When should Codex use multiple agents? A benchmark, not a slogan

More agents do not automatically produce better engineering. They usually add total tokens, duplicated context, handoff delay, and integration risk. Their defensible advantages are narrower: reduced elapsed time for independent work, isolated investigation, or specialist evidence that one agent might omit. The useful question is therefore not “Can this task use subagents?” It is: Does this task contain independent, bounded work whose value exceeds the coordination cost? Codex How To now includes a dependency-free benchmark for testing that question instead of answering it from intuition. Disclosure: I maintain Codex How To , the independent open-source project containing the benchmark, evaluator, and measurements used here. The minimum decision rule Use one agent when the change is small, the interface is unsettled, or several steps must edit the same central files. Consider bounded orchestration only when all of these are true: The task has at least two genuine ownership surfaces. Each writer can own exclusive paths. The interface between those paths is frozen before implementation. The controller retains integration, system checks, and final review. Every worker returns concise evidence rather than a narrative transcript. One external acceptance bar can evaluate every execution method. flowchart TD A["One task contract"] --> B{"Independent write surfaces?"} B -- "No" --> C["One agent or sequential work"] B -- "Yes" --> D{"Frozen interface and exclusive paths?"} D -- "No" --> C D -- "Yes" --> E["Bounded workers"] E --> F["Controller integrates and evaluates"] F --> G{"Coverage or elapsed-time value exceeds coordination cost?"} G -- "Unproven" --> H["Keep measuring"] G -- "Repeated evidence" --> I["Adopt for this task class"] Job titles are not ownership boundaries. “Backend agent,” “test agent,” and “review agent” may still collide on the same files or execute dependent stages. A useful boundary is concrete: one writer owns incident/** , another owns web/** , and n