Unless the client speaks to each micro service independently without going through some kind of gateway or BFF, I'd expect you'd just plonk your MCP in front of it all and expose the same functionality clients call via API (or graphql or RPC or whatever), so it's basically just an LLM-specific interface.

No reason why you couldn't just use tool calls with OpenAPI specs though. Either way, making all your microservices talk to each other over MCP sounds wild.