# Harness

Concept · [Building with AI](https://ecosystem.samcarlton.com/ai)

Also called: agent harness, AI harness, harnesses, scaffolding, agent scaffold

The software wrapped around an [AI model](https://ecosystem.samcarlton.com/model) that [turns](https://ecosystem.samcarlton.com/turn) it into a working [agent](https://ecosystem.samcarlton.com/ai-agent). It supplies the tools, the standing [instructions](https://ecosystem.samcarlton.com/prompt), the permission rules and the handling of the [conversation](https://ecosystem.samcarlton.com/session). When two products use the same model but behave very differently, the harness is usually the reason.

**For example:** Two [coding](https://ecosystem.samcarlton.com/code) tools run the same model, but one can open your files and run your tests while the other can only chat. The difference is the harness.

## Connects to (10)

- [AI agent](https://ecosystem.samcarlton.com/ai-agent): An AI tool that does more than answer a question.
- [Model](https://ecosystem.samcarlton.com/model): The trained system at the center of an AI product.
- [Tool use](https://ecosystem.samcarlton.com/tool-use): A way for a model to ask the software around it to run a defined action, such as a search or a database lookup, and send back the result.
- [System prompt](https://ecosystem.samcarlton.com/system-prompt): Standing instructions that the builder of an AI product gives the model before any user types.
- [Permission mode](https://ecosystem.samcarlton.com/permission-mode): The rules that decide which actions an agent may take on its own and which need your approval first.
- [Coding agent](https://ecosystem.samcarlton.com/coding-agent): An AI agent that writes and changes code.
- [Claude Code](https://ecosystem.samcarlton.com/claude-code): An AI coding agent made by Anthropic.
- [Context window](https://ecosystem.samcarlton.com/context-window): How much text an LLM can take into account at one time.
- [Model provider request](https://ecosystem.samcarlton.com/model-provider-request): One trip from an AI product to the model provider and back.
- [Autocompact](https://ecosystem.samcarlton.com/autocompact): Compaction that the agent's software starts on its own when the context window is nearly full.

Map view: https://ecosystem.samcarlton.com/harness