Harness engineering is everything around your AI agent except the model: memory, tools, permissions, hooks, observability. LangChain gained 13.7 benchmark points changing only the harness. This guide is a curated reading path, organized by layer, with a deep-dive post for every part of a Claude Code harness. Layer 1 only (what most devs have) → Advice the model may ignore All 5 layers (Memory → Tools → → Enforcement the model Permissions → Hooks → Observability) cannot bypass Enter fullscreen mode Exit fullscreen mode LangChain jumped from 52.8% to 66.5% on Terminal Bench 2.0 by changing only the harness. Same model. 13.7 points of pure architecture gain ( LangChain Blog, Feb 2026 ). Most Claude Code users stop at Layer 1. This guide is the reading path to the other four. If you want the theory of harness engineering, read the pillar post . If you want the architecture deep-dive, read the 5 layers post .…