▸ 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.

Initialization

How all Agent Client Protocol connections begin

The Initialization phase allows Clients and Agents to negotiate protocol versions, capabilities, and authentication methods.


sequenceDiagram
    participant Client
    participant Agent

    Note over Client, Agent: Connection established
    Client->>Agent: initialize
    Note right of Agent: Negotiate protocol<br/>version & capabilities
    Agent-->>Client: initialize response
    Note over Client,Agent: Ready for session setup

Before a Session can be created, Clients MUST initialize the connection by calling the initialize method with:

{
  "jsonrpc": "2.0",
  "id": 0,
  "method": "initialize",
  "params": {
    "protocolVersion": 2,
    "capabilities": {},
    "info": {
      "name": "my-client",
      "title": "My Client",
      "version": "1.0.0"
    }
  }
}

The Agent MUST respond with the chosen protocol version, the capabilities it supports, and its implementation information:

{
  "jsonrpc": "2.0",
  "id": 0,
  "result": {
    "protocolVersion": 2,
    "capabilities": {
      "session": {
        "prompt": {
          "image": {},
          "audio": {},
          "embeddedContext": {}
        },
        "mcp": {
          "stdio": {},
          "http": {}
        }
      }
    },
    "info": {
      "name": "my-agent",
      "title": "My Agent",
      "version": "1.0.0"
    },
    "authMethods": []
  }
}

authMethods is optional. Returning one or more valid entries advertises the Agent’s authentication surface and means the Agent MUST implement both auth/login and auth/logout. If authMethods is omitted or empty, Clients MUST NOT call either method.

capabilities.auth is separate from this method surface. It advertises authentication-related extensions and does not change the availability rules for auth/login or auth/logout.

Protocol version

The protocol versions that appear in the initialize requests and responses are a single integer that identifies a MAJOR protocol version. This version is only incremented when breaking changes are introduced.

Clients and Agents MUST agree on a protocol version and act according to its specification.

See Capabilities to learn how non-breaking features are introduced.

Version Negotiation

The initialize request MUST include the latest protocol version the Client supports.

If the Agent supports the requested version, it MUST respond with the same version. Otherwise, the Agent MUST respond with the latest version it supports.

If the Client does not support the version specified by the Agent in the initialize response, the Client SHOULD close the connection and inform the user about it.

Capabilities

Capabilities describe features supported by the Client and the Agent.

All capabilities included in the initialize request are OPTIONAL. Clients and Agents SHOULD support all possible combinations of their peer’s capabilities.

The introduction of new capabilities is not considered a breaking change. Therefore, Clients and Agents MUST treat all capabilities omitted in the initialize request as UNSUPPORTED.

Capabilities may advertise top-level method surfaces or nested features that only apply within a supported surface.

Capabilities may specify the availability of protocol methods, notifications, or a subset of their parameters. They may also signal behaviors of the Agent or Client implementation.

Implementations can also advertise custom capabilities using the _meta field to indicate support for protocol extensions.

Client Capabilities

Clients MAY include defined capability fields in capabilities. Omitted fields mean unsupported. Extension-specific capabilities belong in _meta.

Terminal Authentication

The Client can reproduce the configured Agent invocation in an interactive terminal. Omitted or `null` means unsupported. Supplying `{}` means the Agent may advertise `type: "terminal"` authentication methods. Learn more about Terminal Authentication

Elicitation

The Client advertises its supported `elicitation/create` modes. Omitted or `null` means no modes are advertised. A present object advertises each mode only through its corresponding non-null `form` or `url` field. Empty and all-null objects are valid but advertise no supported modes. Unlike MCP, ACP does not treat `{}` as form support. Learn more about Elicitation

Agent Capabilities

The Agent SHOULD specify whether it supports the following capabilities:

The Agent supports the `session/*` method surface. Omitted or `null` means the Agent does not support session methods. Supplying `{}` means the Agent supports the baseline session methods: `session/new`, `session/list`, `session/resume`, `session/close`, `session/prompt`, `session/cancel`, and `session/update`. Authentication-related extensions supported by the Agent. This field is orthogonal to `authMethods` and does not advertise the availability of `auth/login` or `auth/logout`.

Session Capabilities

Supplying session: {} means the Agent supports session/new, session/list, session/resume, session/close, session/prompt, session/cancel, and session/update.

Optionally, Agents MAY support prompt extensions, MCP server transports, and additional session methods such as session/delete by specifying nested capabilities.

Object indicating the different types of [content](https://agentclientprotocol.com/protocol/v2/content) that may be included in `session/prompt` requests. Omitted or `null` means the Agent does not advertise any prompt extensions beyond the baseline text and resource-link content required by `session/prompt`. Object indicating the MCP server transports that may be included in session lifecycle requests. Omitted or `null` both mean the Agent does not advertise MCP server transport support for sessions. The [`session/delete`](https://agentclientprotocol.com/protocol/v2/session-delete) method is available. Omitted or `null` both mean the Agent does not advertise support. Supplying an empty object means the Agent supports deleting sessions from `session/list`. The Agent supports `additionalDirectories` on supported session lifecycle requests. Omitted or `null` both mean the Agent does not advertise support. Supplying `{}` means the Agent supports additional workspace roots.

Session Prompt Capabilities

As a baseline, Agents that advertise session MUST support ContentBlock::Text and ContentBlock::ResourceLink in session/prompt requests.

Optionally, they MAY support richer types of content by specifying the following capabilities:

The prompt may include `ContentBlock::Image`. Omitted or `null` means the Agent does not advertise support. Supplying `{}` means the Agent supports image content in prompts. The prompt may include `ContentBlock::Audio`. Omitted or `null` means the Agent does not advertise support. Supplying `{}` means the Agent supports audio content in prompts. The prompt may include `ContentBlock::Resource`. Omitted or `null` means the Agent does not advertise support. Supplying `{}` means the Agent supports embedded context in prompts.

Session MCP Capabilities

The Agent supports connecting to MCP servers over stdio. Omitted or `null` means the Agent does not advertise support. Supplying `{}` means the Agent supports stdio MCP server transports. The Agent supports connecting to MCP servers over HTTP. Omitted or `null` means the Agent does not advertise support. Supplying `{}` means the Agent supports HTTP MCP server transports. Learn more about Authentication

Implementation Information

Both Clients and Agents MUST provide information about their implementation in the info field. It takes the following three fields:

Intended for programmatic or logical use, but can be used as a display name fallback if title isn’t present. Intended for UI and end-user contexts — optimized to be human-readable and easily understood. If not provided, the name should be used for display. Version of the implementation. Can be displayed to the user or used for debugging or metrics purposes.

Once the connection is initialized, you’re ready to create a session and begin the conversation with the Agent.

Last updated Oct 08, 2026