base
Base URL to talk to, or null for the protocol's default host.
Non-null is the usual case for a gateway or a proxy - which, along with a provider this framework does not ship, is the reason to write a resolver at all. Set it. Null on an OpenAiCompatible endpoint means OpenAI's own servers, so a null that reached one by accident - an unset property, say - would send a gateway's key to OpenAI under the gateway's name. The one legitimate null is OpenAI itself, which is why the type still permits it.
OpenAiCompatible states it and Anthropic defaults it: "Anthropic's protocol" names a host, "OpenAI-compatible" names none.