If a language model only predicts text, how is a chat assistant searching the web and reading your inbox. The answer is tools, and Bani Chaudhuri spends this one on how they actually work. A tool is an external capability attached to the model by the system around it. The model keeps doing what it is good at, understanding the request and deciding what is needed. The system around it does the part the model cannot.
She traces one weather question end to end, then builds the same thing from the other side, as someone creating a scheduling agent. Define the tool and describe it, the user asks a question, the model decides a tool is needed and with what parameters, the platform or your own code runs it, the result comes back, and the model turns that result into a normal sentence. The line to keep is that the model is the coordinator and not the executor, which is why a tool with a vague description gets called at the wrong moment or never gets called at all.