Keep one active Dynamic Profile

Apple is explicit in WWDC26: a Dynamic Profile resolves to a single active Profile. Conditionals pick it. Nested instructions can compose, but you still have one running configuration.

enum Mode: String, Sendable { case gather, decide, act, review, done }

@Observable
final class AgentState {
    var mode: Mode = .gather
}

struct AgentProfile: LanguageModelSession.DynamicProfile {
    let state: AgentState

    var body: some LanguageModelSession.DynamicProfile {
        switch state.mode {
        case .gather:
            Profile {
                Instructions { "Collect facts. Do not act." }
            }
            .model(SystemLanguageModel())
        case .decide:
            Profile {
                Instructions { "Choose one legal next action." }
            }
            .model(SystemLanguageModel())
        case .act:
            Profile {
                Instructions { "Call only the allowed act tool." }
            }
            .model(SystemLanguageModel())
        case .review:
            Profile {
                Instructions { "Summarize what changed for the user." }
            }
            .model(SystemLanguageModel())
        case .done:
            Profile {
                Instructions { "The task is finished. Do not call tools." }
            }
            .model(SystemLanguageModel())
        }
    }
}

Use the model router if gather should stay on-device and decide should use PCC. The modifier still sits on this one branch. Do not open a second session for a second persona in the same turn.

Next, let tools propose transitions without owning them.