Dynamic mcp routing via middleware

[rrondir]

I'm trying to put together a small PoC for an mcp registry where I can define configurations for MCP servers and proxy requests to them based on the headers.

An example config would look like:

{
  "servers": [
    {
      "name": "payment",
      "urls": {
        "dev": "https://dev-payments.example.com",
        "staging": "https://staging-payments.example.com",
        "prod": "https://payments.example.com"
      }
    },
    {
      "name": "user",
      "urls": {
        "dev": "https://dev-user.example.com",
        "staging": "https://staging-user.example.com",
        "prod": "https://user.example.com"
      }
    }
  ]
}

A client request that only pass something along the lines of X-Backend: user:dev,payment:staging to connect to the relevant tiers. I rather do this by establishing new sessions to avoid mounting all the servers and iterating through the different mounted servers for filtering.

My approach involved using a middleware that on_list_tools utilizes a proxy session on the requested backends. As of now it's not working because the ProxyClient returns mcp.types.Tool instances but the server expect them to be fastmcp.tool.tools.Tool type.

Should I change my approach here? Or could we change the following to:

async def _list_tools_mcp(self) -> list[MCPTool]:
    """
    List all available tools, in the format expected by the low-level MCP
    server.
    """
    logger.debug(f"[{self.name}] Handler called: list_tools")

    async with fastmcp.server.context.Context(fastmcp=self):
        tools = await self._list_tools_middleware()
        return [
            tool if isinstance(tool, MCPTool) else tool.to_mcp_tool(
                name=tool.key,
                include_fastmcp_meta=self.include_fastmcp_meta,
            )
            for tool in tools
        ]