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If the application also fronts your models, the same key configures the provider’s own client, including the base URL. That is the one value no client can work out for itself, because the LLM plane lives on a different host from the MCP one.
Nothing is wrapped. The SDK hands over the values and steps aside, so streaming and every provider option work exactly as the provider’s own client implements them.

What llm() returns

What goes in model depends on how the application routes: "auto" when it load-balances, otherwise a model name. See What goes in model.

Naming the user

The key identifies the application, not the person using it. To attribute a model call to one, send X-NeuralTrust-End-User with it. The value lands on the trace and in Activity, and grants nothing.
A key that reaches no LLM application raises PlaneUnavailableError. For the gateway side of the LLM plane, see Connect your application.