Claude Fable 5 running in Claude Code for web was unable to execute smolmachines.com sandboxes because its Firecracker guest environment lacks nested virtualization support. The model identified that GitHub Actions Ubuntu runners expose /dev/kvm and redirected its test battery to that platform instead. The workaround allowed Claude Fable 5 to validate running untrusted Python and JavaScript code with constrained RAM, CPU, and no network access.
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