args: (Optional if type is "stdio") An array of string arguments to pass to the command.

• env: (Optional if type is "stdio") An object of environment variables (KEY: "value") to set for the server process. These are merged with the proxy server's environment.

• url: (Required if type is "sse" or "http") The full URL of the backend server's endpoint (e.g., SSE endpoint for "sse", MCP endpoint for "http").

• apiKey: (Optional if type is "sse" or "http") An API key to send in the X-Api-Key header when the proxy connects to this specific backend server.

• bearerToken: (Optional if type is "sse" or "http") A token to send in the Authorization: Bearer <token> header when connecting to this specific backend server. (If both apiKey and bearerToken are provided, bearerToken generally takes precedence for that specific backend connection).

• installDirectory: (Optional if type is "stdio") The absolute path where the server itself should be installed (e.g., /opt/my-server-files). Used by the Admin UI's installation feature.

  • If provided in mcp_server.json, this exact path is used.

  • If omitted, the effective directory depends on the TOOLS_FOLDER environment variable (see Environment Variables section).

    • If TOOLS_FOLDER is set and not empty, the server will be installed in a subdirectory named after the server key within this folder (e.g., ${TOOLS_FOLDER}/<server_key>).

    • If TOOLS_FOLDER is also empty or not set, it defaults to a tools subdirectory within the proxy server's working directory (e.g., ./tools/<server_key>).

  • Ensure the parent directory of the target installation path (e.g., TOOLS_FOLDER or ./tools) is writable by the user running the proxy server.

• installCommands: (Optional for Stdio type) An array of shell commands executed sequentially by the Admin UI's installation feature if the target server directory (derived from installDirectory or defaults) does not exist. Commands are executed from the parent directory of the target server installation directory (e.g., if installDirectory resolves to /opt/tools/my-server, commands run in /opt/tools/). Use with extreme caution due to security risks.

2. Tool Configuration (config/tool_config.json)

This file allows overriding properties of tools discovered from backend servers. It is primarily managed via the Admin UI but can be edited manually.

Example config/tool_config.json:

{
  "tools": {
    "unique-server-key1__tool-name-from-server": {
      "enabled": true,
      "displayName": "My Custom Tool Name",
      "description": "A more user-friendly description."
    },
    "another-sse-server__another-tool": {
      "enabled": false
    }
  }
}

• Keys are in the format <server_key><separator><original_tool_name>, where <separator> is the value of the SERVER_TOOLNAME_SEPERATOR environment variable (defaults to __).

• enabled: (Optional, default: true) Set to false to hide this tool from clients connecting to the proxy.

• displayName: (Optional) Override the tool's name in client UIs.

• description: (Optional) Override the tool's description.

3. Environment Variables

• PORT: Port for the proxy server's HTTP-based endpoints (/sse, /mcp, and Admin UI if enabled). Default: 3663. Note: This is only used when running in a mode that starts an HTTP server (e.g., via npm run dev:sse or the Docker container). The npm run dev script runs in Stdio mode.

  export PORT=8080

• ALLOWED_KEYS: (Optional) Comma-separated list of API keys to secure the proxy's HTTP-based endpoints (/sse, /mcp). If neither ALLOWED_KEYS nor ALLOWED_TOKENS are set, authentication is disabled for these endpoints. Clients must provide a key via X-Api-Key header or ?key= query parameter.

  export ALLOWED_KEYS="client_key1,client_key2"

• ALLOWED_TOKENS: (Optional) Comma-separated list of Bearer Tokens to secure the proxy's HTTP-based endpoints (/sse, /mcp). If neither ALLOWED_KEYS nor ALLOWED_TOKENS are set, authentication is disabled. Clients must provide a token via the Authorization: Bearer <token> header. If both ALLOWED_KEYS and ALLOWED_TOKENS are configured, Bearer Token authentication will be attempted first.

  export MCP_PROXY_SSE_ALLOWED_TOKENS="your_bearer_token_1,your_bearer_token_2"

• ENABLE_ADMIN_UI: (Optional) Set to true to enable the Web Admin UI (only applicable in SSE mode). Default: false.

  export ENABLE_ADMIN_UI=true

• ADMIN_USERNAME: (Required if Admin UI enabled) Username for Admin UI login. Default: admin.

• ADMIN_PASSWORD: (Required if Admin UI enabled) Password for Admin UI login. Default: password (Change this!).

  export ADMIN_USERNAME=myadmin
  export ADMIN_PASSWORD=aVerySecurePassword123!

• SESSION_SECRET: (Optional, recommended if Admin UI enabled) Secret used to sign session cookies. If not set, a default, less secure secret is used, and a warning is issued. A secure secret is automatically generated and saved to config/.session_secret on first run if not provided via environment variable.

  # Recommended: Generate a strong secret (e.g., openssl rand -hex 32)
  export SESSION_SECRET='your_very_strong_random_secret_here'

• TOOLS_FOLDER: (Optional) Specifies the base directory for Stdio server installations initiated via the Admin UI, used when installDirectory is not explicitly set in mcp_server.json for a specific server.

  • If set (e.g., /custom/tools_path), installations for servers without a specific installDirectory will target a subdirectory named after the server key within this folder (e.g., ${TOOLS_FOLDER}/<server_key>).

  • If TOOLS_FOLDER is not set or is empty, such installations will default to a tools subdirectory within the proxy server's working directory (e.g., ./tools/<server_key>).

  • The Dockerfile sets this to /tools by default.

  export TOOLS_FOLDER=/srv/mcp_tools

• SERVER_TOOLNAME_SEPERATOR: (Optional) Defines the separator used to combine the server name and tool name when generating the unique key for tools (e.g., server-key__tool-name). This key is used internally and in the tool_config.json file.

  • Default: __.

  • Must be at least 2 characters long and contain only letters (a-z, A-Z), numbers (0-9), hyphens (-), and underscores (_).

  • If the provided value is invalid, the default (__) will be used, and a warning will be logged.

  export SERVER_TOOLNAME_SEPERATOR="___" # Example: using triple underscore

• LOGGING: (Optional) Controls the minimum log level output by the server.

  • Possible values (case-insensitive): error, warn, info, debug.

  • Logs at the specified level and all levels above it will be shown.

  • Default: info.

  export LOGGING="debug"

• RETRY_SSE_TOOL_CALL: (Optional) Controls whether to enable retries for SSE tool calls. Set to "true" to enable, "false" to disable. Default: true. See the "Enhanced Reliability Features" section for details.

  export RETRY_SSE_TOOL_CALL="true"

• SSE_TOOL_CALL_MAX_RETRIES: (Optional) Maximum number of retry attempts for SSE tool calls (after the initial failure). Default: 2. See the "Enhanced Reliability Features" section for details.

  export SSE_TOOL_CALL_MAX_RETRIES="2"

• SSE_TOOL_CALL_RETRY_DELAY_BASE_MS: (Optional) Base delay in milliseconds for SSE tool call retries, used in exponential backoff. Default: 300. See the "Enhanced Reliability Features" section for details.

  export SSE_TOOL_CALL_RETRY_DELAY_BASE_MS="300"

• RETRY_HTTP_TOOL_CALL: (Optional) Controls whether to retry on HTTP tool call connection errors. Set to "true" to enable, "false" to disable. Default: true. See the "Enhanced Reliability Features" section for details.

  export RETRY_HTTP_TOOL_CALL="true"

• HTTP_TOOL_CALL_MAX_RETRIES: (Optional) Maximum number of retry attempts for HTTP tool calls (after the initial failure). Default: 2. See the "Enhanced Reliability Features" section for details.

  export HTTP_TOOL_CALL_MAX_RETRIES="3"

• HTTP_TOOL_CALL_RETRY_DELAY_BASE_MS: (Optional) Base delay in milliseconds for HTTP tool call retries, used in exponential backoff. Default: 300. See the "Enhanced Reliability Features" section for details.

  export HTTP_TOOL_CALL_RETRY_DELAY_BASE_MS="500"

• RETRY_STDIO_TOOL_CALL: (Optional) Controls whether to retry on Stdio tool call connection errors (attempts to restart the process). Set to "true" to enable, "false" to disable. Default: true. See the "Enhanced Reliability Features" section for details.

  export RETRY_STDIO_TOOL_CALL="true"

• STDIO_TOOL_CALL_MAX_RETRIES: (Optional) Maximum number of retry attempts for Stdio tool calls (after the initial failure). Default: 2. See the "Enhanced Reliability Features" section for details.

  export STDIO_TOOL_CALL_MAX_RETRIES="5"

• STDIO_TOOL_CALL_RETRY_DELAY_BASE_MS: (Optional) Base delay in milliseconds for Stdio tool call retries, used in exponential backoff. Default: 300. See the "Enhanced Reliability Features" section for details.

  export STDIO_TOOL_CALL_RETRY_DELAY_BASE_MS="1000"

Enhanced Reliability Features

The MCP Proxy Server includes features to improve its resilience and the reliability of interactions with backend MCP services, ensuring smoother operations and more consistent tool execution.

1. Error Propagation

The proxy server ensures that errors originating from backend MCP services are consistently propagated to the requesting client. These errors are formatted as standard JSON-RPC error responses, making it easier for clients to handle them uniformly.

2. SSE Tool Call Retry

When a tools/call operation is made to an SSE-based backend server, and the underlying connection is lost or experiences an error (including timeouts), the proxy server implements a retry mechanism.

Retry Mechanism: If an initial SSE tool call fails due to a connection error or timeout, the proxy will attempt to re-establish the connection to the SSE backend. If reconnection is successful, it will then retry the original tools/call request using an exponential backoff strategy, similar to HTTP and Stdio retries. This means the delay before each subsequent retry attempt increases exponentially, with a small amount of jitter (randomness) added.

Configuration: These settings are primarily controlled by environment variables. Values in config/mcp_server.json under the proxy object for these specific keys will be overridden by environment variables if set.

• RETRY_SSE_TOOL_CALL (environment variable):

  • Set to "true" to enable retries for SSE tool calls.

  • Set to "false" to disable this feature.

  • Default Behavior: true (if the environment variable is not set, is empty, or is an invalid value).

• SSE_TOOL_CALL_MAX_RETRIES (environment variable):

  • Specifies the maximum number of retry attempts after the initial failed attempt. For example, if set to "2", there will be one initial attempt and up to two retry attempts, totaling a maximum of three attempts.

  • Default Behavior: 2 (if the environment variable is not set, is empty, or is not a valid integer).

• SSE_TOOL_CALL_RETRY_DELAY_BASE_MS (environment variable):

  • The base delay in milliseconds used in the exponential backoff calculation. The delay before the n-th retry (0-indexed) is roughly SSE_TOOL_CALL_RETRY_DELAY_BASE_MS * (2^n) + jitter.

  • Default Behavior: 300 (milliseconds) (if the environment variable is not set, is empty, or is not a valid integer).

Example (Environment Variables):

export RETRY_SSE_TOOL_CALL="true"
export SSE_TOOL_CALL_MAX_RETRIES="3"
export SSE_TOOL_CALL_RETRY_DELAY_BASE_MS="500"

3. HTTP Request Retry for Tool Calls

For tools/call operations directed to HTTP-based backend servers, the proxy implements a retry mechanism for connection errors (e.g., "failed to fetch", network timeouts).

Retry Mechanism: If an initial HTTP request fails due to a connection error, the proxy will retry the request using an exponential backoff strategy. This means the delay before each subsequent retry attempt increases exponentially, with a small amount of jitter (randomness) added to prevent thundering herd scenarios.

Configuration: These settings are primarily controlled by environment variables.

• RETRY_HTTP_TOOL_CALL (environment variable):

  • Set to "true" to enable retries for HTTP tool calls.

  • Set to "false" to disable this feature.

  • Default Behavior: true (if the environment variable is not set, is empty, or is an invalid value).

• HTTP_TOOL_CALL_MAX_RETRIES (environment variable):

  • Specifies the maximum number of retry attempts after the initial failed attempt. For example, if set to "2"，there will be one initial attempt and up to two retry attempts, totaling a maximum of three attempts.

  • Default Behavior: 2 (if the environment variable is not set, is empty, or is not a valid integer).

• HTTP_TOOL_CALL_RETRY_DELAY_BASE_MS (environment variable):

  • The base delay in milliseconds used in the exponential backoff calculation. The delay before the n-th retry (0-indexed) is roughly HTTP_TOOL_CALL_RETRY_DELAY_BASE_MS * (2^n) + jitter.

  • Default Behavior: 300 (milliseconds) (if the environment variable is not set, is empty, or is not a valid integer).

4. Stdio Connection Retry for Tool Calls

For tools/call operations directed to Stdio-based backend servers, the proxy implements a retry mechanism for connection errors (e.g., process crash or unresponsiveness).

**Retry Mecha