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Comment by cle | Hacker News Reader

I'd encourage you to read the MCP specification: https://modelcontextprotocol.io/specification/2025-06-18/ser...

Or write a simple MCP server and a client that uses it. FastMCP is easy: https://gofastmcp.com/getting-started/quickstart

You are quite wrong. The LLM "chooses" to use a tool, but the input (provided by the LLM) is validated with JSON Schema by the server, and the output is validated by the client (Claude Code). The output is not provided back to the LLM if it does not comply with the JSON Schema, instead an error is surfaced.

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> The LLM "chooses" to use a tool

I think the others are trying to point out that statistically speaking, in at least one run the LLM might do something other than choose to use the correct tool. i.e 1 out of (say) 1 million runs it might do something else

No, the discussion is about whether validation is certain to happen when the LLM makes something where the frontend recognizes aa a tool request and calls a tool on behalf of the LLM, not whether the LLM can choose not to make a tool call at all.

The question is whether havign observed Claude Code validating a tool response before handing the response back to the LLM, you can count on that validation on future calls, not whether you can count on the LLM calling a tool in a similar situation.

Why do you think anything you said contradicts what I'm saying? I promise you I'm probably far more experienced in this field than you are.

>The LLM "chooses" to use a tool

Take a minute to just repeat this a few times.

MCP requires that servers providing tools must deterministically validate tool inputs and outputs against the schema.

LLMs cannot decide to skip this validation. They can only decide not to call the tool.

So is your criticism that MCP doesn't specify if and when tools are called? If so then you are essentially asking for a massive expansion of MCP's scope to turn it into an orchestration or workflow platform.

The LLM chooses to call a tool, it doesn't choose how the frontend handles anything about that call between the LLM making a tool request and the frontend, after having done its processing of the response (including any validation), mapping the result into a new prompt and calling the LLM with it.

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