Harness Engineering · 5-Layer Explorer
Agent = Model + Harness
Agent = Model + Harness
Five layers, five owners, five rates of change.
Click a layer to see its examples, common failure mode, and minimum viable version. Switch the metaphor to match your audience.
OpenAI Codex
~1M lines · 0 hand-written
Five months, ~1,500 PRs, zero manually written product code — humans built the harness.
LangChain
52.8% → 66.5%
Same model, harness-only tweaks, +13.7 points on Terminal Bench 2.0.
Vercel v0
−80% tools = better
Removing 80% of available tools measurably improved task completion.
Claude Code
2-3× quality
Boris Cherny: effective verification methods improve final output 2-3×.
The Agentic Principle: any time the agent makes a mistake, engineer it so the agent never makes that mistake again.