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.