Inside Anthropic's Web Sandbox: How Claude Runs Your Code

Anthropic powers Claude Code Web with snapshot-restored MicroVMs that spin up isolated environments in milliseconds.

Developers building ArcBox inspected a live Claude Code Web session using standard Linux tools like strace and objdump. They discovered the entire runtime runs inside a Firecracker MicroVM—the same virtual machine tech powering AWS Lambda. Each session gets 4 vCPUs, 16GB of RAM, and Linux 6.18.5. Instead of standard tools like systemd, a custom Rust binary named process_api acts as PID 1 to supervise processes over WebSockets.

Why it matters: Instant web sandboxes are tricky to build. Anthropic bypasses slow OS boots by creating frozen VM templates, then restoring sessions from snapshot state hours later. At restore time, the host hot-swaps block devices, reseeds random number generators, and hands you a clean Ubuntu environment almost instantly.

Know this: Hardening happens at the kernel layer. Free memory pages are zeroed automatically, PID 1 drops system capabilities after boot, and local WebSocket connections are blocked to prevent unauthorized access.

Turns out even state-of-the-art AI agents rely on clever Linux kernel tricks.