Function Tools

Function tools provide a mechanism for models to retrieve extra information to help them generate a response.

They're useful when it is impractical or impossible to put all the context an agent might need into the system prompt, or when you want to make agents' behavior more deterministic or reliable by deferring some of the logic required to generate a response to another (not necessarily AI-powered) tool.

Function tools vs. RAG

Function tools are basically the "R" of RAG (Retrieval-Augmented Generation) — they augment what the model can do by letting it request extra information.

The main semantic difference between PydanticAI Tools and RAG is RAG is synonymous with vector search, while PydanticAI tools are more general-purpose. (Note: we may add support for vector search functionality in the future, particularly an API for generating embeddings. See #58)

There are a number of ways to register tools with an agent:

via the @agent.tool decorator — for tools that need access to the agent context
via the @agent.tool_plain decorator — for tools that do not need access to the agent context
via the tools keyword argument to Agent which can take either plain functions, or instances of Tool

@agent.tool is considered the default decorator since in the majority of cases tools will need access to the agent context.

Here's an example using both:

dice_game.py


import random

from pydantic_ai import Agent, RunContext

agent = Agent(
    'google-gla:gemini-1.5-flash',  This is a pretty simple task, so we can use the fast and cheap Gemini flash model.

    deps_type=str,  We pass the user's name as the dependency, to keep things simple we use just the name as a string as the dependency.

    system_prompt=(
        "You're a dice game, you should roll the die and see if the number "
        "you get back matches the user's guess. If so, tell them they're a winner. "
        "Use the player's name in the response."
    ),
)


@agent.tool_plain  This tool doesn't need any context, it just returns a random number. You could probably use a dynamic system prompt in this case.

def roll_die() -> str:
    """Roll a six-sided die and return the result."""
    return str(random.randint(1, 6))


@agent.tool  This tool needs the player's name, so it uses RunContext to access dependencies which are just the player's name in this case.

def get_player_name(ctx: RunContext[str]) -> str:
    """Get the player's name."""
    return ctx.deps


dice_result = agent.run_sync('My guess is 4', deps='Anne')  Run the agent, passing the player's name as the dependency.

print(dice_result.data)
#> Congratulations Anne, you guessed correctly! You're a winner!

(This example is complete, it can be run "as is")

Let's print the messages from that game to see what happened:

dice_game_messages.py


from dice_game import dice_result

print(dice_result.all_messages())
"""
[
    ModelRequest(
        parts=[
            SystemPromptPart(
                content="You're a dice game, you should roll the die and see if the number you get back matches the user's guess. If so, tell them they're a winner. Use the player's name in the response.",
                timestamp=datetime.datetime(...),
                dynamic_ref=None,
                part_kind='system-prompt',
            ),
            UserPromptPart(
                content='My guess is 4',
                timestamp=datetime.datetime(...),
                part_kind='user-prompt',
            ),
        ],
        kind='request',
    ),
    ModelResponse(
        parts=[
            ToolCallPart(
                tool_name='roll_die',
                args={},
                tool_call_id='pyd_ai_tool_call_id',
                part_kind='tool-call',
            )
        ],
        model_name='gemini-1.5-flash',
        timestamp=datetime.datetime(...),
        kind='response',
    ),
    ModelRequest(
        parts=[
            ToolReturnPart(
                tool_name='roll_die',
                content='4',
                tool_call_id='pyd_ai_tool_call_id',
                timestamp=datetime.datetime(...),
                part_kind='tool-return',
            )
        ],
        kind='request',
    ),
    ModelResponse(
        parts=[
            ToolCallPart(
                tool_name='get_player_name',
                args={},
                tool_call_id='pyd_ai_tool_call_id',
                part_kind='tool-call',
            )
        ],
        model_name='gemini-1.5-flash',
        timestamp=datetime.datetime(...),
        kind='response',
    ),
    ModelRequest(
        parts=[
            ToolReturnPart(
                tool_name='get_player_name',
                content='Anne',
                tool_call_id='pyd_ai_tool_call_id',
                timestamp=datetime.datetime(...),
                part_kind='tool-return',
            )
        ],
        kind='request',
    ),
    ModelResponse(
        parts=[
            TextPart(
                content="Congratulations Anne, you guessed correctly! You're a winner!",
                part_kind='text',
            )
        ],
        model_name='gemini-1.5-flash',
        timestamp=datetime.datetime(...),
        kind='response',
    ),
]
"""

We can represent this with a diagram:

Registering Function Tools via kwarg

As well as using the decorators, we can register tools via the tools argument to the Agent constructor. This is useful when you want to reuse tools, and can also give more fine-grained control over the tools.

dice_game_tool_kwarg.py


import random

from pydantic_ai import Agent, RunContext, Tool

system_prompt = """\
You're a dice game, you should roll the die and see if the number
you get back matches the user's guess. If so, tell them they're a winner.
Use the player's name in the response.
"""


def roll_die() -> str:
    """Roll a six-sided die and return the result."""
    return str(random.randint(1, 6))


def get_player_name(ctx: RunContext[str]) -> str:
    """Get the player's name."""
    return ctx.deps


agent_a = Agent(
    'google-gla:gemini-1.5-flash',
    deps_type=str,
    tools=[roll_die, get_player_name],  The simplest way to register tools via the Agent constructor is to pass a list of functions, the function signature is inspected to determine if the tool takes RunContext.

    system_prompt=system_prompt,
)
agent_b = Agent(
    'google-gla:gemini-1.5-flash',
    deps_type=str,
    tools=[  agent_a and agent_b are identical — but we can use Tool to reuse tool definitions and give more fine-grained control over how tools are defined, e.g. setting their name or description, or using a custom prepare method.

        Tool(roll_die, takes_ctx=False),
        Tool(get_player_name, takes_ctx=True),
    ],
    system_prompt=system_prompt,
)

dice_result = {}
dice_result['a'] = agent_a.run_sync('My guess is 6', deps='Yashar')
dice_result['b'] = agent_b.run_sync('My guess is 4', deps='Anne')
print(dice_result['a'].data)
#> Tough luck, Yashar, you rolled a 4. Better luck next time.
print(dice_result['b'].data)
#> Congratulations Anne, you guessed correctly! You're a winner!

(This example is complete, it can be run "as is")

Function Tools vs. Structured Results

As the name suggests, function tools use the model's "tools" or "functions" API to let the model know what is available to call. Tools or functions are also used to define the schema(s) for structured responses, thus a model might have access to many tools, some of which call function tools while others end the run and return a result.

Function tools and schema

Function parameters are extracted from the function signature, and all parameters except RunContext are used to build the schema for that tool call.

Even better, PydanticAI extracts the docstring from functions and (thanks to griffe) extracts parameter descriptions from the docstring and adds them to the schema.

Griffe supports extracting parameter descriptions from google, numpy, and sphinx style docstrings. PydanticAI will infer the format to use based on the docstring, but you can explicitly set it using docstring_format. You can also enforce parameter requirements by setting require_parameter_descriptions=True. This will raise a UserError if a parameter description is missing.

To demonstrate a tool's schema, here we use FunctionModel to print the schema a model would receive:

tool_schema.py


from pydantic_ai import Agent
from pydantic_ai.messages import ModelMessage, ModelResponse, TextPart
from pydantic_ai.models.function import AgentInfo, FunctionModel

agent = Agent()


@agent.tool_plain(docstring_format='google', require_parameter_descriptions=True)
def foobar(a: int, b: str, c: dict[str, list[float]]) -> str:
    """Get me foobar.

    Args:
        a: apple pie
        b: banana cake
        c: carrot smoothie
    """
    return f'{a} {b} {c}'


def print_schema(messages: list[ModelMessage], info: AgentInfo) -> ModelResponse:
    tool = info.function_tools[0]
    print(tool.description)
    #> Get me foobar.
    print(tool.parameters_json_schema)
    """
    {
        'additionalProperties': False,
        'properties': {
            'a': {'description': 'apple pie', 'type': 'integer'},
            'b': {'description': 'banana cake', 'type': 'string'},
            'c': {
                'additionalProperties': {'items': {'type': 'number'}, 'type': 'array'},
                'description': 'carrot smoothie',
                'type': 'object',
            },
        },
        'required': ['a', 'b', 'c'],
        'type': 'object',
    }
    """
    return ModelResponse(parts=[TextPart('foobar')])


agent.run_sync('hello', model=FunctionModel(print_schema))

(This example is complete, it can be run "as is")

The return type of tool can be anything which Pydantic can serialize to JSON as some models (e.g. Gemini) support semi-structured return values, some expect text (OpenAI) but seem to be just as good at extracting meaning from the data. If a Python object is returned and the model expects a string, the value will be serialized to JSON.

If a tool has a single parameter that can be represented as an object in JSON schema (e.g. dataclass, TypedDict, pydantic model), the schema for the tool is simplified to be just that object.

Here's an example where we use TestModel.last_model_request_parameters to inspect the tool schema that would be passed to the model.

single_parameter_tool.py


from pydantic import BaseModel

from pydantic_ai import Agent
from pydantic_ai.models.test import TestModel

agent = Agent()


class Foobar(BaseModel):
    """This is a Foobar"""

    x: int
    y: str
    z: float = 3.14


@agent.tool_plain
def foobar(f: Foobar) -> str:
    return str(f)


test_model = TestModel()
result = agent.run_sync('hello', model=test_model)
print(result.data)
#> {"foobar":"x=0 y='a' z=3.14"}
print(test_model.last_model_request_parameters.function_tools)
"""
[
    ToolDefinition(
        name='foobar',
        description='This is a Foobar',
        parameters_json_schema={
            'properties': {
                'x': {'type': 'integer'},
                'y': {'type': 'string'},
                'z': {'default': 3.14, 'type': 'number'},
            },
            'required': ['x', 'y'],
            'title': 'Foobar',
            'type': 'object',
        },
        outer_typed_dict_key=None,
    )
]
"""

(This example is complete, it can be run "as is")

Dynamic Function tools

Tools can optionally be defined with another function: prepare, which is called at each step of a run to customize the definition of the tool passed to the model, or omit the tool completely from that step.

A prepare method can be registered via the prepare kwarg to any of the tool registration mechanisms:

@agent.tool decorator
@agent.tool_plain decorator
Tool dataclass

The prepare method, should be of type ToolPrepareFunc, a function which takes RunContext and a pre-built ToolDefinition, and should either return that ToolDefinition with or without modifying it, return a new ToolDefinition, or return None to indicate this tools should not be registered for that step.

Here's a simple prepare method that only includes the tool if the value of the dependency is 42.

As with the previous example, we use TestModel to demonstrate the behavior without calling a real model.

tool_only_if_42.py


from typing import Union

from pydantic_ai import Agent, RunContext
from pydantic_ai.tools import ToolDefinition

agent = Agent('test')


async def only_if_42(
    ctx: RunContext[int], tool_def: ToolDefinition
) -> Union[ToolDefinition, None]:
    if ctx.deps == 42:
        return tool_def


@agent.tool(prepare=only_if_42)
def hitchhiker(ctx: RunContext[int], answer: str) -> str:
    return f'{ctx.deps} {answer}'


result = agent.run_sync('testing...', deps=41)
print(result.data)
#> success (no tool calls)
result = agent.run_sync('testing...', deps=42)
print(result.data)
#> {"hitchhiker":"42 a"}

(This example is complete, it can be run "as is")

Here's a more complex example where we change the description of the name parameter to based on the value of deps

For the sake of variation, we create this tool using the Tool dataclass.

customize_name.py


from __future__ import annotations

from typing import Literal

from pydantic_ai import Agent, RunContext
from pydantic_ai.models.test import TestModel
from pydantic_ai.tools import Tool, ToolDefinition


def greet(name: str) -> str:
    return f'hello {name}'


async def prepare_greet(
    ctx: RunContext[Literal['human', 'machine']], tool_def: ToolDefinition
) -> ToolDefinition | None:
    d = f'Name of the {ctx.deps} to greet.'
    tool_def.parameters_json_schema['properties']['name']['description'] = d
    return tool_def


greet_tool = Tool(greet, prepare=prepare_greet)
test_model = TestModel()
agent = Agent(test_model, tools=[greet_tool], deps_type=Literal['human', 'machine'])

result = agent.run_sync('testing...', deps='human')
print(result.data)
#> {"greet":"hello a"}
print(test_model.last_model_request_parameters.function_tools)
"""
[
    ToolDefinition(
        name='greet',
        description='',
        parameters_json_schema={
            'additionalProperties': False,
            'properties': {
                'name': {'type': 'string', 'description': 'Name of the human to greet.'}
            },
            'required': ['name'],
            'type': 'object',
        },
        outer_typed_dict_key=None,
    )
]
"""

(This example is complete, it can be run "as is")