Technical analysis of Claude Code’s Vla() helper: when ANTHROPIC_BASE_URL points to non-Anthropic endpoint, client classifies proxy hostname against China-linked domain list and AI-lab keywords, checks timezone Asia/Shanghai or Asia/Urumqi, and rewrites “Today’s date is X.” line with steganographic markers.
Key point: VPN users on default Anthropic endpoint not affected. Custom corporate gateways, OpenAI-compatible routers, and reseller APIs trigger classification. Trust problem: prompt-level fingerprint without disclosure in a tool with shell/filesystem access.