Reverse Engineering Claude Code Web Reveals Anthropic’s Firecracker Runtime and Antspace PaaS

Summary

A reverse-engineering report examines the infrastructure inside Claude Code Web using standard Linux tools, including strace, dmesg, strings, objdump, and Go tooling. It identifies a Firecracker MicroVM with 4 vCPUs, 16 GB of RAM, a 252 GB disk, and Linux 6.18.5. The guest uses a minimal Rust process_api binary as PID 1, providing WebSocket process management on port 2024 and an HTTP container-control API on port 2025. Sessions are restored from frozen VM snapshots rather than booted from scratch; the restore process injects session-specific block devices, refreshes devices and caches, mounts the root filesystem, corrects the wall clock, and applies security measures such as page clearing, capability reduction, JWT authentication, and token scrubbing. The report also analyzes a 27 MB unstripped Go environment-runner binary built from Anthropic’s private api-go module with full symbols and debug information. Its recovered package structure includes Claude execution, sandboxing, Git and credential handling, MCP integrations, Kubernetes lease management, session recording, and deployment actions. Those deployment symbols reveal both a Vercel client and an undocumented Antspace client. Antspace creates deployments, uploads a dist.tar.gz artifact, and streams packaging, uploading, building, deploying, and deployed states through NDJSON; unlike Vercel, it is described as an internal service with a dynamic control-plane URL and bearer-token authentication. The binary also contains resources for Baku, Claude.ai’s web app builder, based on Vite, React, and TypeScript, with automatic Supabase provisioning through MCP tools and Antspace as the default deployment target. A BYOC environment lets enterprise customers run environment-runner on their own infrastructure while Anthropic orchestrates sessions through its API. The author interprets these components as evidence of a vertically integrated AI application platform, while noting that Antspace appears to remain an internal or staging system and that its public product plans are unknown.