Tool use

A Claude API capability in which the model, given a set of tool definitions, emits a structured request to invoke one and resumes once the caller returns the result, letting an agent act on external systems rather than only generating text.

How it works

The caller passes named tool definitions alongside the prompt. When the model decides a tool is needed, it halts and emits a structured call naming the tool and its arguments; the caller runs the tool, returns the result, and the model resumes with that result in its context. Control passes between model and caller turn by turn rather than the model producing a final answer in one pass.

Why it matters

The structured pause is what makes each action inspectable, but it does not make any action safe. A caller that routes every emitted call straight through to execution gets none of the protection the pause affords; the inspection point is real, the validation has to be built on top. The pattern also imposes a round-trip on every action, so a task small enough to answer in one response pays a protocol tax under tool use that direct generation does not.

In practice

A task that needs current data has the model emit a call to a lookup tool; the caller runs it, returns the value, and the model continues from that value rather than guessing, so the answer rests on a real result instead of recall.

Defined by Ready Solutions AI