Microsoft opens its Copilot agent harness to developers through Foundry

On 28 October 2026 Microsoft announced that the Microsoft Copilot harness is available through Microsoft Foundry. The harness is the layer around a model that gives an agent its instructions, context, execution environment and controls. GitHub engineers built it, and Microsoft has adopted it company-wide: it powers Copilot Code, Copilot Studio, the GitHub Copilot app, the CLI, VS Code and other products.

Developers and enterprise customers can download the SDK or run the harness in the cloud, managed by Foundry. Microsoft runs the agent loop and sandbox with governance built in, while customers supply instructions, skills, tools and the choice of model. It works with more than 20 frontier models plus bring-your-own-key and can route work to a suitable model for each task. It supports open standards such as MCP and agent skills, as well as agent plugins.

Microsoft says the harness runtime was recently ported to Rust, which cut session overhead and time to first token. Agents built in Foundry run on the same harness as Microsoft's own products, so later improvements also reach them. Customers can still bring other harnesses and frameworks.