▸ Agent Skills
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Documentation Index

Fetch the complete documentation index at: https://agentclientprotocol.com/llms.txt Use this file to discover all available pages before exploring further.

Overview

How the Agent Client Protocol works

The Agent Client Protocol allows Agents and Clients to communicate by exposing methods that each side can call and sending notifications to inform each other of events.

Communication Model

The protocol follows the JSON-RPC 2.0 specification with two types of messages:

  • Methods: Request-response pairs that expect a result or error
  • Notifications: One-way messages that don’t expect a response

ACP follows JSON-RPC 2.0 batch behavior. See Transports for batch handling requirements.

Message Flow

A typical flow follows this pattern:

<Steps> * Client → Agent: initialize to establish connection * Client → Agent: auth/login if required by the Agent

* Client → Agent: `session/new` to create a new session * Client → Agent: `session/resume` to resume an existing session * Client → Agent: `session/prompt` to send user message * Agent → Client: `session/prompt` response once the prompt is accepted * Agent → Client: `session/update` notifications for accepted messages, state updates, and progress updates * Agent → Client: Permission requests as needed * Client → Agent: `session/cancel` to interrupt processing if needed * Agent → Client: An idle `state_update` when ready for a new prompt, with a stop reason when foreground work ends

Agent

Agents are programs that use generative AI to autonomously modify code. They typically run as subprocesses of the Client.

Baseline Methods

Some baseline methods belong to optional surfaces. An Agent that returns one or more valid entries in authMethods MUST implement both auth/login and auth/logout. If authMethods is omitted or empty, Clients MUST NOT call either method.

<ResponseField name=“initialize” post={[Schema]}> Negotiate versions and exchange capabilities..

<ResponseField name=“auth/login” post={[Schema]}> Authenticate with the Agent (if required).

<ResponseField name=“auth/logout” post={[Schema]}> End the current authenticated state.

<ResponseField name=“session/new” post={[Schema]}> Create a new conversation session.

<ResponseField name=“session/prompt” post={[Schema]}> Send user prompts to the Agent.

<ResponseField name=“session/list” post={[Schema]}> List known sessions.

<ResponseField name=“session/resume” post={[Schema]}> Resume an existing session, optionally replaying history.

<ResponseField name=“session/close” post={[Schema]}> Close an active session.

Notifications

<ResponseField name=“session/cancel” post={[Schema]}> Cancel ongoing operations (no response expected).

Client

Clients provide the interface between users and agents. They are typically code editors (IDEs, text editors) but can also be other UIs for interacting with agents. Clients manage the environment, handle user interactions, and control access to resources.

Baseline Methods

<ResponseField name=“session/request_permission” post={[Schema]}> Request user authorization for operations such as tool calls and commands.

Optional Methods

<ResponseField name=“elicitation/create” post={[Schema]}> Request structured information from the user (requires the matching elicitation mode capability).

Notifications

<ResponseField name=“elicitation/complete” post={[Schema]}> Report completion of an out-of-band URL interaction (no response expected).

<ResponseField name=“session/update” post={[Schema]}> Send session updates to inform the Client of changes (no response expected). This includes message updates and chunks, tool calls, updates, and content chunks, display-only terminal state and output chunks, plans, available commands updates, and config option updates.

Argument requirements

  • All file paths in the protocol MUST be absolute.
  • Line numbers are 1-based

Error Handling

All methods follow standard JSON-RPC 2.0 error handling:

  • Successful responses include a result field
  • Errors include an error object with code and message
  • Notifications never receive responses (success or error)

Conventions

Unless explicitly defined otherwise in the schema, ACP-defined JSON object property keys use camelCase. String values carried by discriminator fields use snake_case. The JSON-RPC envelope fields (jsonrpc, id, method, params, result, and error) follow the JSON-RPC 2.0 specification.

Selected enum-like fields and tagged unions can define custom or future fallbacks. For those fields, _-prefixed values are reserved for implementation-specific extensions, while unknown non-underscore values are reserved for future ACP variants.

Extensibility

The protocol provides built-in mechanisms for adding custom functionality while maintaining compatibility:

  • Add custom data using _meta fields
  • Create custom methods by prefixing their name with underscore (_)
  • Advertise custom capabilities during initialization

Learn about protocol extensibility to understand how to use these mechanisms.

Next Steps

Last updated Oct 08, 2026