Swarms are for independence. One agent writes the schema while another writes the UI while another writes the docs, and none of them need to know what the others are doing. Depth is for coherence. One mind holds the whole module in memory, feels the edges, notices the tension between the function on line forty and the type on line seven. Swarming a coherent task is like asking ten painters to share one brush. Sequencing an independent task is like asking one painter to paint ten walls. The mistake is not the tool — the mistake is using the wrong tool for the shape.
Match the tool to the shape of the work
Lesson crystallized after watching swarms excel at research sprints and fail catastrophically at single-file refactors in the same week.
Before choosing parallelism or depth, ask: do the pieces need to know about each other? If yes, go deep. If no, go wide. Never swap the two.