Can you expand on what you mean by this? If one LLM reads untrusted data then the output from that LLM can't be trusted by other LLMs. (Presume the untrusted data contains instructions to do bad stuff in whatever way is convincing to every LLM in the loop that needs to be convinced.) It seems that it's not possible to separate the data while also using it in a meaningful way, especially given the whole point of an MCP server is to automate agents.
I agree that LLMs are useful but until LLM architecture makes prompt injections impossible, I don't see how an agent can possibly be secure to this, nor do I see how it helps to blame the user. The real problem with them is that they will decide what to do based on untrusted input. A program that has its own workflow but uses LLMs can have pretty much the same benefit without introducing the problem that a support ticket can tell it to exfiltrate data or delete data or whatever, simply because that workflow is more specialized in what it does.