> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/agentclientprotocol/typescript-sdk/llms.txt
> Use this file to discover all available pages before exploring further.

# Agent Interface

> Interface that all ACP-compliant agents must implement

## Overview

The Agent interface defines the interface that all ACP-compliant agents must implement.

Agents are programs that use generative AI to autonomously modify code. They handle requests from clients and execute tasks using language models and tools.

## Type Definition

```typescript theme={null}
export interface Agent {
  // Required methods
  initialize(params: InitializeRequest): Promise<InitializeResponse>;
  newSession(params: NewSessionRequest): Promise<NewSessionResponse>;
  authenticate(params: AuthenticateRequest): Promise<AuthenticateResponse | void>;
  prompt(params: PromptRequest): Promise<PromptResponse>;
  cancel(params: CancelNotification): Promise<void>;
  
  // Optional methods
  loadSession?(params: LoadSessionRequest): Promise<LoadSessionResponse>;
  unstable_forkSession?(params: ForkSessionRequest): Promise<ForkSessionResponse>;
  unstable_listSessions?(params: ListSessionsRequest): Promise<ListSessionsResponse>;
  unstable_resumeSession?(params: ResumeSessionRequest): Promise<ResumeSessionResponse>;
  setSessionMode?(params: SetSessionModeRequest): Promise<SetSessionModeResponse | void>;
  unstable_setSessionModel?(params: SetSessionModelRequest): Promise<SetSessionModelResponse | void>;
  setSessionConfigOption?(params: SetSessionConfigOptionRequest): Promise<SetSessionConfigOptionResponse>;
  extMethod?(method: string, params: Record<string, unknown>): Promise<Record<string, unknown>>;
  extNotification?(method: string, params: Record<string, unknown>): Promise<void>;
}
```

## Required Methods

### initialize

```typescript theme={null}
initialize(params: InitializeRequest): Promise<InitializeResponse>
```

Establishes the connection with a client and negotiates protocol capabilities.

This method is called once at the beginning of the connection to:

* Negotiate the protocol version to use
* Exchange capability information between client and agent
* Determine available authentication methods

The agent should respond with its supported protocol version and capabilities.

<ParamField path="params" type="InitializeRequest" required>
  The initialization request parameters
</ParamField>

<ResponseField name="response" type="InitializeResponse">
  The agent's capabilities and protocol version
</ResponseField>

See protocol docs: [Initialization](https://agentclientprotocol.com/protocol/initialization)

### newSession

```typescript theme={null}
newSession(params: NewSessionRequest): Promise<NewSessionResponse>
```

Creates a new conversation session with the agent.

Sessions represent independent conversation contexts with their own history and state.

The agent should:

* Create a new session context
* Connect to any specified MCP servers
* Return a unique session ID for future requests

May return an `auth_required` error if the agent requires authentication.

<ParamField path="params" type="NewSessionRequest" required>
  The new session request parameters
</ParamField>

<ResponseField name="response" type="NewSessionResponse">
  The new session information including session ID
</ResponseField>

See protocol docs: [Session Setup](https://agentclientprotocol.com/protocol/session-setup)

### authenticate

```typescript theme={null}
authenticate(params: AuthenticateRequest): Promise<AuthenticateResponse | void>
```

Authenticates the client using the specified authentication method.

Called when the agent requires authentication before allowing session creation. The client provides the authentication method ID that was advertised during initialization.

After successful authentication, the client can proceed to create sessions with `newSession` without receiving an `auth_required` error.

<ParamField path="params" type="AuthenticateRequest" required>
  The authentication request parameters
</ParamField>

<ResponseField name="response" type="AuthenticateResponse | void">
  An empty object on success, or void
</ResponseField>

See protocol docs: [Initialization](https://agentclientprotocol.com/protocol/initialization)

### prompt

```typescript theme={null}
prompt(params: PromptRequest): Promise<PromptResponse>
```

Processes a user prompt within a session.

This method handles the whole lifecycle of a prompt:

* Receives user messages with optional context (files, images, etc.)
* Processes the prompt using language models
* Reports language model content and tool calls to the Clients
* Requests permission to run tools
* Executes any requested tool calls
* Returns when the turn is complete with a stop reason

<ParamField path="params" type="PromptRequest" required>
  The prompt request parameters
</ParamField>

<ResponseField name="response" type="PromptResponse">
  The prompt response with stop reason
</ResponseField>

See protocol docs: [Prompt Turn](https://agentclientprotocol.com/protocol/prompt-turn)

### cancel

```typescript theme={null}
cancel(params: CancelNotification): Promise<void>
```

Cancels ongoing operations for a session.

This is a notification sent by the client to cancel an ongoing prompt turn.

Upon receiving this notification, the Agent SHOULD:

* Stop all language model requests as soon as possible
* Abort all tool call invocations in progress
* Send any pending `session/update` notifications
* Respond to the original `session/prompt` request with `StopReason::Cancelled`

<ParamField path="params" type="CancelNotification" required>
  The cancellation notification parameters
</ParamField>

<ResponseField name="Promise<void>" type="Promise<void>">
  A promise that resolves when the notification has been processed
</ResponseField>

See protocol docs: [Cancellation](https://agentclientprotocol.com/protocol/prompt-turn#cancellation)

## Optional Methods

### loadSession

```typescript theme={null}
loadSession?(params: LoadSessionRequest): Promise<LoadSessionResponse>
```

Loads an existing session to resume a previous conversation.

This method is only available if the agent advertises the `loadSession` capability.

The agent should:

* Restore the session context and conversation history
* Connect to the specified MCP servers
* Stream the entire conversation history back to the client via notifications

<ParamField path="params" type="LoadSessionRequest" required>
  The load session request parameters
</ParamField>

<ResponseField name="response" type="LoadSessionResponse">
  An empty object on success
</ResponseField>

See protocol docs: [Loading Sessions](https://agentclientprotocol.com/protocol/session-setup#loading-sessions)

### unstable\_forkSession

```typescript theme={null}
unstable_forkSession?(params: ForkSessionRequest): Promise<ForkSessionResponse>
```

**UNSTABLE** - This capability is not part of the spec yet, and may be removed or changed at any point.

Forks an existing session to create a new independent session.

Creates a new session based on the context of an existing one, allowing operations like generating summaries without affecting the original session's history.

This method is only available if the agent advertises the `session.fork` capability.

<ParamField path="params" type="ForkSessionRequest" required>
  The fork session request parameters
</ParamField>

<ResponseField name="response" type="ForkSessionResponse">
  The forked session information
</ResponseField>

### unstable\_listSessions

```typescript theme={null}
unstable_listSessions?(params: ListSessionsRequest): Promise<ListSessionsResponse>
```

**UNSTABLE** - This capability is not part of the spec yet, and may be removed or changed at any point.

Lists existing sessions from the agent.

This method is only available if the agent advertises the `listSessions` capability.

Returns a list of sessions with metadata like session ID, working directory, title, and last update time. Supports filtering by working directory and cursor-based pagination.

<ParamField path="params" type="ListSessionsRequest" required>
  The list sessions request parameters
</ParamField>

<ResponseField name="response" type="ListSessionsResponse">
  The list of sessions with metadata
</ResponseField>

### unstable\_resumeSession

```typescript theme={null}
unstable_resumeSession?(params: ResumeSessionRequest): Promise<ResumeSessionResponse>
```

**UNSTABLE** - This capability is not part of the spec yet, and may be removed or changed at any point.

Resumes an existing session without returning previous messages.

This method is only available if the agent advertises the `session.resume` capability.

The agent should resume the session context, allowing the conversation to continue without replaying the message history (unlike `session/load`).

<ParamField path="params" type="ResumeSessionRequest" required>
  The resume session request parameters
</ParamField>

<ResponseField name="response" type="ResumeSessionResponse">
  The resumed session information
</ResponseField>

### setSessionMode

```typescript theme={null}
setSessionMode?(params: SetSessionModeRequest): Promise<SetSessionModeResponse | void>
```

Sets the operational mode for a session.

Allows switching between different agent modes (e.g., "ask", "architect", "code") that affect system prompts, tool availability, and permission behaviors.

The mode must be one of the modes advertised in `availableModes` during session creation or loading. Agents may also change modes autonomously and notify the client via `current_mode_update` notifications.

This method can be called at any time during a session, whether the Agent is idle or actively generating a turn.

<ParamField path="params" type="SetSessionModeRequest" required>
  The set session mode request parameters
</ParamField>

<ResponseField name="response" type="SetSessionModeResponse | void">
  An empty object on success, or void
</ResponseField>

See protocol docs: [Session Modes](https://agentclientprotocol.com/protocol/session-modes)

### unstable\_setSessionModel

```typescript theme={null}
unstable_setSessionModel?(params: SetSessionModelRequest): Promise<SetSessionModelResponse | void>
```

**UNSTABLE** - This capability is not part of the spec yet, and may be removed or changed at any point.

Select a model for a given session.

<ParamField path="params" type="SetSessionModelRequest" required>
  The set session model request parameters
</ParamField>

<ResponseField name="response" type="SetSessionModelResponse | void">
  An empty object on success, or void
</ResponseField>

### setSessionConfigOption

```typescript theme={null}
setSessionConfigOption?(params: SetSessionConfigOptionRequest): Promise<SetSessionConfigOptionResponse>
```

Set a configuration option for a given session.

The response contains the full set of configuration options and their current values, as changing one option may affect the available values or state of other options.

<ParamField path="params" type="SetSessionConfigOptionRequest" required>
  The set config option request parameters
</ParamField>

<ResponseField name="response" type="SetSessionConfigOptionResponse">
  The full set of configuration options and their current values
</ResponseField>

### extMethod

```typescript theme={null}
extMethod?(method: string, params: Record<string, unknown>): Promise<Record<string, unknown>>
```

Extension method. Allows the Agent to send an arbitrary request that is not part of the ACP spec.

To help avoid conflicts, it's a good practice to prefix extension methods with a unique identifier such as domain name.

<ParamField path="method" type="string" required>
  The extension method name
</ParamField>

<ParamField path="params" type="Record<string, unknown>" required>
  The extension method parameters
</ParamField>

<ResponseField name="response" type="Record<string, unknown>">
  The extension method response
</ResponseField>

### extNotification

```typescript theme={null}
extNotification?(method: string, params: Record<string, unknown>): Promise<void>
```

Extension notification. Allows the Agent to send an arbitrary notification that is not part of the ACP spec.

<ParamField path="method" type="string" required>
  The extension notification name
</ParamField>

<ParamField path="params" type="Record<string, unknown>" required>
  The extension notification parameters
</ParamField>

<ResponseField name="Promise<void>" type="Promise<void>">
  A promise that resolves when the notification has been processed
</ResponseField>