> No they're literally just skipping an entire step into how LLM's actually "use" MCP.
No,you are literally misunderstanding the entire control flow of how an LLM toolchain uses both the model and any external tools (whether specified via MCP or not, but the focus of the conversation is MCP.)
> MCP is just a standard, largely for humans.
The standard is for humans implementing both tools and the toolchains that call them.
> LLM's do not give a singular fuck about it.
Correct. LLM toolchains, which if they can connect to tools via MCP, are also MCP clients care about it. LLMs don't care abojt it because the toolchain is the thing that actually calls both the LLM and the tools. And that's true whether the toolchain is a desktop frontend with a local, in process llama.cpp backend for running the LLM or if its the Claude Desktop app with a remote connection to the Anthropic API for calling the LLM or whatever.
> Some might be fine tuned for it to decrease erroneous output,
No, they aren't. Most models that are used to call tools now are specially trained for tool calling with a well-defined format for requesting tool calls from the toolchain a mnd receiving results back from it (though this isn't necessary for tool calling to work, people were using the ReAct pattern in toolchains to do it with regular chat models without any training or prespecified prompt/response format for tool calls just by having the toolchain inject tool-related instructions in the prompt, and read LLM responses to see if it was asking for tool calls), none of them that exist now are fine tuned for MCP, nor do they need to be because they literally never see it. The toolchain reads LLM responses, identifies tool call requests, takes any that map to tools defined via MCP and routes them down the channel (http or subprocess stdio) specified by the MCP, and does the reverse woth responses from the MCP server, validating responses and then mapping them into a prompt template that specifies where tool responses go and how they are formatted. It does the same thing (minus the MCP parts) for tools that aren’t specified by MCP (frontends might have their own built-tools, or have other mechanisms for custom tools that predate MCP support.) The LLM doesn't see any difference between MCP tools and other tools or a human reading the message with the tool request and manually creating a response that goes directly back.
> LLM's use MCP to discover tools they can call,
No, they don't. LLM frontends, which are traditional deterministic programs, use MCP to do that, and to find schemas for what should be sent to and expected from the tools. LLMs don’t see the MCP specs, and get information from the toolchain in prompts in formats that are model-specific and unrelated to MCP that tell them what tools they can request calls be made to and what they can expect back.
> an LLM interacting with other tools via MCP still needs system prompts or fine tuning to do so. Both of those things are not predictable or deterministic. They will fail at some point in the future. That is indisputable.
That's not, contrary to your description, a point of contention.
The point of contention is that the validation of data returned by an MCP server against the schema provided by the server is not predictable or deterministic. Confusing these two issues can only happen if you think the model does something with each response that controls whether or not the toolchain validates it, which is impossible, because the toolchain does whatever validation it is programmed to do before the model sees the data. The model has no way to know there is a response until that happens.
Now,can the model make requests that the don't fit the toolchain’s expectations due to unpredictable model behavior? Sure. Can the model do dumb things with the post-validation reaponse data after the toolchain has validated it and mapped it into the models prompt template and called the model with that prompt, for the same reason? Abso-fucking-lutely.
Can the model do anything to tell the toolchain not to validate response data for a tool call that it did decide to make on behalf of the model if the toolchain is programmed to validate the response data against the schema provided by the tool server? No, it can't. It can't even know that the tool was provided by an MCP and that that might be an issue, not can it know that the toolchain made the request, nor can it know that the toolchain received a response until the toolchain has done what it is programmed to do with the response through the point of populating the prompt template and calling the model with the resulting prompt, by which point any validation it was programmed to do has been done and is an immutable part of history.
>No, they don't. LLM frontends, which are traditional deterministic programs, use MCP to do that, and to find schemas for what should be sent to and expected from the tools.
You are REALLY, REALLY misunderstanding how this works. Like severely.
You think MCP is being used for some other purpose despite the one it was explicitly designed for... which is just weird and silly.
>Confusing these two issues can only happen if you think the model does something with each response that controls whether or not the toolchain validates it
No, you're still just arguing against something no one is arguing for the sake of pretending like MCP is doing something it literally cannot do or fundamentally fix about how LLM's operate.
I promise you if you read this a month from now with a fresh pair of eyes you will see your mistake.
What do you think the `tools/call` MCP flow is between the LLM and an MCP server? For example, if I had the GitHub MCP server configured on Claude Code and prompted "Show me the most recent pull requests on the torvalds/linux repository".
Hum, I'm not sure if everyone is simply unable to understand what you are saying, including me, but if the MCP client validates the MCP server response against the schema before passing the response to the LLM model, the model doesn't even matter, your MCP client could choose to report an error and interrupt the agentic flow.
That will depend on what MCP client you are using and how they've handled it.
No,you are literally misunderstanding the entire control flow of how an LLM toolchain uses both the model and any external tools (whether specified via MCP or not, but the focus of the conversation is MCP.)
> MCP is just a standard, largely for humans.
The standard is for humans implementing both tools and the toolchains that call them.
> LLM's do not give a singular fuck about it.
Correct. LLM toolchains, which if they can connect to tools via MCP, are also MCP clients care about it. LLMs don't care abojt it because the toolchain is the thing that actually calls both the LLM and the tools. And that's true whether the toolchain is a desktop frontend with a local, in process llama.cpp backend for running the LLM or if its the Claude Desktop app with a remote connection to the Anthropic API for calling the LLM or whatever.
> Some might be fine tuned for it to decrease erroneous output,
No, they aren't. Most models that are used to call tools now are specially trained for tool calling with a well-defined format for requesting tool calls from the toolchain a mnd receiving results back from it (though this isn't necessary for tool calling to work, people were using the ReAct pattern in toolchains to do it with regular chat models without any training or prespecified prompt/response format for tool calls just by having the toolchain inject tool-related instructions in the prompt, and read LLM responses to see if it was asking for tool calls), none of them that exist now are fine tuned for MCP, nor do they need to be because they literally never see it. The toolchain reads LLM responses, identifies tool call requests, takes any that map to tools defined via MCP and routes them down the channel (http or subprocess stdio) specified by the MCP, and does the reverse woth responses from the MCP server, validating responses and then mapping them into a prompt template that specifies where tool responses go and how they are formatted. It does the same thing (minus the MCP parts) for tools that aren’t specified by MCP (frontends might have their own built-tools, or have other mechanisms for custom tools that predate MCP support.) The LLM doesn't see any difference between MCP tools and other tools or a human reading the message with the tool request and manually creating a response that goes directly back.
> LLM's use MCP to discover tools they can call,
No, they don't. LLM frontends, which are traditional deterministic programs, use MCP to do that, and to find schemas for what should be sent to and expected from the tools. LLMs don’t see the MCP specs, and get information from the toolchain in prompts in formats that are model-specific and unrelated to MCP that tell them what tools they can request calls be made to and what they can expect back.
> an LLM interacting with other tools via MCP still needs system prompts or fine tuning to do so. Both of those things are not predictable or deterministic. They will fail at some point in the future. That is indisputable.
That's not, contrary to your description, a point of contention.
The point of contention is that the validation of data returned by an MCP server against the schema provided by the server is not predictable or deterministic. Confusing these two issues can only happen if you think the model does something with each response that controls whether or not the toolchain validates it, which is impossible, because the toolchain does whatever validation it is programmed to do before the model sees the data. The model has no way to know there is a response until that happens.
Now,can the model make requests that the don't fit the toolchain’s expectations due to unpredictable model behavior? Sure. Can the model do dumb things with the post-validation reaponse data after the toolchain has validated it and mapped it into the models prompt template and called the model with that prompt, for the same reason? Abso-fucking-lutely.
Can the model do anything to tell the toolchain not to validate response data for a tool call that it did decide to make on behalf of the model if the toolchain is programmed to validate the response data against the schema provided by the tool server? No, it can't. It can't even know that the tool was provided by an MCP and that that might be an issue, not can it know that the toolchain made the request, nor can it know that the toolchain received a response until the toolchain has done what it is programmed to do with the response through the point of populating the prompt template and calling the model with the resulting prompt, by which point any validation it was programmed to do has been done and is an immutable part of history.