Session event “session.model_deselected”. The model the user had explicitly selected is no longer available, because the host that published it withdrew it, so the session no longer has an explicit selection. The next turn resolves a default as though the user had never chosen a model. Clients should stop presenting the previous model as selected. This event is durable because resume rebuilds the selected model from the event log; without it a resumed session would restore a model its provider no longer serves. Reasoning effort, verbosity, and other session-level preferences are deliberately unchanged, because they belong to the session rather than to the model.