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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.

Session Setup

Creating and resuming sessions

Sessions represent a specific conversation or thread between the Client and Agent. Each session maintains its own context, conversation history, and state, allowing multiple independent interactions with the same Agent.

Before creating a session, Clients MUST first complete the initialization phase to establish protocol compatibility and capabilities.

sequenceDiagram
    participant Client
    participant Agent

    Note over Agent,Client: Initialized

    alt
        Client->>Agent: session/new
        Note over Agent: Create session context
        Note over Agent: Connect to MCP servers
        Agent-->>Client: session/new response (sessionId)
    else
        Client->>Agent: session/resume (sessionId)
        Note over Agent: Restore session context
        Note over Agent: Connect to MCP servers
        opt replayFrom: start
            Note over Agent,Client: Replay conversation history...
            Agent->>Client: session/update
            Agent->>Client: session/update
            Note over Agent,Client: All content reported
        end
        Agent-->>Client: session/resume response
    end

    Note over Client,Agent: Ready for prompts

Creating a Session

Clients create a new session by calling the session/new method with:

The working directory for the session
A list of MCP servers the Agent should connect to

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "session/new",
  "params": {
    "cwd": "/home/user/project",
    "mcpServers": [
      {
        "type": "stdio",
        "name": "workspace-tools",
        "command": "/path/to/mcp-server",
        "args": ["--stdio"],
        "env": []
      }
    ]
  }
}

The Agent MUST respond with a unique Session ID that identifies this conversation:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "sessionId": "sess_abc123def456"
  }
}

Resuming Sessions

Agents that support the session method surface MUST support session/resume, allowing Clients to reconnect to existing sessions. By default, resume restores the session context without replaying prior conversation history. Clients that need history replay can request it with replayFrom.

To resume an existing session, Clients MUST call the session/resume method with:

The Session ID to resume
MCP servers to connect to
The working directory
Optionally, a replayFrom cursor

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "session/resume",
  "params": {
    "sessionId": "sess_789xyz",
    "cwd": "/home/user/project",
    "mcpServers": [
      {
        "type": "stdio",
        "name": "workspace-tools",
        "command": "/path/to/mcp-server",
        "args": ["--mode", "workspace"],
        "env": []
      }
    ]
  }
}

When replayFrom is omitted or null, the Agent MUST NOT replay the conversation history via session/update notifications before responding. It restores the session context, reconnects to the requested MCP servers, and returns once the session is ready to continue.

To request full history replay, Clients set replayFrom to { "type": "start" }:

Replay cursors are inclusive: replay includes the position identified by the cursor. For { "type": "start" }, that means replaying the whole conversation.

{
  "jsonrpc": "2.0",
  "id": 3,
  "method": "session/resume",
  "params": {
    "sessionId": "sess_789xyz",
    "cwd": "/home/user/project",
    "replayFrom": {
      "type": "start"
    }
  }
}

When replayFrom.type is "start", the Agent MUST replay the entire conversation to the Client in the form of session/update notifications before responding. User, agent, and thought messages can each be replayed as message updates with full content arrays or as chunks.

For example, a user message from the conversation history:

{
  "jsonrpc": "2.0",
  "method": "session/update",
  "params": {
    "sessionId": "sess_789xyz",
    "update": {
      "sessionUpdate": "user_message",
      "messageId": "msg_user_8f7a1",
      "content": [
        {
          "type": "text",
          "text": "What's the capital of France?"
        }
      ]
    }
  }
}

Followed by the agent’s response:

{
  "jsonrpc": "2.0",
  "method": "session/update",
  "params": {
    "sessionId": "sess_789xyz",
    "update": {
      "sessionUpdate": "agent_message",
      "messageId": "msg_agent_c42b9",
      "content": [
        {
          "type": "text",
          "text": "The capital of France is Paris."
        }
      ]
    }
  }
}

During replay, the Agent MUST include an opaque, unique messageId for each replayed message. user_message, agent_message, and agent_thought updates are upserts keyed by messageId; their content arrays replace the whole message content, while chunk updates with the same messageId append content.

Clients apply replayed message updates and chunks in the order they are received. If a message update with content arrives after chunks for the same messageId, it replaces the content accumulated from those chunks. If chunks arrive after a message update, they append to that update’s current content. Message updates that omit content can update other optional fields without changing the current content.

When all requested replay entries have been reported to the Client, the Agent MUST respond to the original session/resume request.

{
  "jsonrpc": "2.0",
  "id": 3,
  "result": {}
}

The response MAY also include initial session configuration state when that feature is supported by the Agent.

Closing Active Sessions

Agents that support the session method surface MUST support session/close, allowing Clients to tell the Agent to cancel any ongoing work for a session and free any resources associated with that active session.

To close an active session, Clients MUST call the session/close method with the session ID:

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "session/close",
  "params": {
    "sessionId": "sess_789xyz"
  }
}

The ID of the active session to close.

The Agent MUST cancel any ongoing work for that session as if session/cancel had been called, then free the resources associated with the session.

On success, the Agent responds with an empty result object:

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {}
}

Agents MAY return an error if the session does not exist or is not currently active.

Additional Workspace Roots

Agents that advertise session.additionalDirectories allow Clients to include additionalDirectories on supported session lifecycle requests to expand the session’s effective workspace root set. Supported stable lifecycle requests include session/new and session/resume.

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "session/resume",
  "params": {
    "sessionId": "sess_789xyz",
    "cwd": "/home/user/project",
    "additionalDirectories": [
      "/home/user/shared-lib",
      "/home/user/product-docs"
    ]
  }
}

When present, additionalDirectories has the following behavior:

cwd remains the primary working directory and the base for relative paths
each additionalDirectories entry MUST be an absolute path
omitting the field or providing an empty array activates no additional roots for the resulting session
on session/resume, Clients must send the full intended additional-root list again; that list may differ from any previous or reported list as long as the request cwd matches the session’s cwd, and omitting the field or providing an empty array does not restore stored roots implicitly

Clients MUST only send additionalDirectories when the Agent advertises session.additionalDirectories.

Session ID

The session ID returned by session/new is a unique identifier for the conversation context.

Clients use this ID to:

Send prompt requests via session/prompt
Cancel ongoing operations via session/cancel
Resume previous sessions via session/resume
Close active sessions via session/close

Working Directory

The cwd (current working directory) parameter establishes the primary file system context for the session. This directory:

MUST be an absolute path
MUST be used for the session regardless of where the Agent subprocess was spawned
MUST remain the base for relative-path resolution
MUST be part of the session’s effective root set

When session.additionalDirectories is in use, the session’s effective root set is [cwd, ...additionalDirectories]. This root set SHOULD serve as a boundary for tool operations on the file system.

MCP Servers

The Model Context Protocol (MCP) allows Agents to access external tools and data sources. When creating a session, Clients MAY include connection details for MCP servers that the Agent should connect to.

MCP servers can be connected to using different transports. Agents advertise supported transports during initialization using session.mcp.stdio, session.mcp.http, and any extension-specific transport capabilities.

Transport Types

Every MCP server object has a type discriminator that identifies its transport. Custom implementation-specific transport types MUST begin with _; unknown non-underscore transport types are reserved for future ACP variants.

Stdio Transport

When the Agent supports session.mcp.stdio, Clients can specify MCP servers configurations using the stdio transport.

Must be `"stdio"` to indicate stdio transport A human-readable identifier for the server The absolute path to the MCP server executable Command-line arguments to pass to the server. Omitted and empty values are equivalent. Environment variables to set when launching the server. Omitted and empty values are equivalent. The name of the environment variable.

<ParamField path="value" type="string">
  The value of the environment variable.
</ParamField>

Example stdio transport configuration:

{
  "type": "stdio",
  "name": "workspace-tools",
  "command": "/path/to/mcp-server",
  "args": ["--stdio"],
  "env": [
    {
      "name": "API_KEY",
      "value": "secret123"
    }
  ]
}

HTTP Transport

When the Agent supports session.mcp.http, Clients can specify MCP servers configurations using the HTTP transport.

Must be `"http"` to indicate HTTP transport A human-readable identifier for the server The URL of the MCP server HTTP headers to include in requests to the server. Omitted and empty values are equivalent. The name of the HTTP header.

<ParamField path="value" type="string">
  The value to set for the HTTP header.
</ParamField>

Example HTTP transport configuration:

{
  "type": "http",
  "name": "api-server",
  "url": "https://api.example.com/mcp",
  "headers": [
    {
      "name": "Authorization",
      "value": "Bearer token123"
    },
    {
      "name": "Content-Type",
      "value": "application/json"
    }
  ]
}

Checking Transport Support

Before using stdio or HTTP transports, Clients MUST verify the Agent’s capabilities during initialization:

{
  "jsonrpc": "2.0",
  "id": 0,
  "result": {
    "protocolVersion": 2,
    "capabilities": {
      "session": {
        "mcp": {
          "stdio": {},
          "http": {}
        }
      }
    }
  }
}

If session.mcp.stdio is omitted or null, the Agent does not support stdio transport. If session.mcp.http is omitted or null, the Agent does not support HTTP transport. Supplying {} means the Agent supports the corresponding transport.

Agents SHOULD connect to all MCP servers specified by the Client.

Clients MAY use this ability to provide tools directly to the underlying language model by including their own MCP server.

Last updated Oct 08, 2026