When the model asks the harness to do something — like read a file, run a command, or check a calendar. The model describes the action in text; the harness executes it.
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A tool call is how the model’s text output becomes an action in the outside world. The model does not execute tool calls — it produces them as part of its response. The runtime reads the tool call, executes the tool, and feeds the result back as context for the next turn. A single response can contain zero, one, or multiple tool calls; zero means the loop ends, one or more means the runtime executes them and starts a new turn.
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