# Harness

> The runtime in Copilot Studio that sits between what you build and the AI model, deciding when to call the model, what to send it and which tools to use.

**Category:** Copilot

**Whatever you build in Copilot Studio runs on a harness.** You design the agent or [workflow](/glossary#workflow), the model supplies the reasoning, and the harness sits in between. Microsoft offers three: the **GitHub Copilot harness** for reasoning-heavy, multi-step processes, the standard harness for rule-based agents and structured, repeatable conversations, and the Copilot chat harness for extending Copilot Chat with your organisation's knowledge.

The choice affects how the work gets done, what you can build out of the box and how it is billed. Billing is the difference to watch. The standard harness starts charging [Copilot Credits](/glossary#copilot-credits) after you publish; the GitHub Copilot harness charges from the moment you start building, previews and tests included.

## At work

Before a colleague spends an afternoon testing a new agent, the team lead checks which harness it runs on. On the GitHub Copilot harness, every test round already draws on the tenant's credits.

## Often confused with

The model. The [model](/glossary#model) is the AI that reasons and writes; the harness is the runtime around it that calls the model, passes it the right components and runs the tools.

## Related terms

- Copilot Studio
- Workflow
- Copilot Credits
- Model

## See it in a lesson

[Copilot Studio: build and automate](/en/learn/copilot/advanced/copilot-studio)

## Further reading

- [Harnesses in Copilot Studio (Microsoft Learn)](https://learn.microsoft.com/en-us/microsoft-copilot-studio/harnesses-overview): The three harnesses and what each choice affects.
- [Billing for agents powered by the GitHub Copilot harness (Microsoft Learn)](https://learn.microsoft.com/en-us/microsoft-copilot-studio/agents-experience/billing-credit-overview): Why agents on this harness are billed from build and test onward.

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Last updated: 14/10/2026  
Source: Rollo Academy glossary