You only need the frontier model for one single edit

Summary

Monkey see, monkey do! 🍌97%of frontier performance41%cheaper in $$1.9×faster completion~3×less likely to cheatCost vs pass rate, 7 arms · SWE-Bench Pro · a bare model = oneshot · $/task includes the frontier model's opening turns · † executes with Flash 3.5 · ‡ executes with 5.6 Luna/plan makes perfect sense. It really shouldn't!You've heard this pitch; you may have even shipped it. The expensive model is clearly the better architect, but it feels like a waste to use it for the entire pipeline. Why not let it do the "hard part"?Read the code, think deeply, write a precise plan. Then a model a tenth the price executes the plan. Senior architect, junior engineer.Sounds great, right?Find the red dot above, labeled Opus 4.8 + /plan†. Opus plans read-only, Gemini Flash implements: lands at $3.18 per task, 12.7 minutes, 84.6% pass.Opus doing the entire task by itself, no handoff, no junior: $2.78, 10.1 minutes, 84.6%.The "cost-saving" measure costs 14% more than not saving. Huh?reads it all · $$$reads it all again · $$The mistake is upstream of the architecture diagram. People price agents the way they price people: senior time is expensive, so minimize senior involvement.But the expensive part of an agent's day is not the fixing, building, or even the thinking. Opus fixing things does not cost money. Opus reading things costs money.Take a look at this admittedly anecdotal distribution of where our tokens went; fully automated agents look no different.1.81B tokens across ~2M tool calls · "doing the task" (every edit and write) is 9%; reading is what scales the bill, and both models pay full price for itNine percent of the tokens are edits. The rest is reading, and this split is not a quirk of one harness, or something you can "fix". Trust us, we've tried — that's how snapcompact happened.Any agent, any model, any scaffold: the bill is essentially O(reads).Now walk through every reason you'd reach for /plan, with that in mind: "I want the deep understanding of the big mode...