What Is a Harness

In English, a harness is the gear strapped onto a horse — it lets the rider control and guide the animal. In the AI world, a Harness (also called an Agent Harness) is the "middleware layer" between the LLM and the real world. It's not the AI itself — it's the AI's infrastructure. The core problem a Harness solves: how do you turn a "brain that can only think" into a "person who can act"? The answer: give it five core capabilities.

1. 🔧 Tools — The Agent's hands Let the AI read/write files, execute commands, search the web, etc. 2. 📚 Knowledge — The Agent's supplementary brain Load domain-specific knowledge and guidance on demand 3. 👀 Observation — The Agent's eyes Get real-time results from tool execution, perceive environment changes 4. ⚡ Action — The Agent's execution power Actually execute decisions, turning ideas into reality 5. 🛡️ Permissions — The Agent's rules Ensure the AI operates within safe boundaries and doesn't perform dangerous actions

There are already some commercial Agent products (like Claude Code, Cursor), but their internals are closed — you can't see how they work. OpenHarness is an open-source project that implements a complete Agent Harness in about 11,700 lines of Python code. Its purpose is to: • Help developers understand the underlying principles of Agents • Provide a reference implementation for learning, experimenting, and extending • Be compatible with Claude Code's tool and plugin ecosystem If Claude Code is a complete car, then OpenHarness is a transparent training vehicle — you can see how every part works.