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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.

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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
ToolCallId
required
A unique identifier for this tool call within the session
​
title
string
required
A human-readable title describing what the tool is doing
​
kind
ToolKind
The category of tool being invoked.Tool kinds help Clients choose appropriate icons and optimize how they display tool execution progress.
​
status
ToolCallStatus
The current execution status (defaults to pending)
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content
ToolCallContent[]
Content produced by the tool call
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locations
ToolCallLocation[]
File locations affected by this tool call
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rawInput
object
The raw input parameters sent to the tool
​
rawOutput
object
The raw output returned by the tool

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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.

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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
SessionId
required
The session ID for this request
​
toolCall
ToolCallUpdate
required
The tool call update containing details about the operation
​
options
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"
    }
  }
}
​
outcome
RequestPermissionOutcome
required
The user’s decision, either: - cancelled - The prompt turn was cancelled - selected with an optionId - The ID of the selected permission option

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Permission Options

Each permission option provided to the Client contains:
​
optionId
string
required
Unique identifier for this option
​
name
string
required
Human-readable label to display to the user
​
kind
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

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Status

Tool calls progress through different statuses during their lifecycle:
​
pending
The tool call hasn’t started running yet because the input is either streaming or awaiting approval
​
in_progress
The tool call is currently running
​
completed
The tool call completed successfully
​
failed
The tool call failed with an error

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Content

Tool calls can produce different types of content:

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Regular Content

Standard content blocks like text, images, or resources: 
{
  "type": "content",
  "content": {
    "type": "text",
    "text": "Analysis complete. Found 3 issues."
  }
}

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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}"
}
​
path
string
required
The absolute file path being modified
​
oldText
string
The original content (null for new files)
​
newText
string
required
The new content after modification

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Terminals

Live terminal output from command execution: 
{
  "type": "terminal",
  "terminalId": "term_xyz789"
}
​
terminalId
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

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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
}
​
path
string
required
The absolute file path being accessed or modified
​
line
number
Optional line number within the file
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