Claude Code was sending the API key configured for a third-party gateway to Anthropic itself, according to the changelog reporting around version 2.1.299. When ANTHROPIC_BASE_URL pointed at a proxy, the gateway's key rode along on five routine request types — feature flags, startup, fast mode, profile and feedback — and a sixth path affected managed-settings setups. Version 2.1.299 stops all of it.

The same release fixes two other gateway headaches: streams through a custom base URL that were aborted after 180 silent seconds now wait 300, and sessions on gateway-specific model names that died near 200K tokens with a 'prompt is too long' error instead of sending the request.

Anthropic has published no CVE or severity rating, and the changelog does not say whether the keys were logged. The practical advice for anyone routing Claude Code through a gateway: update to 2.1.299 or later — the stable channel was still on 2.1.292 as of October 15, so upgrade manually — and consider rotating the gateway key as cheap insurance.