Backends

Microagent has one backend per host OS. The --backend flag exists for lower-level request compatibility and backend smoke tests, not as a user-facing backend selector.

Backend	Host OS	Implementation	connect	Process model
firecracker	Linux	In-process Go	not supported (use logs)	Microagent records VM PID; stop sends SIGTERM, kill sends SIGKILL
apple-vf	macOS	Swift supervisor (microagent-applevf-supervisor)	supported	One supervisor invocation per request

Both backends expose the same lifecycle surface: run, create, start, status, stop, kill, delete. Both record state files and emit lifecycle events.

Firecracker (Linux)

• Requires /dev/kvm and the firecracker binary on PATH (or under <prefix>/libexec/firecracker, or MICROAGENT_FIRECRACKER).
• delete refuses to remove state while the recorded VM process is still running. Use stop or kill first.
• Does not support interactive connect. Use logs for serial output.
• Default kernel SHA is pinned. See Smoke tests.

Apple VF (macOS)

• Uses Apple Virtualization.framework via the Swift supervisor.
• Supports interactive connect and connect --send.
• The supervisor is packaged as microagent-applevf-supervisor. Override with --supervisor or MICROAGENT_APPLEVF_SUPERVISOR.
• Default kernel for arm64 lives at ~/.microagent/kernels/apple-vf/arm64/Image.

Selecting a host

microagent doctor reports the active backend, the binary it found, KVM/VF availability, and the default kernel status.