What is Open Mcp Proxy

open-mcp-proxy is an open-source bidirectional proxy designed for MCP servers, allowing for seamless communication between different protocols such as SSE and STDIO.

Use cases

Use cases include integrating MCP clients that only support STDIO with servers that use SSE, and implementing custom message handling for specific application needs.

How to use

To use open-mcp-proxy, subclass one of the provided Proxy classes and override the necessary callback methods to implement custom logic during the MCP protocol lifecycle.

Key features

Key features include support for both SSE and STDIO protocols, lightweight design leveraging the official mcp-python-sdk, and the ability to handle custom logic through callback methods.

Where to use

open-mcp-proxy can be used in applications that require communication between MCP clients and servers, particularly in environments where different protocols need to be bridged.

Open MCP Proxy

A simple and lightweight open-source bidirectional proxy for MCP servers.

It exposes several callbacks that you can use to implement your own logic and do your own actions during the MCP protocol lifecycle.

It supports both SSE and STDIO protocols so it can be used to enable MCP Client supporting only stdio (like the Claude Desktop App) to also support SSE.

It leverages as much as possible the official mcp-python-sdk to remains simple, lightweight and future-proof.

it declines in 2 versions to support both SSE and STDIO protocols:

SSE Proxy

graph LR
    STDIN -->|stdio| Proxy[Open MCP Proxy]
    Proxy -->|SSE| Server[Remote MCP Server]

graph RL
    Server[Remote MCP Server] -->|SSE| Proxy[Open MCP Proxy]
    Proxy -->|stdio| STDOUT

STDIO Proxy

graph LR
    STDIN -->|stdio| Proxy[Open MCP Proxy]
    Proxy -->|stdio| Server[Local MCP Server]

graph RL
    Server[Local MCP Server] -->|stdio| Proxy[Open MCP Proxy]
    Proxy -->|stdio| STDOUT

Note that in both cases the proxy listen to STDIN and write to STDOUT to work seemlessly with stdio MCP Clients.

Usage

Hook into the MCP protocol lifecycle

The recommended approach to hook into the MCP protocol lifecycle is to subclass one of the Proxy class that we expose and override the callback methods you need.

At the moment, we expose 4 callback methods:

For example if you need a proxy over the stdio protocol:

from omproxy.proxy import StdioProxy

class MyStdioProxy(StdioProxy):
    def _on_start(self):
        print("Starting proxy", file=sys.stderr)

    def _on_mcp_client_message(self, message: JSONRPCMessage):
        print(message, file=sys.stderr)

    def _on_mcp_server_message(self, message: JSONRPCMessage | Exception):
        print(message, file=sys.stderr)

    def _on_close(self):
        print("Closing proxy", file=sys.stderr)

if __name__ == "__main__":
    proxy = MyStdioProxy()
    proxy.run(StdioServerParameters(command="uv", args=["run", "src/example_server.py"]))

tip: dont write to stdout in your callbacks or anywhere really as it will mess with the stdio MCP communication.

MCP Client supporting only STDIO to your SSE MCP Server

We provide a simple CLI to start the proxy if you have an SSE MCP server running and you want to make it available to an MCP Client supporting only stdio you can simply do:

uvx omproxy@latest sse --url https://yourssemcpserver.io

see uvx omproxy@latest sse --help for more information including setting headers for authorization for example.