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AgentSubscriber

The AgentSubscriber interface provides a comprehensive event-driven system for handling agent lifecycle events, message updates, and state mutations during agent execution. It allows you to hook into various stages of the agent’s operation and modify its behavior. 
import { AgentSubscriber } from "@ag-ui/client"

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Overview

AgentSubscriber defines a collection of optional event handlers and lifecycle hooks that can respond to different stages of agent execution. All methods in the interface are optional, allowing you to implement only the events you need to handle. All subscriber methods can be either synchronous or asynchronous - if they return a Promise, the agent will await their completion before proceeding.

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Adding Subscribers to Agents

Subscribers can be added to agents in two ways:

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Permanent Subscription

Use the subscribe() method to add a subscriber that will persist across multiple agent runs: 
const agent = new HttpAgent({ url: "https://api.example.com/agent" })

const subscriber: AgentSubscriber = {
  onTextMessageContentEvent: ({ textMessageBuffer }) => {
    console.log("Streaming text:", textMessageBuffer)
  },
}

// Add permanent subscriber
const subscription = agent.subscribe(subscriber)

// Later, remove the subscriber if needed
subscription.unsubscribe()

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Temporary Subscription

Pass a subscriber directly to runAgent() for one-time use: 
const temporarySubscriber: AgentSubscriber = {
  onRunFinishedEvent: ({ result }) => {
    console.log("Run completed with result:", result)
  },
}

// Use subscriber for this run only
await agent.runAgent({ tools: [myTool] }, temporarySubscriber)

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Core Interfaces

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AgentSubscriber

The main interface that defines all available event handlers and lifecycle hooks. All methods in the interface are optional, allowing you to implement only the events you need to handle.

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AgentStateMutation

Event handlers can return an AgentStateMutation object to modify the agent’s state and control event processing: 
interface AgentStateMutation {
  messages?: Message[] // Update the message history
  state?: State // Update the agent state
  stopPropagation?: boolean // Prevent further subscribers from processing this event
}
When a subscriber returns a mutation:
  • messages: Replaces the current message history
  • state: Replaces the current agent state
  • stopPropagation: If true, prevents subsequent subscribers from handling the event (useful for overriding default behavior)

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AgentSubscriberParams

Common parameters passed to most subscriber methods: 
interface AgentSubscriberParams {
  messages: Message[] // Current message history
  state: State // Current agent state
  agent: AbstractAgent // The agent instance
  input: RunAgentInput // The original input parameters
}

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Event Handlers

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Lifecycle Events

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onRunInitialized()

Called when the agent run is first initialized, before any processing begins. 
onRunInitialized?(params: AgentSubscriberParams): MaybePromise<Omit<AgentStateMutation, "stopPropagation"> | void>

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onRunFailed()

Called when the agent run encounters an error. 
onRunFailed?(params: { error: Error } & AgentSubscriberParams): MaybePromise<Omit<AgentStateMutation, "stopPropagation"> | void>

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onRunFinalized()

Called when the agent run completes, regardless of success or failure. 
onRunFinalized?(params: AgentSubscriberParams): MaybePromise<Omit<AgentStateMutation, "stopPropagation"> | void>

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Event Handlers

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onEvent()

General event handler that receives all events during agent execution. 
onEvent?(params: { event: BaseEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onRunStartedEvent()

Triggered when an agent run begins execution. 
onRunStartedEvent?(params: { event: RunStartedEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onRunFinishedEvent()

Called when an agent run completes successfully. 
onRunFinishedEvent?(params: { event: RunFinishedEvent; result?: any } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onRunErrorEvent()

Triggered when an agent run encounters an error. 
onRunErrorEvent?(params: { event: RunErrorEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onStepStartedEvent()

Called when a step within an agent run begins. 
onStepStartedEvent?(params: { event: StepStartedEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onStepFinishedEvent()

Triggered when a step within an agent run completes. 
onStepFinishedEvent?(params: { event: StepFinishedEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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Message Events

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onTextMessageStartEvent()

Triggered when a text message starts being generated. 
onTextMessageStartEvent?(params: { event: TextMessageStartEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onTextMessageContentEvent()

Called for each chunk of text content as it’s generated. 
onTextMessageContentEvent?(params: { event: TextMessageContentEvent; textMessageBuffer: string } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onTextMessageEndEvent()

Called when a text message generation is complete. 
onTextMessageEndEvent?(params: { event: TextMessageEndEvent; textMessageBuffer: string } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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Tool Call Events

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onToolCallStartEvent()

Triggered when a tool call begins. 
onToolCallStartEvent?(params: { event: ToolCallStartEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onToolCallArgsEvent()

Called as tool call arguments are being parsed, providing both raw and parsed argument data. 
onToolCallArgsEvent?(params: { event: ToolCallArgsEvent; toolCallBuffer: string; toolCallName: string; partialToolCallArgs: Record<string, any> } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onToolCallEndEvent()

Called when a tool call is complete with final arguments. 
onToolCallEndEvent?(params: { event: ToolCallEndEvent; toolCallName: string; toolCallArgs: Record<string, any> } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onToolCallResultEvent()

Triggered when a tool call result is received. 
onToolCallResultEvent?(params: { event: ToolCallResultEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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State Events

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onStateSnapshotEvent()

Called when a complete state snapshot is provided. 
onStateSnapshotEvent?(params: { event: StateSnapshotEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onStateDeltaEvent()

Triggered when partial state changes are applied. 
onStateDeltaEvent?(params: { event: StateDeltaEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onMessagesSnapshotEvent()

Called when a complete message history snapshot is provided. 
onMessagesSnapshotEvent?(params: { event: MessagesSnapshotEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onRawEvent()

Handler for raw, unprocessed events. 
onRawEvent?(params: { event: RawEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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onCustomEvent()

Handler for custom application-specific events. 
onCustomEvent?(params: { event: CustomEvent } & AgentSubscriberParams): MaybePromise<AgentStateMutation | void>

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State Change Handlers

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onMessagesChanged()

Called when the agent’s message history is updated. 
onMessagesChanged?(params: Omit<AgentSubscriberParams, "input"> & { input?: RunAgentInput }): MaybePromise<void>

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onStateChanged()

Triggered when the agent’s state is modified. 
onStateChanged?(params: Omit<AgentSubscriberParams, "input"> & { input?: RunAgentInput }): MaybePromise<void>

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onNewMessage()

Called when a new message is added to the conversation. 
onNewMessage?(params: { message: Message } & Omit<AgentSubscriberParams, "input"> & { input?: RunAgentInput }): MaybePromise<void>

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onNewToolCall()

Triggered when a new tool call is added to a message. 
onNewToolCall?(params: { toolCall: ToolCall } & Omit<AgentSubscriberParams, "input"> & { input?: RunAgentInput }): MaybePromise<void>

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Async Support

All subscriber methods support both synchronous and asynchronous execution: 
const subscriber: AgentSubscriber = {
  // Synchronous handler
  onTextMessageContentEvent: ({ textMessageBuffer }) => {
    updateUI(textMessageBuffer)
  },

  // Asynchronous handler
  onStateChanged: async ({ state }) => {
    await saveStateToDatabase(state)
  },

  // Async handler with mutation
  onRunInitialized: async ({ messages, state }) => {
    const enrichedState = await loadUserPreferences()
    return {
      state: { ...state, ...enrichedState },
    }
  },
}

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Multiple Subscribers

Agents can have multiple subscribers, which are processed in the order they were added: 
// First subscriber modifies state
const stateEnricher: AgentSubscriber = {
  onRunInitialized: ({ state }) => ({
    state: { ...state, timestamp: new Date().toISOString() },
  }),
}

// Second subscriber sees the modified state
const logger: AgentSubscriber = {
  onRunInitialized: ({ state }) => {
    console.log("State after enrichment:", state)
  },
}

agent.subscribe(stateEnricher)
agent.subscribe(logger)

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Integration with Agents

Basic usage pattern: 
const agent = new HttpAgent({ url: "https://api.example.com/agent" })

// Add persistent subscriber
agent.subscribe({
  onTextMessageContentEvent: ({ textMessageBuffer }) => {
    updateStreamingUI(textMessageBuffer)
  },
  onRunFinishedEvent: ({ result }) => {
    displayFinalResult(result)
  },
})

// Run agent (subscriber will be called automatically)
const result = await agent.runAgent({
  tools: [myTool],
})