What if the harness mattered more than the model? - Aditya Bhargava, Etsy
Jul 7, 2026 · 32:04
Aditya Bhargava, a Staff Engineer at Etsy, argues that the harness—the tooling around a model—matters more than the model itself, especially for weaker models, and introduces Agency, a language he built for constructing agents. He cites a benchmark where only the harness changed, producing a 52.4% to 76.2% score range (over 20 points), showing that a better harness can make local open-source models perform near proprietary ones. Through seven evolving examples of a coding agent, he demonstrates key harness improvements: adding tools, ensuring safety via handlers and partial function application (PFA) to lock directory access, implementing a reason-act-feedback loop, using subagents to add capabilities without context bloat, and self-optimization via a built-in optimizer to systematically measure and improve prompts. He advocates for building expertise in harnesses to reduce reliance on proprietary models, enabling use of locally runnable ones. Agency provides simple tool syntax, safety features like interrupts and true pause/resume, and built-in optimizers for measured improvement.
