smolmachines / smolvm as a sandbox for untrusted Python & JavaScript

Claude Fable 5, running in Claude Code for web, was tasked with thoroughly testing https://smolmachines.com as a fast and secure sandbox. The goal was to determine how to run untrusted Python and JavaScript code within this environment, limiting its access to RAM and CPU time (preventing potential infinite loops) while providing controlled filesystem access to designated files. The primary objective was to use this sandbox to execute user-provided tasks, such as data transformations.

The Claude Code for web environment, however, proved to be incompatible with smol machines. The notes from the tests revealed that this Linux 6.18.5-fc-v20 container, which serves as a Firecracker guest, had 4 virtual CPUs and 15GB of RAM. Crucially, it did not have access to /dev/kvm or the vmx/svm CPU flags for nested virtualization. As a result, attempts to run the smolvm machine within this environment consistently failed due to the unavailability of kvm.

In response to this limitation, the team devised Plan B. They leveraged GitHub Actions Ubuntu runners, which do provide access to /dev/kvm. By creating a temporary workflow on this branch, they were able to execute the comprehensive test battery and collect logs. Following the completion of the tests, they removed the workflow in the final commit. This solution effectively bypassed the environmental constraints posed by Claude Code for web, demonstrating Fable's proactive approach to overcoming obstacles.