How a model picks a tool from written descriptions, and how the pick breaks when the descriptions are vague.

A tool is a description

The model cannot see your code, your database, or your API. All it receives is a list of names, sentences and parameter shapes. It picks by reading. So the description is not documentation — it is the entire interface.

Two tools with the same description are the same tool as far as the model is concerned. Nothing else distinguishes them — the code behind the name is invisible from where it sits.