Tool call

The model's output naming a tool and its arguments — just structured text. It doesn't do anything on its own; the harness has to read it and execute. Produced by the model in one model provider request.

The lifecycle of a tool call:

Step	Who	What happens
1	Model	Learns which tools exist from descriptions in the system prompt
2	Model	Emits a call — tool name plus arguments, usually JSON — and stops
3	Harness	Parses the call and checks it against the permission mode
4	Harness	Executes it if allowed
5	Harness	Sends the outcome back as a tool result in the next request

One turn of agent work is usually many of these round trips chained together.

Because the call is generated by next-token prediction like everything else, it can be wrong the way any model output can be wrong: a path that doesn't exist, a flag the command doesn't have, arguments that are plausible rather than correct. The harness executes what was written, not what was meant — a mistyped path doesn't error gracefully, it edits the wrong file.