One task, many simultaneous workers
For tasks with independent workstreams — analyze 50 documents, evaluate 5 vendor proposals, process 200 contracts — Cowork splits the job across parallel sub-agents and merges their outputs. A serial 30-minute task becomes a parallel 10-minute task.
The pattern that matters: parallel works when units are independent (fan-out / merge), it doesn't when each step depends on the previous (sequential refinement). Cowork is smart about picking the shape, but you can hint by phrasing the goal — "analyze each independently, then synthesize" leans parallel; "draft, review, polish" leans serial.
Monitoring lives in the Cowork panel: each sub-agent shows status (Planning / Working / Waiting / Merging / Complete). When a sub-agent finishes early, it parks; the merge step kicks off only after every sub-agent reports done.