Mcp Proxy

mcp-proxy

[](https://codecov.io/gh/sparfenyuk/mcp-proxy)

• [mcp-proxy](#mcp-proxy)
• [About](#about)
• [1. stdio to SSE/StreamableHTTP](#1-stdio-to-ssestreamablehttp)
• [1.1 Configuration](#11-configuration)
• [1.2 Example usage](#12-example-usage)
• [2. SSE to stdio](#2-sse-to-stdio)
• [2.1 Configuration](#21-configuration)
• [2.2 Example usage](#22-example-usage)
• [Named Servers](#named-servers)
• [Installation](#installation)
• [Installing via PyPI](#installing-via-pypi)
• [Installing via Github repository (latest)](#installing-via-github-repository-latest)
• [Installing as container](#installing-as-container)
• [Troubleshooting](#troubleshooting)
• [Extending the container image](#extending-the-container-image)
• [Docker Compose Setup](#docker-compose-setup)
• [Command line arguments](#command-line-arguments)
• [Example config file](#example-config-file)
• [Testing](#testing)

About

The mcp-proxy is a tool that lets you switch between server transports. There are two supported modes:

1. stdio to SSE/StreamableHTTP
2. SSE to stdio

1. stdio to SSE/StreamableHTTP

Run a proxy server from stdio that connects to a remote SSE server.

This mode allows clients like Claude Desktop to communicate to a remote server over SSE even though it is not supported natively.

graph LR
    A["Claude Desktop"]  |stdio| B["mcp-proxy"]
    B  |SSE| C["External MCP Server"]

    style A fill:#ffe6f9,stroke:#333,color:black,stroke-width:2px
    style B fill:#e6e6ff,stroke:#333,color:black,stroke-width:2px
    style C fill:#e6ffe6,stroke:#333,color:black,stroke-width:2px

1.1 Configuration

This mode requires providing the URL of the MCP Server's SSE endpoint as the program’s first argument. If the server uses Streamable HTTP transport, make sure to enforce it on the mcp-proxy side by passing --transport=streamablehttp.

Arguments

| Name | Required | Description | Example | | ---------------- | -------- | ----------------------------------------------------------------------------------------------------------------- | --------------------------------------------- | | command_or_url | Yes | The MCP server SSE endpoint to connect to | http://example.io/sse | | --headers | No | Headers to use for the MCP server SSE connection | Authorization 'Bearer my-secret-access-token' | | --transport | No | Decides which transport protocol to use when connecting to an MCP server. Can be either 'sse' or 'streamablehttp' | streamablehttp | | --client-id | No | OAuth2 client ID for authentication | yourclientid | | --client-secret| No | OAuth2 client secret for authentication | yourclientsecret | | --token-url | No | OAuth2 token endpoint URL for authentication | https://auth.example.com/oauth/token |

Environment Variables

| Name | Required | Description | Example | | ------------------ | -------- | ---------------------------------------------------------------------------- | ---------- | | API_ACCESS_TOKEN | No | Can be used instead of --headers Authorization 'Bearer ' | YOUR_TOKEN |

1.2 Example usage

mcp-proxy is supposed to be started by the MCP Client, so the configuration must be done accordingly.

For Claude Desktop, the configuration entry can look like this:

{
  "mcpServers": {
    "mcp-proxy": {
      "command": "mcp-proxy",
      "args": [
        "http://example.io/sse"
      ],
      "env": {
        "API_ACCESS_TOKEN": "access-token"
      }
    }
  }
}

2. SSE to stdio

Run a proxy server exposing a SSE server that connects to a local stdio server.

This allows remote connections to the local stdio server. The mcp-proxy opens a port to listen for SSE requests, spawns a local stdio server that handles MCP requests.

graph LR
    A["LLM Client"] |SSE| B["mcp-proxy"]
    B |stdio| C["Local MCP Server"]

    style A fill:#ffe6f9,stroke:#333,color:black,stroke-width:2px
    style B fill:#e6e6ff,stroke:#333,color:black,stroke-width:2px
    style C fill:#e6ffe6,stroke:#333,color:black,stroke-width:2px

2.1 Configuration

This mode requires the --sse-port argument to be set. The --sse-host argument can be set to specify the host IP address that the SSE server will listen on. Additional environment variables can be passed to the local stdio server using the --env argument. The command line arguments for the local stdio server must be passed after the -- separator.

Arguments

| Name | Required | Description | Example | | ------------------------------------ | -------------------------- | --------------------------------------------------------------------------------------------- | ------------------------------------------- | | command_or_url | Yes | The command to spawn the MCP stdio server | uvx mcp-server-fetch | | --port | No, random available | The MCP server port to listen on | 8080 | | --host | No, 127.0.0.1 by default | The host IP address that the MCP server will listen on | 0.0.0.0 | | --env | No | Additional environment variables to pass to the MCP stdio server. Can be used multiple times. | FOO BAR | | --cwd | No | The working directory to pass to the MCP stdio server process. | /tmp | | --pass-environment | No | Pass through all environment variables when spawning the server | --no-pass-environment | | --allow-origin | No | Allowed origins for the SSE server. Can be used multiple times. Default is no CORS allowed. | --allow-origin "\*" | | --expose-header | No | Headers added to Access-Control-Expose-Headers. Can be used multiple times. Defaults to mcp-session-id. | --expose-header Custom-Header | | --stateless | No | Enable stateless mode for streamable http transports. Default is False | --no-stateless | | --named-server NAME COMMAND_STRING | No | Defines a named stdio server. | --named-server fetch 'uvx mcp-server-fetch' | | --named-server-config FILE_PATH | No | Path to a JSON file defining named stdio servers. | --named-server-config /path/to/servers.json | | --sse-port (deprecated) | No, random available | The SSE server port to listen on | 8080 | | --sse-host (deprecated) | No, 127.0.0.1 by default | The host IP address that the SSE server will listen on | 0.0.0.0 |

2.2 Example usage

To start the mcp-proxy server that listens on port 8080 and connects to the local MCP server:

# Start the MCP server behind the proxy
mcp-proxy uvx mcp-server-fetch

# Start the MCP server behind the proxy with a custom port
# (deprecated) mcp-proxy --sse-port=8080 uvx mcp-server-fetch
mcp-proxy --port=8080 uvx mcp-server-fetch

# Start the MCP server behind the proxy with a custom host and port
# (deprecated) mcp-proxy --sse-host=0.0.0.0 --sse-port=8080 uvx mcp-server-fetch
mcp-proxy --host=0.0.0.0 --port=8080 uvx mcp-server-fetch

# Start the MCP server behind the proxy with a custom user agent
# Note that the `--` separator is used to separate the `mcp-proxy` arguments from the `mcp-server-fetch` arguments
# (deprecated) mcp-proxy --sse-port=8080 -- uvx mcp-server-fetch --user-agent=YourUserAgent
mcp-proxy --port=8080 -- uvx mcp-server-fetch --user-agent=YourUserAgent

# Start multiple named MCP servers behind the proxy
mcp-proxy --port=8080 --named-server fetch 'uvx mcp-server-fetch' --named-server fetch2 'uvx mcp-server-fetch'

# Start multiple named MCP servers using a configuration file
mcp-proxy --port=8080 --named-server-config ./servers.json

# Start the MCP server with CORS enabled and custom exposed headers
mcp-proxy --port=8080 --allow-origin='*' --expose-header Custom-Header uvx mcp-server-fetch

Named Servers

• NAME is used in the URL path /servers/NAME/.
• COMMAND_STRING is the command to start the server (e.g., 'uvx mcp-server-fetch').
• Can be used multiple times.
• This argument is ignored if --named-server-config is used.
• FILE_PATH - If provided, this is the exclusive source for named servers, and --named-server CLI arguments are ignored.

If a default server is specified (the command_or_url argument without --named-server or --named-server-config), it will be accessible at the root paths (e.g., http://127.0.0.1:8080/sse).

Named servers (whether defined by --named-server or --named-server-config) will be accessible under /servers// (e.g., http://127.0.0.1:8080/servers/fetch1/sse). The /status endpoint provides global status.

JSON Configuration File Format for --named-server-config:

The JSON file should follow this structure:

{
  "mcpServers": {
    "fetch": {
      "disabled": false,
      "timeout": 60,
      "command": "uvx",
      "args": [
        "mcp-server-fetch"
      ],
      "transportType": "stdio"
    },
    "github": {
      "timeout": 60,
      "command": "npx",
      "args": [
        "-y",
        "@modelcontextprotocol/server-github"
      ],
      "env": {
        "GITHUB_PERSONAL_ACCESS_TOKEN": ""
      },
      "transportType": "stdio"
    }
  }
}

• mcpServers: A dictionary where each key is the server name (used in the URL path, e.g., /servers/fetch/) and the value is an object defining the server.
• command: (Required) The command to execute for the stdio server.
• args: (Optional) A list of arguments for the command. Defaults to an empty list.
• enabled: (Optional) If false, this server definition will be skipped. Defaults to true.
• timeout and transportType: These fields are present in standard MCP client configurations but are currently ignored by mcp-proxy when loading named servers. The transport type is implicitly "stdio".

Installation

Installing via PyPI

The stable version of the package is available on the PyPI repository. You can install it using the following command:

# Option 1: With uv (recommended)
uv tool install mcp-proxy

# Option 2: With pipx (alternative)
pipx install mcp-proxy

Once installed, you can run the server using the mcp-proxy command. See configuration options for each mode above.

Installing via Github repository (latest)

The latest version of the package can be installed from the git repository using the following command:

uv tool install git+https://github.com/sparfenyuk/mcp-proxy

> [!NOTE] > If you have already installed the server, you can update it using uv tool upgrade --reinstall command.

> [!NOTE] > If you want to delete the server, use the uv tool uninstall mcp-proxy command.

Installing as container

Starting from version 0.3.2, it's possible to pull and run the corresponding container image. Release images are published to GHCR and Docker Hub as multi-platform manifests for linux/amd64 and linux/arm64; Docker selects the matching image for the host architecture automatically.

docker run --rm -t ghcr.io/sparfenyuk/mcp-proxy:v0.12.0 --help
docker run --rm -t sparfenyuk/mcp-proxy:v0.12.0 --help

Troubleshooting

• Problem: Claude Desktop can't start the server: ENOENT code in the logs

Solution: Try to use the full path to the binary. To do so, open a terminal and run the commandwhich mcp-proxy ( macOS, Linux) or where.exe mcp-proxy (Windows). Then, use the output path as a value for 'command' attribute: ``json "fetch": { "command": "/full/path/to/bin/mcp-proxy", "args": [ "http://localhost:8932/sse" ] } ``

Extending the container image

You can extend the mcp-proxy container image to include additional executables. For instance, uv is not included by default, but you can create a custom image with it:

# file: mcp-proxy.Dockerfile

FROM ghcr.io/sparfenyuk/mcp-proxy:latest

# Install the 'uv' package
RUN python3 -m ensurepip && pip install --no-cache-dir uv

ENV PATH="/usr/local/bin:$PATH" \
    UV_PYTHON_PREFERENCE=only-system

ENTRYPOINT ["catatonit", "--", "mcp-proxy"]

Docker Compose Setup

With the custom Dockerfile, you can define a service in your Docker Compose file:

services:
  mcp-proxy-custom:
    build:
      context: .
      dockerfile: mcp-proxy.Dockerfile
    network_mode: host
    restart: unless-stopped
    ports:
      - 8096:8096
    command: "--pass-environment --port=8096 --sse-host 0.0.0.0 uvx mcp-server-fetch"

> [!NOTE] > Don't forget to set --pass-environment argument, otherwise you'll end up with the error "No interpreter found in > managed installations or search path"

Command line arguments

usage: mcp-proxy [-h] [--version] [-H KEY VALUE]
                 [--transport {sse,streamablehttp}] [--verify-ssl [VALUE]]
                 [--no-verify-ssl] [-e KEY VALUE] [--cwd CWD]
                 [--client-id CLIENT_ID] [--client-secret CLIENT_SECRET] [--token-url TOKEN_URL]
                 [--pass-environment | --no-pass-environment]
                 [--log-level LEVEL] [--debug | --no-debug]
                 [--named-server NAME COMMAND_STRING]
                 [--named-server-config FILE_PATH] [--port PORT] [--host HOST]
                 [--stateless | --no-stateless] [--sse-port SSE_PORT]
                 [--sse-host SSE_HOST]
                 [--allow-origin ALLOW_ORIGIN [ALLOW_ORIGIN ...]]
                 [--expose-header HEADER]
                 [command_or_url] [args ...]

Start the MCP proxy in one of two possible modes: as a client or a server.

positional arguments:
  command_or_url        Command or URL to connect to. When a URL, will run an SSE/StreamableHTTP client. Otherwise, if --named-server is not used, this will be the command for the default stdio client. If --named-server is used, this argument is ignored for stdio mode unless no default server is desired. See corresponding options for more details.

options:
  -h, --help            show this help message and exit
  --vers

…

## Source & license

This open-source MCP server is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [sparfenyuk](https://github.com/sparfenyuk)
- **Source:** [sparfenyuk/mcp-proxy](https://github.com/sparfenyuk/mcp-proxy)
- **License:** MIT

Install and usage instructions live in the source repository linked above.