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

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

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.

params · InitializeRequest · required

The initialization request parameters

response · InitializeResponse

The agent’s capabilities and protocol version

See protocol docs: Initialization

newSession

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.

params · NewSessionRequest · required

The new session request parameters

response · NewSessionResponse

The new session information including session ID

See protocol docs: Session Setup

authenticate

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.

params · AuthenticateRequest · required

The authentication request parameters

response · AuthenticateResponse | void

An empty object on success, or void

See protocol docs: Initialization

prompt

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

params · PromptRequest · required

The prompt request parameters

response · PromptResponse

The prompt response with stop reason

See protocol docs: Prompt Turn

cancel

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

params · CancelNotification · required

The cancellation notification parameters

Promise<void> · Promise<void>

A promise that resolves when the notification has been processed

See protocol docs: Cancellation

Optional Methods

loadSession

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

params · LoadSessionRequest · required

The load session request parameters

response · LoadSessionResponse

An empty object on success

See protocol docs: Loading Sessions

unstable_forkSession

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.

params · ForkSessionRequest · required

The fork session request parameters

response · ForkSessionResponse

The forked session information

unstable_listSessions

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.

params · ListSessionsRequest · required

The list sessions request parameters

response · ListSessionsResponse

The list of sessions with metadata

unstable_resumeSession

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).

params · ResumeSessionRequest · required

The resume session request parameters

response · ResumeSessionResponse

The resumed session information

setSessionMode

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.

params · SetSessionModeRequest · required

The set session mode request parameters

response · SetSessionModeResponse | void

An empty object on success, or void

See protocol docs: Session Modes

unstable_setSessionModel

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.

params · SetSessionModelRequest · required

The set session model request parameters

response · SetSessionModelResponse | void

An empty object on success, or void

setSessionConfigOption

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.

params · SetSessionConfigOptionRequest · required

The set config option request parameters

response · SetSessionConfigOptionResponse

The full set of configuration options and their current values

extMethod

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.

method · string · required

The extension method name

params · Record<string, unknown> · required

The extension method parameters

response · Record<string, unknown>

The extension method response

extNotification

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.

method · string · required

The extension notification name

params · Record<string, unknown> · required

The extension notification parameters

Promise<void> · Promise<void>

A promise that resolves when the notification has been processed