Hmm, but you can't write a standard MCP (e.g. batch_tool_call) that calls other MCPs because the protocol doesn't give you a way to know what other MCPs are loaded in the runtime with you or any means to call them? Or have I got that wrong?
So I guess you had to modify the agent harness to do this? or I guess you could use... mcp-cli ... ??
I don't maintain this anymore but I experimented with this a while back: https://github.com/jx-codes/lootbox
Essentially you give the agent a way to run code that calls MCP servers, then it can use them like any other API.
Nowadays small bash/bun scripts and an MCP gateway proxy gets me the same exact thing.
So yeah at some level you do have to build out your own custom functionality.