Claude Code's Leaked Source: A Real-World Masterclass in Harness Engineering

Earlier this year, Mitchell Hashimoto coined the term "harness engineering" — the discipline of building everything around the model that makes an AI agent actually work in production. OpenAI wrote about it. Anthropic published guides. Martin Fowler analyzed it. Then Claude Code's source leaked. 512K lines of TypeScript. And suddenly we have the first real look at what production harness engineering looks like at scale. The Evolution: From Prompt to Harness The AI engineering discipline has shifted rapidly: 2023-2024: Prompt Engineering → "How to ask the model" 2025: Context Engineering → "What information to feed the model" 2026: Harness Engineering → "How the entire system runs around the model" Prompt engineering is the question. Context engineering is the blueprint. Harness engineering is the construction site — tools, permissions, safety checks, cost controls, feedback loops, and state management that let the agent operate reliably. The leaked Claude Code source is a concrete case