Tool Calls

Tool calls represent actions that language models request Agents to perform during a prompt turn. When an LLM determines it needs to interact with external systems—like reading files, running code, or fetching data—it generates tool calls that the Agent executes on its behalf. Agents report tool calls through session/update notifications, allowing Clients to display real-time progress and results to users. While Agents handle the actual execution, they may leverage Client capabilities like permission requests or file system access to provide a richer, more integrated experience.

Creating

When the language model requests a tool invocation, the Agent SHOULD report it to the Client:

{
  "jsonrpc": "2.0",
  "method": "session/update",
  "params": {
    "sessionId": "sess_abc123def456",
    "update": {
      "sessionUpdate": "tool_call",
      "toolCallId": "call_001",
      "title": "Reading configuration file",
      "kind": "read",
      "status": "pending"
    }
  }
}

ToolCallId

required

A unique identifier for this tool call within the session

string

required

A human-readable title describing what the tool is doing

ToolKind

The category of tool being invoked.

Show kinds

read - Reading files or data - edit - Modifying files or content - delete - Removing files or data - move - Moving or renaming files - search - Searching for information - execute - Running commands or code - think - Internal reasoning or planning - fetch - Retrieving external data
other - Other tool types (default)

Tool kinds help Clients choose appropriate icons and optimize how they display tool execution progress.

ToolCallStatus

The current execution status (defaults to pending)

ToolCallContent[]

Content produced by the tool call

ToolCallLocation[]

File locations affected by this tool call

object

The raw input parameters sent to the tool

object

The raw output returned by the tool

Updating

As tools execute, Agents send updates to report progress and results. Updates use the session/update notification with tool_call_update:

{
  "jsonrpc": "2.0",
  "method": "session/update",
  "params": {
    "sessionId": "sess_abc123def456",
    "update": {
      "sessionUpdate": "tool_call_update",
      "toolCallId": "call_001",
      "status": "in_progress",
      "content": [
        {
          "type": "content",
          "content": {
            "type": "text",
            "text": "Found 3 configuration files..."
          }
        }
      ]
    }
  }
}

All fields except toolCallId are optional in updates. Only the fields being changed need to be included.

Requesting Permission

The Agent MAY request permission from the user before executing a tool call by calling the session/request_permission method:

{
  "jsonrpc": "2.0",
  "id": 5,
  "method": "session/request_permission",
  "params": {
    "sessionId": "sess_abc123def456",
    "toolCall": {
      "toolCallId": "call_001"
    },
    "options": [
      {
        "optionId": "allow-once",
        "name": "Allow once",
        "kind": "allow_once"
      },
      {
        "optionId": "reject-once",
        "name": "Reject",
        "kind": "reject_once"
      }
    ]
  }
}

SessionId

required

The session ID for this request

ToolCallUpdate

required

The tool call update containing details about the operation

PermissionOption[]

required

Available permission options for the user to choose from

The Client responds with the user’s decision:

{
  "jsonrpc": "2.0",
  "id": 5,
  "result": {
    "outcome": {
      "outcome": "selected",
      "optionId": "allow-once"
    }
  }
}

Clients MAY automatically allow or reject permission requests according to the user settings. If the current prompt turn gets cancelled, the Client MUST respond with the "cancelled" outcome:

{
  "jsonrpc": "2.0",
  "id": 5,
  "result": {
    "outcome": {
      "outcome": "cancelled"
    }
  }
}

RequestPermissionOutcome

required

The user’s decision, either: - cancelled - The prompt turn was cancelled - selected with an optionId - The ID of the selected permission option

Permission Options

Each permission option provided to the Client contains:

string

required

Unique identifier for this option

string

required

Human-readable label to display to the user

PermissionOptionKind

required

A hint to help Clients choose appropriate icons and UI treatment for each option.

allow_once - Allow this operation only this time
allow_always - Allow this operation and remember the choice
reject_once - Reject this operation only this time
reject_always - Reject this operation and remember the choice

Status

Tool calls progress through different statuses during their lifecycle:

The tool call hasn’t started running yet because the input is either streaming or awaiting approval

The tool call is currently running

The tool call completed successfully

The tool call failed with an error

Content

Tool calls can produce different types of content:

Regular Content

Standard content blocks like text, images, or resources:

{
  "type": "content",
  "content": {
    "type": "text",
    "text": "Analysis complete. Found 3 issues."
  }
}

Diffs

File modifications shown as diffs:

{
  "type": "diff",
  "path": "/home/user/project/src/config.json",
  "oldText": "{\n  \"debug\": false\n}",
  "newText": "{\n  \"debug\": true\n}"
}

string

required

The absolute file path being modified

string

The original content (null for new files)

string

required

The new content after modification

Terminals

Live terminal output from command execution:

{
  "type": "terminal",
  "terminalId": "term_xyz789"
}

string

required

The ID of a terminal created with terminal/create

When a terminal is embedded in a tool call, the Client displays live output as it’s generated and continues to display it even after the terminal is released.

Learn more about Terminals

Following the Agent

Tool calls can report file locations they’re working with, enabling Clients to implement “follow-along” features that track which files the Agent is accessing or modifying in real-time.

{
  "path": "/home/user/project/src/main.py",
  "line": 42
}

string

required

The absolute file path being accessed or modified

number

Optional line number within the file

Get Started

Protocol

Libraries

Creating

Updating

Requesting Permission

Permission Options

Status

Content

Regular Content

Diffs

Terminals

Following the Agent

​Creating

​Updating

​Requesting Permission

​Permission Options

​Status

​Content

​Regular Content

​Diffs

​Terminals

​Following the Agent

Creating

Updating

Requesting Permission

Permission Options

Status

Content

Regular Content

Diffs

Terminals

Following the Agent