Agent scaffolding, not MCP vs CLI, is what actually drives cost
A controlled study ran one software task across seven agent scaffoldings and five models, and found that tool interface was nearly irrelevant to cost. Two scaffolds shipping no MCP support finished every run 5x to 28x cheaper than the five that support it, and paired MCP-to-CLI cost ratios swung from 0.43x to 29x. Interface choice alone is a weak predictor; the harness is the lever. Harness, task, and data are released.