solver, output, or neither (they’re mutually exclusive).
solver — an LLM-written report
data goes to the solver model, which writes prose. In plan run the report streams to stdout as it generates; as a plan tool, it returns to the agent as {"answer": …}. One LLM call.
Use for: human-readable reports, analysis, anything with judgment in the presentation.
output — structured JSON, zero LLM
Silent — side effects only
A plan with neithersolver nor output runs its steps and exits 0, printing only a ✓ plan completed (N steps) note to stderr. Combined with write-capable tools (linear__save_comment, an exec tool that deploys, …), this is a complete zero-inference automation job.
Empty data behaves differently per mode
When a step’s data runs out mid-plan (a.0. reference into an empty search result):
Rationale: a narrator can explain emptiness; a JSON contract can’t. More in Errors & replanning.