When running Claude Code CLI inside a VSCode devcontainer, the OAuth authentication flow failed silently. The browser would complete authentication and redirect to localhost:43149, but the CLI never received the callback—it just hung.
After debugging with curl -v, I discovered the request was accepted on IPv4 but received no response. Running ss -tlnp inside the container revealed the culprit: Claude Code's callback server binds exclusively to ::1 (IPv6 localhost), while VSCode's port forwarding sends traffic to 127.0.0.1 (IPv4).
The fix: add this to your devcontainer.json:
{
"containerEnv": {
"NODE_OPTIONS": "--dns-result-order=ipv4first"
}
}This forces Node.js to prefer IPv4, making the callback server bind to 127.0.0.1 where VSCode's port forwarding can reach it.