It mustn't always nor never. It should follow a best judgement based on the .md, toml or whatever you use; in the end it's up to the LLM to decide which registered tools/mcps are used, and if the LLM is confident about some bs it will use that confidence instead of the tool.

When people complain about it, it's more often a gap between different knowledge domains and hard to measure characteristics of the environment, than it is an actual "you're using it wrong".