# Anatomy of an agent

> Source: https://trigger.dev/docs/ai-chat/anatomy

The AI Agents and Prompts surface ships as part of the **v4.5 release candidate**. Install with `@trigger.dev/sdk@rc` (or pin `4.5.0-rc.0` or later) to use these features — they aren’t yet on the latest stable, and APIs may still change before the 4.5.0 GA. See [supported AI SDK versions](https://trigger.dev/docs/ai-chat/reference#compatibility)
 and the [AI chat changelog](https://trigger.dev/docs/ai-chat/changelog)
 for details.

**A chat agent is three parts: a long-lived agent task that runs the turn loop, a durable Session carrying messages in and the response stream out, and a frontend transport that plugs the session into `useChat`.** The pages in this section each own one part of that picture. This page is the map — if you’d rather read mechanics end to end, skip to [How it works](https://trigger.dev/docs/ai-chat/how-it-works)
. Everything below maps onto one annotated agent:

trigger/my-agent.ts

    import { chat } from "@trigger.dev/sdk/ai";
    import { streamText, stepCountIs } from "ai";
    import { anthropic } from "@ai-sdk/anthropic";
    
    export const myAgent = chat.agent({
      id: "my-agent",
    
      // Tools declared on the config survive history re-conversion
      // across turns — see Tools.
      tools: { searchDocs },
    
      // Hooks fire around each turn: validation, persistence,
      // post-turn work — see Lifecycle hooks.
      onTurnComplete: async ({ responseMessage }) => {
        await db.messages.save(responseMessage);
      },
    
      // The turn loop. Messages arrive accumulated; you stream back.
      // Options, levels, and alternatives — see Backend.
      run: async ({ messages, tools, signal }) =>
        streamText({
          ...chat.toStreamTextOptions({ tools }),
          model: anthropic("claude-sonnet-4-5"),
          messages,
          abortSignal: signal,
          stopWhen: stepCountIs(15),
        }),
    });
    

The frontend side is one hook — `useTriggerChatTransport` connects `useChat` to the agent’s session, no API routes ([Frontend](https://trigger.dev/docs/ai-chat/frontend)
). Underneath, the conversation lives on a [Session](https://trigger.dev/docs/ai-chat/sessions)
: a pair of durable streams keyed on your `chatId` that survives refreshes, deploys, and run boundaries.

[​](https://trigger.dev/docs/ai-chat/anatomy#where-each-part-is-covered)

Where each part is covered
------------------------------------------------------------------------------------------------------

| Part | Page |
| --- | --- |
| `chat.agent()` options, the turn loop, piping | [Backend](https://trigger.dev/docs/ai-chat/backend) |
| Hooks around each turn (`onTurnComplete`, hydration) | [Lifecycle hooks](https://trigger.dev/docs/ai-chat/lifecycle-hooks) |
| Declaring tools, typed payloads, `toModelOutput` | [Tools](https://trigger.dev/docs/ai-chat/tools) |
| `useChat` wiring, tokens, starting sessions | [Frontend](https://trigger.dev/docs/ai-chat/frontend) |
| Driving a chat from your server instead of a browser | [Server-side chat](https://trigger.dev/docs/ai-chat/server-chat) |
| The durable substrate under every agent | [Sessions](https://trigger.dev/docs/ai-chat/sessions) |
| Per-run typed state inside the loop | [chat.local](https://trigger.dev/docs/ai-chat/chat-local) |
| Type-safe payloads, client data, and messages | [Types](https://trigger.dev/docs/ai-chat/types) |
| Building without the managed lifecycle | [Custom agents](https://trigger.dev/docs/ai-chat/custom-agents) |
| End-to-end mechanics: what survives a refresh and why | [How it works](https://trigger.dev/docs/ai-chat/how-it-works) |

Beyond this section: [Features](https://trigger.dev/docs/ai-chat/fast-starts)
 covers opt-in capabilities (Head Start, compaction, steering, actions), and [Patterns](https://trigger.dev/docs/ai-chat/patterns/sub-agents)
 covers production recipes (sub-agents, HITL approvals, persistence, recovery).

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