CtrlK
BlogDocsLog inGet started
Tessl Logo

tessl/pypi-anthropic

The official Python library for the anthropic API

—

Pending

Quality

Pending

Does it follow best practices?

Impact

Pending

No eval scenarios have been run

Overview
Eval results
Files

tool-usage.mddocs/guides/

Tool Usage Guide

Comprehensive guide to using tools (function calling) with Claude for building agentic workflows.

Tool Basics

Tools let Claude call functions you define, enabling it to take actions and retrieve information.

Manual Tool Definition

message = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[{
        "name": "get_weather",
        "description": "Get weather for a location",
        "input_schema": {
            "type": "object",
            "properties": {
                "location": {"type": "string", "description": "City and state"}
            },
            "required": ["location"]
        }
    }],
    messages=[{"role": "user", "content": "What's the weather in SF?"}]
)

Using Tool Decorators

from anthropic import beta_tool

@beta_tool
def get_weather(location: str, unit: str = "fahrenheit") -> dict:
    """
    Get weather for a location.

    Args:
        location: City and state, e.g. San Francisco, CA
        unit: Temperature unit (celsius or fahrenheit)
    """
    # Implementation
    return {"temperature": 72, "condition": "sunny", "unit": unit}

message = client.beta.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[get_weather],
    messages=[{"role": "user", "content": "What's the weather in NYC?"}]
)

Tool Execution Flow

Manual Execution

# 1. Send initial request
message = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[...],
    messages=[{"role": "user", "content": "What's the weather in Paris?"}]
)

# 2. Extract tool use
tool_use = next(block for block in message.content if block.type == "tool_use")

# 3. Execute function
result = get_weather(location=tool_use.input["location"])

# 4. Send result back
message = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[...],
    messages=[
        {"role": "user", "content": "What's the weather in Paris?"},
        {"role": "assistant", "content": message.content},
        {
            "role": "user",
            "content": [{
                "type": "tool_result",
                "tool_use_id": tool_use.id,
                "content": str(result)
            }]
        }
    ]
)

Automatic Execution with Tool Runner

@beta_tool
def get_weather(location: str) -> dict:
    """Get weather for location."""
    return {"temp": 72, "condition": "sunny"}

@beta_tool
def get_time(timezone: str = "UTC") -> str:
    """Get current time in timezone."""
    from datetime import datetime
    return datetime.now().strftime("%H:%M")

for message in client.beta.messages.tool_runner(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[get_weather, get_time],
    messages=[{"role": "user", "content": "What's the weather and time in SF?"}]
):
    if message.stop_reason == "end_turn":
        print(message.content[0].text)

Tool Choice Control

Auto (Default)

Let Claude decide whether to use tools:

message = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[...],
    tool_choice={"type": "auto"},
    messages=[...]
)

Force Any Tool

Require Claude to use at least one tool:

message = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[...],
    tool_choice={"type": "any"},
    messages=[...]
)

Force Specific Tool

Require Claude to use a specific tool:

message = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[...],
    tool_choice={"type": "tool", "name": "get_weather"},
    messages=[...]
)

Disable All Tools

message = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[...],
    tool_choice={"type": "none"},
    messages=[...]
)

Disable Parallel Tool Use

Force sequential tool calls:

message = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[...],
    tool_choice={
        "type": "auto",
        "disable_parallel_tool_use": True
    },
    messages=[...]
)

Advanced Patterns

Stateful Tools

class Calculator:
    def __init__(self):
        self.memory = 0

    @beta_tool
    def calculate(self, expression: str) -> float:
        """Evaluate mathematical expression."""
        result = eval(expression)
        self.memory = result
        return result

    @beta_tool
    def recall(self) -> float:
        """Recall last calculation result."""
        return self.memory

calc = Calculator()

for message in client.beta.messages.tool_runner(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[calc.calculate, calc.recall],
    messages=[{"role": "user", "content": "Calculate 5*8, then add 10"}]
):
    if message.stop_reason == "end_turn":
        print(message.content[0].text)

Error Handling in Tools

@beta_tool
def divide(a: float, b: float) -> float:
    """Divide two numbers."""
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

# In tool result
try:
    result = divide(10, 0)
    tool_result = str(result)
    is_error = False
except Exception as e:
    tool_result = str(e)
    is_error = True

message = client.messages.create(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[...],
    messages=[
        ...,
        {
            "role": "user",
            "content": [{
                "type": "tool_result",
                "tool_use_id": tool_use.id,
                "content": tool_result,
                "is_error": is_error
            }]
        }
    ]
)

Async Tools

from anthropic import beta_async_tool

@beta_async_tool
async def fetch_data(url: str) -> dict:
    """Fetch data from URL."""
    async with httpx.AsyncClient() as client:
        response = await client.get(url)
        return response.json()

# Use with async tool runner
async for message in client.beta.messages.tool_runner(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    tools=[fetch_data],
    messages=[...]
):
    if message.stop_reason == "end_turn":
        print(message.content[0].text)

Best Practices

1. Clear Tool Descriptions

@beta_tool
def search_database(
    query: str,
    limit: int = 10,
    category: str | None = None
) -> list[dict]:
    """
    Search database for items matching query.

    Args:
        query: Search keywords or phrase
        limit: Maximum results to return (1-100)
        category: Optional category filter (e.g., "electronics", "books")

    Returns:
        List of matching items with id, name, price
    """
    ...

2. Validate Tool Inputs

@beta_tool
def set_temperature(degrees: float, unit: str = "celsius") -> dict:
    """Set thermostat temperature."""
    if unit not in ["celsius", "fahrenheit"]:
        raise ValueError(f"Invalid unit: {unit}")
    if degrees < -50 or degrees > 50:
        raise ValueError(f"Temperature out of range: {degrees}")
    # Set temperature
    return {"status": "success", "temperature": degrees, "unit": unit}

3. Return Structured Data

@beta_tool
def get_user_info(user_id: str) -> dict:
    """Get user information."""
    return {
        "id": user_id,
        "name": "Alice Smith",
        "email": "alice@example.com",
        "created_at": "2024-01-15"
    }

See Also

  • Tool Use API - Complete API reference
  • Beta Features - Tool runner and advanced features
  • Messages API - Message creation

Install with Tessl CLI

npx tessl i tessl/pypi-anthropic@0.75.0

docs

guides

batch-processing.md

error-handling.md

getting-started.md

multimodal.md

streaming-guide.md

tool-usage.md

index.md

tile.json