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Claude tool calling lets you define callable functions that Claude can choose to invoke during generation. You define a Tool with a name, description, and input schema, attach it to a generation request, and Claude responds with a tool_use content block when it decides to call the tool. The SDK maps that block to a ToolCallPart in the response, which your application can inspect and execute.

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Basic Tool

Define a tool with Tool::make(), declare its input schema, and register a handler with ->run(). Attach the tool to a generation request using ->tool(). 
use AiSdk\Anthropic;
use AiSdk\Generate;
use AiSdk\Schema;
use AiSdk\Tool;

$weather = Tool::make('weather', 'Get current weather')
    ->input(Schema::string(name: 'city')->required())
    ->run(fn (string $city): string => "Sunny in {$city}");

$result = Generate::text()
    ->model(Anthropic::model('claude-sonnet-4'))
    ->prompt('What is the weather in Lahore?')
    ->tool($weather)
    ->run();
When Claude decides to call the tool, $result->finishReason is FinishReason::ToolCalls and $result->parts contains a ToolCallPart with the tool name and decoded argument array.

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Wire Format

AnthropicToolConverter::convert() maps each Tool object to the shape required by the Anthropic API: 
{
  "name": "weather",
  "description": "Get current weather",
  "input_schema": {
    "type": "object",
    "properties": {
      "city": { "type": "string" }
    },
    "required": ["city"]
  }
}
Tool results are returned to Anthropic in the next generation turn as tool_result blocks nested inside a user message: 
{
  "role": "user",
  "content": [
    {
      "type": "tool_result",
      "tool_use_id": "<id from tool_use block>",
      "content": "Sunny in Lahore"
    }
  ]
}

​
Multiple Tools

Pass multiple tools at once using ->tools() with an array: 
$result = Generate::text()
    ->model(Anthropic::model('claude-sonnet-4'))
    ->prompt('What is the weather in Lahore and what time is it?')
    ->tools([$weatherTool, $timeTool])
    ->run();
Tools are sent to the API in the order they are registered. Claude may call one or more of them depending on the prompt.
Tool result messages use Message::ROLE_TOOL internally. The SDK converts them to Anthropic’s user / tool_result message format automatically — you do not need to construct the wire format yourself.

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