A new internal check gates a capability to first-party accounts using the default API endpoint
A new helper function checks whether the account's auth type is firstParty and several other conditions, including that ANTHROPIC_BASE_URL (which overrides the API endpoint, for example to route through a proxy) is not set, plus region and configuration checks, before allowing some capability to proceed.
The finding doesn't say what capability this gates, but requiring first-party auth and the default API endpoint fits the existing pattern where certain features are disabled when requests are routed through a non-Anthropic endpoint.
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
| `400` errors naming `thinking` or `adaptive`, such as `Input tag 'adaptive' found` | The upstream model build doesn't accept adaptive reasoning, which Claude Code requests for Claude 4.6 and later models | Upgrade the gateway's upstream.…llm-gateway-connect see the edit
What capability this eligibility check controls is not stated.
Anthropic's documentation has since written up ANTHROPIC_BASE_URL, on Connect Claude Code to an LLM gateway.