Closed-loop development: how we shipped Mac runners in 3 weeks

Blacksmith has launched generally available M4-powered Mac CI runners that it says can more than double build speeds by using each Mac mini’s local SSD rather than network-attached storage, while maintaining compatibility with GitHub Actions through near-identical runner images. The company attributes its rapid three-week development timeline, compared with roughly three months for Windows runners, to structuring AI-assisted engineering around closed feedback loops in which agents can independently test, diagnose, and iterate without frequent human intervention. Development was divided into self-verifying stages covering Mac VM booting through Apple’s Virtualization.framework, filesystem image creation and validation, fleet deployment with Ansible and Argo, and end-to-end GitHub Actions testing using retained failed VMs and monitoring alerts. Blacksmith also highlights Testboxes, remote environments that replicate CI infrastructure for agents, as a way to provide the dependencies, secrets, and isolation needed for such iterative work, arguing that the key to faster delivery is giving agents clear constraints and reliable ways to verify results.

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