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Agents

Agents are the core components in the AG-UI protocol that process requests and generate responses. They establish a standardized way for front-end applications to communicate with AI services through a consistent interface, regardless of the underlying implementation.

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What is an Agent?

In AG-UI, an agent is a class that:
  1. Manages conversation state and message history
  2. Processes incoming messages and context
  3. Generates responses through an event-driven streaming interface
  4. Follows a standardized protocol for communication
Agents can be implemented to connect with any AI service, including:
  • Large language models (LLMs) like GPT-4 or Claude
  • Custom AI systems
  • Retrieval augmented generation (RAG) systems
  • Multi-agent systems

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Agent Architecture

All agents in AG-UI extend the AbstractAgent class, which provides the foundation for:
  • State management
  • Message history tracking
  • Event stream processing
  • Tool usage
import { AbstractAgent } from "@ag-ui/client"

class MyAgent extends AbstractAgent {
  protected run(input: RunAgentInput): RunAgent {
    // Implementation details
  }
}

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Core Components

AG-UI agents have several key components:
  1. Configuration: Agent ID, thread ID, and initial state
  2. Messages: Conversation history with user and assistant messages
  3. State: Structured data that persists across interactions
  4. Events: Standardized messages for communication with clients
  5. Tools: Functions that agents can use to interact with external systems

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Agent Types

AG-UI provides different agent implementations to suit various needs:

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AbstractAgent

The base class that all agents extend. It handles core event processing, state management, and message history.

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HttpAgent

A concrete implementation that connects to remote AI services via HTTP: 
import { HttpAgent } from "@ag-ui/client"

const agent = new HttpAgent({
  url: "https://your-agent-endpoint.com/agent",
  headers: {
    Authorization: "Bearer your-api-key",
  },
})

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Custom Agents

You can create custom agents to integrate with any AI service by extending AbstractAgent: 
class CustomAgent extends AbstractAgent {
  // Custom properties and methods

  protected run(input: RunAgentInput): RunAgent {
    // Implement the agent's logic
  }
}

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Implementing Agents

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Basic Implementation

To create a custom agent, extend the AbstractAgent class and implement the required run method: 
import {
  AbstractAgent,
  RunAgent,
  RunAgentInput,
  EventType,
  BaseEvent,
} from "@ag-ui/client"
import { Observable } from "rxjs"

class SimpleAgent extends AbstractAgent {
  protected run(input: RunAgentInput): RunAgent {
    const { threadId, runId } = input

    return () =>
      new Observable<BaseEvent>((observer) => {
        // Emit RUN_STARTED event
        observer.next({
          type: EventType.RUN_STARTED,
          threadId,
          runId,
        })

        // Send a message
        const messageId = Date.now().toString()

        // Message start
        observer.next({
          type: EventType.TEXT_MESSAGE_START,
          messageId,
          role: "assistant",
        })

        // Message content
        observer.next({
          type: EventType.TEXT_MESSAGE_CONTENT,
          messageId,
          delta: "Hello, world!",
        })

        // Message end
        observer.next({
          type: EventType.TEXT_MESSAGE_END,
          messageId,
        })

        // Emit RUN_FINISHED event
        observer.next({
          type: EventType.RUN_FINISHED,
          threadId,
          runId,
        })

        // Complete the observable
        observer.complete()
      })
  }
}

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Agent Capabilities

Agents in the AG-UI protocol provide a rich set of capabilities that enable sophisticated AI interactions:

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Interactive Communication

Agents establish bi-directional communication channels with front-end applications through event streams. This enables:
  • Real-time streaming responses character-by-character
  • Immediate feedback loops between user and AI
  • Progress indicators for long-running operations
  • Structured data exchange in both directions

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Tool Usage

Agents can use tools to perform actions and access external resources. Importantly, tools are defined and passed in from the front-end application to the agent, allowing for a flexible and extensible system: 
// Tool definition
const confirmAction = {
  name: "confirmAction",
  description: "Ask the user to confirm a specific action before proceeding",
  parameters: {
    type: "object",
    properties: {
      action: {
        type: "string",
        description: "The action that needs user confirmation",
      },
      importance: {
        type: "string",
        enum: ["low", "medium", "high", "critical"],
        description: "The importance level of the action",
      },
      details: {
        type: "string",
        description: "Additional details about the action",
      },
    },
    required: ["action"],
  },
}

// Running an agent with tools from the frontend
agent.runAgent({
  tools: [confirmAction], // Frontend-defined tools passed to the agent
  // other parameters
})
Tools are invoked through a sequence of events:
  1. TOOL_CALL_START: Indicates the beginning of a tool call
  2. TOOL_CALL_ARGS: Streams the arguments for the tool call
  3. TOOL_CALL_END: Marks the completion of the tool call
Front-end applications can then execute the tool and provide results back to the agent. This bidirectional flow enables sophisticated human-in-the-loop workflows where:
  • The agent can request specific actions be performed
  • Humans can execute those actions with appropriate judgment
  • Results are fed back to the agent for continued reasoning
  • The agent maintains awareness of all decisions made in the process
This mechanism is particularly powerful for implementing interfaces where AI and humans collaborate. For example, CopilotKit leverages this exact pattern with their useCopilotAction hook, which provides a simplified way to define and handle tools in React applications. By keeping the AI informed about human decisions through the tool mechanism, applications can maintain context and create more natural collaborative experiences between users and AI assistants.

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State Management

Agents maintain a structured state that persists across interactions. This state can be:
  • Updated incrementally through STATE_DELTA events
  • Completely refreshed with STATE_SNAPSHOT events
  • Accessed by both the agent and front-end
  • Used to store user preferences, conversation context, or application state
// Accessing agent state
console.log(agent.state.preferences)

// State is automatically updated during agent runs
agent.runAgent().subscribe((event) => {
  if (event.type === EventType.STATE_DELTA) {
    // State has been updated
    console.log("New state:", agent.state)
  }
})

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Multi-Agent Collaboration

AG-UI supports agent-to-agent handoff and collaboration:
  • Agents can delegate tasks to other specialized agents
  • Multiple agents can work together in a coordinated workflow
  • State and context can be transferred between agents
  • The front-end maintains a consistent experience across agent transitions
For example, a general assistant agent might hand off to a specialized coding agent when programming help is needed, passing along the conversation context and specific requirements.

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Human-in-the-Loop Workflows

Agents support human intervention and assistance:
  • Agents can request human input on specific decisions
  • Front-ends can pause agent execution and resume it after human feedback
  • Human experts can review and modify agent outputs before they’re finalized
  • Hybrid workflows combine AI efficiency with human judgment
This enables applications where the agent acts as a collaborative partner rather than an autonomous system.

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Conversational Memory

Agents maintain a complete history of conversation messages:
  • Past interactions inform future responses
  • Message history is synchronized between client and server
  • Messages can include rich content (text, structured data, references)
  • The context window can be managed to focus on relevant information
// Accessing message history
console.log(agent.messages)

// Adding a new user message
agent.messages.push({
  id: "msg_123",
  role: "user",
  content: "Can you explain that in more detail?",
})

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Metadata and Instrumentation

Agents can emit metadata about their internal processes:
  • Reasoning steps through custom events
  • Performance metrics and timing information
  • Source citations and reference tracking
  • Confidence scores for different response options
This allows front-ends to provide transparency into the agent’s decision-making process and help users understand how conclusions were reached.

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Using Agents

Once you’ve implemented or instantiated an agent, you can use it like this: 
// Create an agent instance
const agent = new HttpAgent({
  url: "https://your-agent-endpoint.com/agent",
})

// Add initial messages if needed
agent.messages = [
  {
    id: "1",
    role: "user",
    content: "Hello, how can you help me today?",
  },
]

// Run the agent
agent
  .runAgent({
    runId: "run_123",
    tools: [], // Optional tools
    context: [], // Optional context
  })
  .subscribe({
    next: (event) => {
      // Handle different event types
      switch (event.type) {
        case EventType.TEXT_MESSAGE_CONTENT:
          console.log("Content:", event.delta)
          break
        // Handle other events
      }
    },
    error: (error) => console.error("Error:", error),
    complete: () => console.log("Run complete"),
  })

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Agent Configuration

Agents accept configuration through the constructor: 
interface AgentConfig {
  agentId?: string // Unique identifier for the agent
  description?: string // Human-readable description
  threadId?: string // Conversation thread identifier
  initialMessages?: Message[] // Initial messages
  initialState?: State // Initial state object
}

// Using the configuration
const agent = new HttpAgent({
  agentId: "my-agent-123",
  description: "A helpful assistant",
  threadId: "thread-456",
  initialMessages: [
    { id: "1", role: "system", content: "You are a helpful assistant." },
  ],
  initialState: { preferredLanguage: "English" },
})

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Agent State Management

AG-UI agents maintain state across interactions: 
// Access current state
console.log(agent.state)

// Access messages
console.log(agent.messages)

// Clone an agent with its state
const clonedAgent = agent.clone()

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Conclusion

Agents are the foundation of the AG-UI protocol, providing a standardized way to connect front-end applications with AI services. By implementing the AbstractAgent class, you can create custom integrations with any AI service while maintaining a consistent interface for your applications. The event-driven architecture enables real-time, streaming interactions that are essential for modern AI applications, and the standardized protocol ensures compatibility across different implementations.