type Agent interface {
    Name(ctx context.Context) string
    Description(ctx context.Context) string

    // Run runs the agent.
    // The returned AgentEvent within the AsyncIterator must be safe to modify.
    // If the returned AgentEvent within the AsyncIterator contains MessageStream,
    // the MessageStream MUST be exclusive and safe to be received directly.
    // NOTE: it's recommended to use SetAutomaticClose() on the MessageStream of AgentEvents emitted by AsyncIterator,
    // so that even the events are not processed, the MessageStream can still be closed.
    Run(ctx context.Context, input *AgentInput, options ...AgentRunOption) *AsyncIterator[*AgentEvent]
}

type ChatModelAgentConfig struct {
    // Name of the agent. Better be unique across all agents.
    Name string
    // Description of the agent's capabilities.
    // Helps other agents determine whether to transfer tasks to this agent.
    Description string
    // Instruction used as the system prompt for this agent.
    // Optional. If empty, no system prompt will be used.
    // Supports f-string placeholders for session values in default GenModelInput, for example:
    // "You are a helpful assistant. The current time is {Time}. The current user is {User}."
    // These placeholders will be replaced with session values for "Time" and "User".
    Instruction string

    Model model.ToolCallingChatModel

    ToolsConfig ToolsConfig

    // GenModelInput transforms instructions and input messages into the model's input format.
    // Optional. Defaults to defaultGenModelInput which combines instruction and messages.
    GenModelInput GenModelInput

    // Exit defines the tool used to terminate the agent process.
    // Optional. If nil, no Exit Action will be generated.
    // You can use the provided 'ExitTool' implementation directly.
    Exit tool.BaseTool

    // OutputKey stores the agent's response in the session.
    // Optional. When set, stores output via AddSessionValue(ctx, outputKey, msg.Content).
    OutputKey string

    // MaxIterations defines the upper limit of ChatModel generation cycles.
    // The agent will terminate with an error if this limit is exceeded.
    // Optional. Defaults to 20.
    MaxIterations int
}

func NewChatModelAgent(_ context.Context, config *ChatModelAgentConfig) (*ChatModelAgent, error) {
    // omit code
}

type RunnerConfig struct {
    Agent           Agent
    EnableStreaming bool

    CheckPointStore compose.CheckPointStore
}

func NewRunner(_ context.Context, conf RunnerConfig) *Runner {
    // omit code
}