Tool call
A tool call is a structured request from a model to run something outside itself, such as reading a file or executing a command, whose result is fed back into the context.
Tools are what let a model act rather than only answer. The cost is that every tool has to be described to the model, in the request, before it can be used. A registered tool that the current task never touches still occupies input tokens on every turn.
The results cost more than the definitions. A command that prints two thousand lines contributes two thousand lines of context, and keeps contributing them for the rest of the session.
Filtering output before it reaches the model is therefore one of the few interventions that is both simple and worth a lot. head, grep, a --quiet flag or a JSON filter all cost less than asking the model to ignore the noise.