"A decent model with a great harness consistently beats a great model with a bad harness." @addyosmani wrote that, and 857K people saw it on X. Addy spent over a decade on Chrome at Google and now works on Claude Code. When someone who builds these tools says the setup matters more than the model, I listen. So how do you actually build a great harness? Here's the roadmap I'd follow if I were starting today: 1. Learn what a harness is Agent = Model + Harness. It's everything that isn't the…