--- title: Agent (Interface) description: API Reference for the Agent interface. --- # `Agent` (interface) The `Agent` interface defines a contract for agents that can generate or stream AI-generated responses in response to prompts. Agents may encapsulate advanced logic such as tool usage, multi-step workflows, or prompt handling, enabling both simple and autonomous AI agents. Implementations of the `Agent` interface—such as `ToolLoopAgent`—fulfill the same contract and integrate seamlessly with all SDK APIs and utilities that expect an agent. This design allows users to supply custom agent classes or wrappers for third-party chains, while maximizing compatibility with AI SDK features. ## Interface Definition ```ts import { ModelMessage } from '@ai-sdk/provider-utils'; import { ToolSet } from '../generate-text/tool-set'; import { Output } from '../generate-text/output'; import { GenerateTextResult } from '../generate-text/generate-text-result'; import { StreamTextResult } from '../generate-text/stream-text-result'; export type AgentCallParameters = ([CALL_OPTIONS] extends [never] ? { options?: never } : { options: CALL_OPTIONS }) & ( | { /** * A prompt. It can be either a text prompt or a list of messages. * * You can either use `prompt` or `messages` but not both. */ prompt: string | Array; /** * A list of messages. * * You can either use `prompt` or `messages` but not both. */ messages?: never; } | { /** * A list of messages. * * You can either use `prompt` or `messages` but not both. */ messages: Array; /** * A prompt. It can be either a text prompt or a list of messages. * * You can either use `prompt` or `messages` but not both. */ prompt?: never; } ) & { /** * Abort signal. */ abortSignal?: AbortSignal; }; /** * An Agent receives a prompt (text or messages) and generates or streams an output * that consists of steps, tool calls, data parts, etc. * * You can implement your own Agent by implementing the `Agent` interface, * or use the `ToolLoopAgent` class. */ export interface Agent< CALL_OPTIONS = never, TOOLS extends ToolSet = {}, OUTPUT extends Output = never, > { /** * The specification version of the agent interface. This will enable * us to evolve the agent interface and retain backwards compatibility. */ readonly version: 'agent-v1'; /** * The id of the agent. */ readonly id: string | undefined; /** * The tools that the agent can use. */ readonly tools: TOOLS; /** * Generates an output from the agent (non-streaming). */ generate( options: AgentCallParameters, ): PromiseLike>; /** * Streams an output from the agent (streaming). */ stream( options: AgentCallParameters, ): PromiseLike>; } ``` ## Core Properties & Methods | Name | Type | Description | | ------------ | ------------------------------------------------ | ------------------------------------------------------------------- | | `version` | `'agent-v1'` | Interface version for compatibility. | | `id` | `string \| undefined` | Optional agent identifier. | | `tools` | `ToolSet` | The set of tools available to this agent. | | `generate()` | `PromiseLike>` | Generates full, non-streaming output for a text prompt or messages. | | `stream()` | `PromiseLike>` | Streams output (chunks or steps) for a text prompt or messages. | ## Generic Parameters | Parameter | Default | Description | | -------------- | ------- | -------------------------------------------------------------------------- | | `CALL_OPTIONS` | `never` | Optional type for additional call options that can be passed to the agent. | | `TOOLS` | `{}` | The type of the tool set available to this agent. | | `OUTPUT` | `never` | The type of additional output data that the agent can produce. | ## Method Parameters Both `generate()` and `stream()` accept an `AgentCallParameters` object with: - `prompt` (optional): A string prompt or array of `ModelMessage` objects - `messages` (optional): An array of `ModelMessage` objects (mutually exclusive with `prompt`) - `options` (optional): Additional call options when `CALL_OPTIONS` is not `never` - `abortSignal` (optional): An `AbortSignal` to cancel the operation ## Example: Custom Agent Implementation Here's how you might implement your own Agent: ```ts import { Agent, GenerateTextResult, StreamTextResult } from 'ai'; import type { ModelMessage } from '@ai-sdk/provider-utils'; class MyEchoAgent implements Agent { version = 'agent-v1' as const; id = 'echo'; tools = {}; async generate({ prompt, messages, abortSignal }) { const text = prompt ?? JSON.stringify(messages); return { text, steps: [] }; } async stream({ prompt, messages, abortSignal }) { const text = prompt ?? JSON.stringify(messages); return { textStream: (async function* () { yield text; })(), }; } } ``` ## Usage: Interacting with Agents All SDK utilities that accept an agent—including [`createAgentUIStream`](/docs/reference/ai-sdk-core/create-agent-ui-stream), [`createAgentUIStreamResponse`](/docs/reference/ai-sdk-core/create-agent-ui-stream-response), and [`pipeAgentUIStreamToResponse`](/docs/reference/ai-sdk-core/pipe-agent-ui-stream-to-response)—expect an object adhering to the `Agent` interface. You can use the official [`ToolLoopAgent`](/docs/reference/ai-sdk-core/tool-loop-agent) (recommended for multi-step AI workflows with tool use), or supply your own implementation: ```ts import { ToolLoopAgent, createAgentUIStream } from "ai"; const agent = new ToolLoopAgent({ ... }); const stream = await createAgentUIStream({ agent, messages: [{ role: "user", content: "What is the weather in NYC?" }] }); for await (const chunk of stream) { console.log(chunk); } ``` ## See Also - [`ToolLoopAgent`](/docs/reference/ai-sdk-core/tool-loop-agent) — Official multi-step agent implementation - [`createAgentUIStream`](/docs/reference/ai-sdk-core/create-agent-ui-stream) - [`GenerateTextResult`](/docs/reference/ai-sdk-core/generate-text) - [`StreamTextResult`](/docs/reference/ai-sdk-core/stream-text) ## Notes - Agents should define their `tools` property, even if empty (`{}`), for compatibility with SDK utilities. - The interface accepts both plain prompts and message arrays as input, but only one at a time. - The `CALL_OPTIONS` generic parameter allows agents to accept additional call-specific options when needed. - The `abortSignal` parameter enables cancellation of agent operations. - This design is extensible for both complex autonomous agents and simple LLM wrappers.

Agent Interface